Experiments on Plant Hybridization — TEI-tagged view

Gregor MendelpersName (1866date). “Experiments on Plant Hybridization.” Verhandlungen des naturforschenden Vereines in Brünntitle·j, BrnoplaceName.

Object of study — The inheritance of discrete traits across generationsterm·key of hybridised pea plantsterm·key.

Research question — By what regular law are inherited charactersterm·key transmitted and recombined in hybridterm·dom offspringterm·key?

Contribution — Establishes the laws of segregationterm·dom and independent assortmentterm·dom, founding genetics through quantitative breeding experimentsterm·mth.

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EXPERIMENTSterm·mth IN PLANTterm·key HYBRIDIZATION (1865) GREGOR MENDELpersName Read at the February 8th, and March 8th, 1865, meetings of the Brünn Natural History Society MendelpersName, Gregor. 1866. Versuche über Plflanzenhybriden. Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr 1865, Abhandlungen, 3–47.

MendelpersName’s paper was first translated into English by William Bateson in 1901. This present version derives from the Bateson translation, with some minor corrections and changes provided by Roger Blumberg as part of the MendelWeb project. A few additional corrections have been made in this version. For an annotated copy, which the Bateson translation and Blumberg's corrections, see the MendelWeb site: includes comments on (http://www.netspace.org./MendelWeb/).

© 1996, Electronic Scholarly Publishing Project http://www.esp.org This electronic edition is made freely available for scholarly or educational purposes, provided that this copyright notice is included. The manuscript may not be reprinted or redistributed for commercial purposes without permission.

MendelpersName, Gregor. 1866. Versuche über Plflanzen-hybriden. Verhandlungen des naturforschenden Ver-eines in Brünn, Bd. IV für das Jahr 1865, Abhand-lungen, 3–47.

EXPERIMENTSterm·mth IN PLANTterm·key HYBRIDIZATION (1865) GREGOR MENDELpersName Read at the February 8th, and March 8th, 1865, meetings of the Brünn Natural History Society INTRODUCTORY REMARKS EXPERIENCE OF ARTIFICIAL FERTILIZATION, such as is effected with ornamental plantsterm·key in order to obtain new variations in color, has led to the experimentsterm·mth which will here be discussed. The striking regularity with which the same hybridterm·dom forms always reappeared whenever fertilization took place between the same species induced further experimentsterm·mth to be undertaken, the object of which was to follow up the developments of the hybridsterm·key in their progeny.

To this object numerous careful observers, such as Kölreuter, Gärtner, Herbert, Lecoq, Wichura and others, have devoted a part of their lives with inexhaustible perseverance. Gärtner especially in his work Die Bastarderzeugung im Pflanzenreiche [The Production of Hybridsterm·key in the Vegetable Kingdom], has recorded very valuable observations; and quite recently Wichura published the results of some profound investigations into the hybridsterm·key of the Willow. That, so far, no generally applicable law governing the formation and development of hybridsterm·key has been successfully formulated can hardly be wondered at by anyone who is acquainted with the extent of the task, and can appreciate the difficulties with which experimentsterm·mth of this class have to contend. A final decision can only be arrived at when we shall have before us the results of detailed experimentsterm·mth made on plantsterm·key belonging to the most diverse orders.

© 1996, Electronic Scholarly Publishing Project This electronic edition is made freely available for scholarly or educational purposes, provided that this copyright notice is included. The manuscript may not be reprinted or redistributed for commercial purposes without permission.

GREGOR MENDELpersName (1865) Those who survey the work done in this department will arrive at the conviction that among all the numerous experimentsterm·mth made, not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspringterm·key of the hybridsterm·key appear, or to arrange these forms with certainty according to their separate generationsterm·key, or definitely to ascertain their statistical relations.

It requires indeed some courage to undertake a labor of such far– reaching extent; this appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be overestimated in connection with the history of the evolution of organic forms.

The paper now presented records the results of such a detailed experimentterm·mth. This experimentterm·mth was practically confined to a small plantterm·key group, and is now, after eight years’ pursuit, concluded in all essentials. Whether the plan upon which the separate experimentsterm·mth were conducted and carried out was the best suited to attain the desired end is left to the friendly decision of the reader.

SELECTION OF THE EXPERIMENTAL PLANTSterm·key The value and utility of any experimentterm·mth are determined by the fitness of the material to the purpose for which it is used, and thus in the case before us it cannot be immaterial what plantsterm·key are subjected to experimentterm·mth and in what manner such experimentterm·mth is conducted.

The selection of the plantterm·key group which shall serve for experimentsterm·mth of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results.

The experimental plantsterm·key must necessarily: 1. Possess constant differentiating characteristicsterm·dom 2. The hybridsterm·key of such plantsterm·key must, during the flowering period, be protected from the influence of all foreign pollenterm·key, or be easily capable of such protection.

The hybridsterm·key and their offspringterm·key should suffer no marked disturbance in their fertility in the successive generationsterm·key.

Accidental impregnation by foreign pollenterm·key, if it occurred during the experimentsterm·mth and were not recognized, would lead to entirely erroneous conclusions. Reduced fertility or entire sterility of certain forms, such as occurs in the offspringterm·key of many hybridsterm·key, would render ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden the experimentsterm·mth very difficult or entirely frustrate them. In order to discover the relations in which the hybridterm·dom forms stand towards each other and also towards their progenitors it appears to be necessary that all member of the series developed in each successive generationsterm·key should be, without exception, subjected to observation.

At the very outset special attention was devoted the Leguminosae on account of floral structure. Experimentsterm·mth which were made with several members of this family led to the result that the genus Pisum was found to possess the necessary qualifications.

their peculiar Some thoroughly distinct forms of this genus possess charactersterm·key which are constant, and easily and certainly recognizable, and when their hybridsterm·key are mutually crossed they yield perfectly fertile progeny. Furthermore, a disturbance through foreign pollenterm·key cannot easily occur, since the fertilizing organs are closely packed inside the keel and the anthers burst within the bud, so that the stigma becomes covered with pollenterm·key even before the flower opens. This circumstance is especially important. As additional advantages worth mentioning, there may be cited the easy culture of these plantsterm·key in the open ground and in pots, and also their relatively short period of growth. Artificial fertilization is certainly a somewhat elaborate process, but nearly always succeeds. For this purpose the bud is opened before it is perfectly developed, the keel is removed, and each stamen carefully extracted by means of forceps, after which the stigma can at once be dusted over with the foreign pollenterm·key.

In all, thirty–four more or less distinct varieties of Peas were obtained from several seedsmen and subjected to a two year’s trial. In the case of one variety there were noticed, among a larger number of plantsterm·key all alike, a few forms which were markedly different. These, however, did not vary in the following year, and agreed entirely with another variety obtained from the same seedsman; the seeds were therefore doubtless merely accidentally mixed. All the other varieties yielded perfectly constant and similar offspringterm·key; at any rate, no essential difference was observed during two trial years. For fertilization twenty–two of these were selected and cultivated during the whole period of the experimentsterm·mth. They remained constant without any exception.

Their systematic classification is difficult and uncertain. If we adopt the strictest definition of a species, according to which only those individuals belong to a species which under precisely the same circumstances display precisely similar charactersterm·key, no two of these varieties could be referred to one species. According to the opinion of CLASSICAL GENETICS GREGOR MENDELpersName (1865) experts, however, the majority belong to the species Pisum sativum; while the rest are regarded and classed, some as sub–species of P. sativum, and some as independent species, such as P. quadratum, P. saccharatum, and P. umbellatum. The positions, however, which may be assigned to them in a classificatory system are quite immaterial for the purposes of the experimentsterm·mth in question. It has so far been found to be just as impossible to draw a sharp line between the hybridsterm·key of species and varieties as between species and varieties themselves.

DIVISION AND ARRANGEMENT OF THE EXPERIMENTSterm·mth If two plantsterm·key which differ constantly in one or several charactersterm·key be crossed, numerous experimentsterm·mth have demonstrated that the common charactersterm·key are transmitted unchanged to the hybridsterm·key and their progeny; but each pair of differentiating charactersterm·key, on the other hand, unite in the hybridterm·dom to form a new characterterm·key, which in the progeny of the hybridterm·dom is usually variable. The object of the experimentterm·mth was to observe these variations in the case of each pair of differentiating charactersterm·key, and to deduce the law according to which they appear in successive generationsterm·key. The experimentterm·mth resolves itself therefore into just as many separate experimentsterm·mth are there are constantly differentiating charactersterm·key presented in the experimental plantsterm·key.

The various forms of Peas selected for crossing showed differences in length and color of the stem; in the size and form of the leaves; in the position, color, size of the flowers; in the length of the flower stalk; in the color, form, and size of the pods; in the form and size of the seeds; and in the color of the seed–coats and of the albumen (endosperm). Some of the charactersterm·key noted do not permit of a sharp and certain separation, since the difference is of a “more or less” nature, which is often difficult to define. Such charactersterm·key could not be utilized for the separate experimentsterm·mth; these could only be applied to charactersterm·key which stand out clearly and definitely in the plantsterm·key. Lastly, the result must show whether they, in their entirety, observe a regular behavior in their hybridterm·dom unions, and whether from these facts any conclusion can be reached regarding those charactersterm·key which possess a subordinate significance in the type.

The charactersterm·key which were selected for experimentterm·mth relate: 1. To the difference in the form of the ripe seeds. These are either round or roundish, the depressions, if any, occur ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden on the surface, being always only shallow; or they are irregularly angular and deeply wrinkled (P. quadratum). 2. To the difference in the color of the seed albumen (endosperm). The albumen of the ripe seeds is either pale yellow, bright yellow and orange colored, or it possesses a more or less intense green tint. This difference of color is easily seen in the seeds as their coats are transparent. 3. To the difference in the color of the seed–coat. This is either white, with which characterterm·key white flowers are constantly correlated; or it is gray, gray–brown, leather– brown, with or without violet spotting, in which case the color of the standards is violet, that of the wings purple, and the stem in the axils of the leaves is of a reddish tint. The gray seed–coats become dark brown in boiling water.

4. To the difference in the form of the ripe pods. These are either simply inflated, not contracted in places; or they are deeply constricted between the seeds and more or less wrinkled (P. saccharatum).

5. To the difference in the color of the unripe pods. They are either light to dark green, or vividly yellow, in which coloring the stalks, leaf–veins, and calyx participate.* 6. To the difference in the position of the flowers. They are either axial, that is, distributed along the main stem; or they are terminal, that is, bunched at the top of the stem and arranged almost in a false umbel; in this case the upper part of the stem is more or less widened in section (P. umbellatum).

7. To the difference in the length of the stem. The length of the stem is very various in some forms; it is, however, a constant characterterm·key for each, in so far that healthy plantsterm·key, grown in the same soil, are only subject to unimportant variations in this characterterm·key. In experimentsterm·mth with this characterterm·key, in order to be able to discriminate with certainty, the long axis of 6 to 7 ft. was always crossed with the short one of ¾ ft. to 1½ ft.

One species possesses a beautifully brownish–red colored pod, which when ripening turns to violet and blue. Trials with this characterterm·key were only begun last year.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) Each two of the differentiating charactersterm·key enumerated above were united by cross–fertilization. There were made for the experimentterm·mth 60 fertilizations on 15 plantsterm·key. 1st 2nd experimentterm·mth 58 fertilizations on 10 plantsterm·key. 3rd experimentterm·mth 35 fertilizations on 10 plantsterm·key. 4th experimentterm·mth 40 fertilizations on 10 plantsterm·key. 5th experimentterm·mth 23 fertilizations on 5 plantsterm·key. 6th experimentterm·mth 34 fertilizations on 10 plantsterm·key. 7th experimentterm·mth 37 fertilizations on 10 plantsterm·key.

From a larger number of plantsterm·key of the same variety only the most vigorous were chosen for fertilization. Weakly plantsterm·key always afford uncertain results, because even in the first generationterm·key of hybridsterm·key, and still more so in the subsequent ones, many of the offspringterm·key either entirely fail to flower or only form a few and inferior seeds.

Furthermore, in all the experimentsterm·mth reciprocal crossings were effected in such a way that each of the two varieties which in one set of fertilizations served as seed–bearer in the other set was used as the pollenterm·key plantterm·key.

The plantsterm·key were grown in garden beds, a few also in pots, and were maintained in their natural upright position by means of sticks, branches of trees, and strings stretched between. For each experimentterm·mth a number of pot plantsterm·key were placed during the blooming period in a greenhouse, to serve as control plantsterm·key for the main experimentterm·mth in the open as regards possible disturbance by insects. Among the insects which visit Peas the beetle Buchus pisi might be detrimental to the experimentsterm·mth should it appear in numbers. The female of this species is known to lay the eggs in the flower, and in so doing opens the keel; upon the tarsi of one specimen, which was caught in a flower, some pollenterm·key grains could clearly be seen under a lens. Mention must also be made of a circumstance which possibly might lead to the introduction of foreign pollenterm·key. It occurs, for instance, in some rare cases that certain parts of an otherwise normally developed flower wither, resulting in a partial exposure of the fertilizing organs. A defective development of the keel has also been observed, owing to which the stigma and anthers remained partially covered. It also sometimes happens that the pollenterm·key does not reach full perfection. In this event there occurs a gradual lengthening of the pistil during the blooming period, until the stigmatic tip protrudes at the point of the keel. This remarkable appearance has also been observed in hybridsterm·key of Phaseolus and Lathyrus.

ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden The risk of false impregnation by foreign pollenterm·key is, however, a very slight one with Pisum, and is quite incapable of disturbing the general result. Among more than 10,000 plantsterm·key which were carefully examined there were only a very few cases where an indubitable false impregnation had occurred. Since in the greenhouse such a case was never remarked, it may well be supposed that Brucus pisi, and possibly also the described abnormalities in the floral structure, were to blame.

THE FORMS OF THE HYBRIDSterm·key Experimentsterm·mth which in previous years were made with ornamental plantsterm·key have already afforded evidence that the hybridsterm·key, as a rule, are not exactly intermediate between the parental species. With some of the more striking charactersterm·key, those, for instance, which relate to the form and size of the leaves, the pubescence of the several parts, etc., the intermediate, indeed, is nearly always to be seen; in other cases, however, one of the two parental charactersterm·key is so preponderant that it is difficult, or quite impossible, to detect the other in the hybridterm·dom.

This is precisely the case with the Pea hybridsterm·key. In the case of each of the seven crosses the hybridterm·dom–characterterm·key resembles that of one of the parental forms so closely that the other either escapes observation completely or cannot be detected with certainty. This circumstance is of great importance in the determination and classification of the forms under which the offspringterm·key of the hybridsterm·key appear. Henceforth in this paper those charactersterm·key which are transmitted entire, or almost unchanged in the hybridization, and therefore in themselves constitute the charactersterm·key of the hybridterm·dom, are termed the dominantterm·dom, and those which become latent in the process recessiveterm·dom. The expression recessiveterm·dom has been chosen because the charactersterm·key thereby designated withdraw or entirely disappear in the hybridsterm·key, but nevertheless reappear unchanged in their progeny, as will be demonstrated later on.

It was furthermore shown by the whole of the experimentsterm·mth that it is perfectly immaterial whether the dominant characterterm·def belongs to the seed plantterm·key or to the pollenterm·key plantterm·key; the form of the hybridterm·dom remains identical in both cases. This interesting fact was also emphasized by Gärtner, with the remark that even the most practiced expert is not in a position to determine in a hybridterm·dom which of the two parental species was the seed or the pollenterm·key plantterm·key.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) Of the differentiating charactersterm·key which were used the experimentsterm·mth the following are dominantterm·dom: 1. The round or roundish form of the seed with or without shallow depressions.

2. The yellow coloring of the seed albumen. 3. The gray, gray–brown, or leather brown color of the seed–coat, in association with violet–red blossoms and reddish spots in the leaf axils. 4. The simply inflated form of the pod. 5. The green coloring of the unripe pod in association with the same color of the stems, the leaf–veins and the calyx.

6. The distribution of the flowers along the stem. 7. The greater length of stem.

With regard to this last characterterm·key it must be stated that the longer of the two parental stems is usually exceeded by the hybridterm·dom, a fact which is possibly only attributable to the greater luxuriance which appears in all parts of plantsterm·key when stems of very different lengths are crossed. Thus, for instance, in repeated experimentsterm·mth, stems of 1 ft. and 6 ft. in length yielded without exception hybridsterm·key which varied in length between 6 ft. and 7½ ft.

The hybridterm·dom seeds in the experimentsterm·mth with seed–coat are often more spotted, and the spots sometimes coalesce into small bluish– violet patches. The spotting also frequently appears even when it is absent as a parental characterterm·key.

The hybridterm·dom forms of the seed–shape and of the [color of the] albumen are developed immediately after the artificial fertilization by the mere influence of the foreign pollenterm·key. They can, therefore, be observed even in the first year of experimentterm·mth, whilst all the other charactersterm·key naturally only appear in the following year in such plantsterm·key as have been raised from the crossed seed.

THE FIRST GENERATIONterm·key FROM THE HYBRIDSterm·key In this generationterm·key there reappear, together with the dominant charactersterm·def, also the recessiveterm·dom ones with their peculiarities fully developed, and this occurs in the definitely expressed average proportionterm of three to one, so that among each four plantsterm·key of this generationterm·key three display the dominant characterterm·def and one the recessiveterm·dom.

ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden This relates without exception to all the charactersterm·key which were investigated in the experimentsterm·mth. The angular wrinkled form of the seed, the green color of the albumen, the white color of the seed–coats and the flowers, the constrictions of the pods, the yellow color of the unripe pod, of the stalk, of the calyx, and of the leaf venation, the umbel–like form of the inflorescence, and the dwarfed stem, all reappear in the numerical proportion given, without any essential alteration. Transitional forms were not observed in any experimentterm·mth.

Since the hybridsterm·key resulting from reciprocal crosses are formed alike and present no appreciable difference in their subsequent development, consequently these results can be reckoned together in each experimentterm·mth. The relative numbers which were obtained for each pair of differentiating charactersterm·key are as follows: Expt 1: Expt 2: Form of seed. –– From 253 hybridsterm·key 7,324 seeds were obtained in the second trial year. Among them were 5,474 round or roundish ones and 1,850 angular wrinkled ones. Therefrom the ratioterm·key 2.96:1 is deduced. Color of albumen. –– 258 plantsterm·key yielded 8,023 seeds, 6,022 yellow, and 2,001 green; their ratioterm·key, therefore, is as 3.01:1.

In these two experimentsterm·mth each pod yielded usually both kinds of seed. In well–developed pods which contained on the average six to nine seeds, it often happened that all the seeds were round (Expt. 1) or all yellow (Expt. 2); on the other hand there were never observed more than five wrinkled or five green ones on one pod. It appears to make no difference whether the pods are developed early or later in the hybridterm·dom or whether they spring from the main axis or from a lateral one. In some few plantsterm·key only a few seeds developed in the first formed pods, and these possessed exclusively one of the two charactersterm·key, but in the subsequently developed pods the normal proportions were maintained nevertheless.

As in separate pods, so did the distribution of the charactersterm·key vary in separate plantsterm·key. By way of illustration the first ten individuals from both series of experimentsterm·mth may serve.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) Experimentterm·mth 1 Form of the Seed round 45 27 24 19 32 26 88 22 28 25 wrinkled 12 8 7 10 11 6 24 10 6 7 Plantsterm·key 1 2 3 4 5 6 7 8 9 10 Experimentterm·mth 2 Color of the Albumen yellow 25 32 14 70 24 20 32 44 50 44 green 11 7 5 27 13 6 13 9 14 18 As extremes in the distribution of the two seed charactersterm·key in one plantterm·key, there were observed in Expt. 1 an instance of 43 round and only two angular, and another of 14 round and 15 angular seeds. In Expt. 2 there was a case of 32 yellow and only one green seed, but also one of 20 yellow and 19 green.

These two experimentsterm·mth are important for the determination of the average ratiosterm·key, because with a smaller number of experimental plantsterm·key they show that very considerable fluctuations may occur. In counting the seeds, also, especially in Expt. 2, some care is requisite, since in some of the seeds of many plantsterm·key the green color of the albumen is less developed, and at first may be easily overlooked. The cause of this partial disappearance of the green coloring has no connection with the hybridterm·dom–characterterm·key of the plantsterm·key, as it likewise occurs in the parental variety. This peculiarity is also confined to the individual and is not inherited by the offspringterm·key. In luxuriant plantsterm·key this appearance was frequently noted. Seeds which are damaged by insects during their development often vary in color and form, but with a little practice in sorting, errors are easily avoided. It is almost superfluous to mention that the pods must remain on the plantsterm·key until they are thoroughly ripened and have become dried, since it is only then that the shape and color of the seed are fully developed.

Expt. 3: Color of the seed–coats. –– Among 929 plantsterm·key, 705 bore violet–red flowers and gray–brown seed– coats; 224 had white flowers and white seed– coats, giving the proportion 3.15:1. Form of pods. –– Of 1,181 plantsterm·key, 882 had them simply inflated, and in 299 they were constricted. Resulting ratioterm·key, 2.95:1.

Expt. 4: ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden Expt. 5: Color of the unripe pods. –– The number of trial plantsterm·key was 580, of which 428 had green pods and 152 yellow ones. Consequently these stand in the ratioterm·key of 2.82:1. Position of flowers. –– Among 858 cases 651 had inflorescences axial and 207 terminal. Ratioterm·key, 3.14:1.

Expt. 6: Expt. 7: Length of stem. –– Out of 1,064 plantsterm·key, in 787 cases the stem was long, and in 277 short. Hence a mutual ratioterm·key of 2.84:1. In this experimentterm·mth the and dwarfed plantsterm·key were transferred to a special bed. This precaution was necessary, as otherwise they would have perished through being overgrown by their tall relatives. Even in their quite young state they can be easily picked out by their compact growth and thick dark–green foliage.

carefully lifted If now the results of the whole of the experimentsterm·mth be brought together, there is found, as between the number of forms with the dominantterm·dom and recessive charactersterm·def, an average ratioterm·key of 2.98:1, or 3:1.

The dominant characterterm·def can have here a double signification –– viz. that of a parental characterterm·key or a hybridterm·dom–characterterm·key. In which of the two significations it appears in each separate case can only be determined by the following generationterm·key. As a parental characterterm·key it must pass over unchanged to the whole of the offspringterm·key; as a hybridterm·dom– characterterm·key, on the other hand, it must maintain the same behavior as in the first generationterm·key.

THE SECOND GENERATIONterm·key FROM THE HYBRIDSterm·key Those forms which in the first generationterm·key exhibit the recessive characterterm·def do not further vary in the second generationterm·key as regards this characterterm·key; they remain constant in their offspringterm·key.

It is otherwise with those which possess the dominant characterterm·def in the first generationterm·key [bred from the hybridsterm·key –– i.e., the F2 in modern terminology]. Of these two–thirds yield offspringterm·key which display the dominantterm·dom and recessive charactersterm·def in the proportion of three to one, and thereby show exactly the same ratioterm·key as the hybridterm·dom forms, while only one–third remains with the dominant characterterm·def constant.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) The separate experimentsterm·mth yielded the following results: Expt. 1: Among 565 plantsterm·key which were raised from round seeds of the first generationterm·key, 193 yielded round seeds only, and remained therefore constant in this characterterm·key; 372, however, gave both round and wrinkled seeds, in the proportion of 3:1. The number of the hybridsterm·key, therefore, as compared with the constants is 1.93:1.

Expt. 2: Of 519 plantsterm·key which were raised from seeds whose albumen was of yellow color the first generationterm·key, 166 yielded exclusively yellow, while 353 yielded yellow and green seeds in the proportion of 3:1. There resulted, therefore, a division into hybridterm·dom and constant forms in the proportion of 2.13:1.

For each separate trial in the following experimentsterm·mth 100 plantsterm·key were selected which displayed the dominant characterterm·def in the first generationterm·key, and in order to ascertain the significance of this, ten seeds of each were cultivated.

Expt. 3: Expt. 4: Expt. 5: The offspringterm·key of 36 plantsterm·key yielded exclusively gray–brown seed–coats, while of the offspringterm·key of 64 plantsterm·key some had gray–brown and some had white.

The offspringterm·key of 29 plantsterm·key had only simply inflated pods; of the offspringterm·key of 71, on the other hand, some had inflated and some constricted.

The offspringterm·key of 40 plantsterm·key had only green pods; of the offspringterm·key of 60 plantsterm·key some had green, some yellow ones.

Expt. 6.: The offspringterm·key of 33 plantsterm·key had only axial flowers; of the offspringterm·key of 67, on the other hand, some had axial and some terminal flowers. The offspringterm·key of 28 plantsterm·key inherited the long axis, of those of 72 plantsterm·key some the long and some the short axis.

Expt. 7: In each of these experimentsterm·mth a certain number of the plantsterm·key came constant with the dominant characterterm·def. For the determination of the proportion in which the separation of the forms with the constantly ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden persistent characterterm·key results, the two first experimentsterm·mth are especially important, since in these a larger number of plantsterm·key can be compared. The ratiosterm·key 1.93:1 and 2.13:1 gave together almost exactly the average ratioterm·key of 2:1. Experimentterm·mth 6 gave a quite concordant result; in the others the ratioterm·key varies more or less, as was only to be expected in view of the smaller number of 100 trial plantsterm·key. Experimentterm·mth 5, which shows the greatest departure, was repeated, and then in lieu of the ratioterm·key of 60:40, that of 65:35 resulted. The average ratioterm·key of 2 to 1 appears, therefore, as fixed with certainty. It is therefore demonstrated that, of those forms which possess the dominant characterterm·def in the first generationterm·key, two–thirds have the hybridterm·dom–characterterm·key, while one–third remains constant with the dominant characterterm·def.

The ratioterm·key 3:1, in accordance with which the distribution of the dominantterm·dom and recessive charactersterm·def results in the first generationterm·key, resolves itself therefore in all experimentsterm·mth into the ratioterm·key of 2:1:1, if the dominant characterterm·def be differentiated according to its significance as a hybridterm·dom–characterterm·key or as a parental one. Since the members of the first generationterm·key spring directly from the seed of the hybridsterm·key, it is now clear that the hybridsterm·key form seeds having one or other of the two differentiating charactersterm·key, and of these one–half develop again the hybridterm·dom form, while the other half yield plantsterm·key which remain constant and receive the dominantterm·dom or the recessive charactersterm·def in equal numbers.

THE SUBSEQUENT GENERATIONSterm·key FROM THE HYBRIDSterm·key The proportions in which the descendants of the hybridsterm·key develop and split up in the first and second generationsterm·key presumably hold good for all subsequent progeny. Experimentsterm·mth 1 and 2 have already been carried through six generationsterm·key; 3 and 7 through five; and 4, 5, and 6 through four; these experimentsterm·mth being continued from the third generationterm·key with a small number of plantsterm·key, and no departure from the rule has been perceptible. The offspringterm·key of the hybridsterm·key separated in each generationterm·key in the ratioterm·key of 2:1:1 into hybridsterm·key and constant forms.

If A be taken as denoting one of the two constant charactersterm·key, for instance the dominantterm·dom, a, the recessiveterm·dom, and Aa the hybridterm·dom form in which both are conjoined, the expression A + 2Aa + a CLASSICAL GENETICS GREGOR MENDELpersName (1865) shows the terms in the series for the progeny of the hybridsterm·key of two differentiating charactersterm·key.

The observation made by Gärtner, Kölreuter, and others, that hybridsterm·key are inclined to revert to the parental forms, is also confirmed by the experimentsterm·mth described. It is seen that the number of the hybridsterm·key which arise from one fertilization, as compared with the number of forms which become constant, and their progeny from generationterm·key to generationterm·key, is continually diminishing, but that nevertheless they could not entirely disappear. If an average equality of fertility in all plantsterm·key in all generationsterm·key be assumed, and if, furthermore, each hybridterm·dom forms seed of which one–half yields hybridsterm·key again, while the other half is constant to both charactersterm·key in equal proportions, the ratioterm·key of numbers for the offspringterm·key in each generationterm·key is seen by the following summary, in which A and a denote again the two parental charactersterm·key, and Aa the hybridterm·dom forms. For brevity’s sake it may be assumed that each plantterm·key in each generationterm·key furnishes only four seeds.

Generationterm·key 1 2 3 4 5 n A 1 6 28 120 496 Aa 2 4 8 16 32 a 1 6 28 120 496 Ratiosterm·key a A : Aa : 1 : 1 : 3 : 3 : : 7 : 7 : 15 15 : : 31 31 : : 2n – 1 2n – 1 : 2 2 2 2 2 2 In the tenth generationterm·key, for instance, 2n – 1 = 1,023. There result, therefore, in each 2,048 plantsterm·key which arise in this generationterm·key 1,023 with the constant dominant characterterm·def, 1,023 with the recessive characterterm·def, and only two hybridsterm·key.

THE OFFSPRINGterm·key OF HYBRIDSterm·key IN WHICH SEVERAL DIFFERENTIATING CHARACTERSterm·key ARE ASSOCIATED.

In the experimentsterm·mth above described plantsterm·key were used which differed only on one essential characterterm·key. The next task consisted in ascertaining whether the law of development discovered in these applied to each pair of differentiating charactersterm·key when several diverse charactersterm·key are united in the hybridterm·dom by crossing.

As regards the form of the hybridsterm·key in these cases, the experimentsterm·mth showed throughout that this invariably more nearly approaches to that one of the two parental plantsterm·key which possesses the greater number of ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden dominant charactersterm·def. If, for instance, the seed plantterm·key has a short stem, terminal white flowers, and simply inflated pods; the pollenterm·key plantterm·key, on the other hand, a long stem, violet–red flowers distributed along the stem, and constricted pods; the hybridterm·dom resembles the seed parent only in the form of the pod; in the other charactersterm·key it agrees with the pollenterm·key parent. Should one of the two parental types possess only dominant charactersterm·def, then the hybridterm·dom is scarcely or not at all distinguishable from it.

Two experimentsterm·mth were made with a considerable number of plantsterm·key. In the first experimentterm·mth the parental plantsterm·key differed in the form of the seed and in the color of the albumen; in the second in the form of the seed, in the color of the albumen, and in the color of the seed– coats. Experimentsterm·mth with seed charactersterm·key give the result in the simplest and most certain way.

In order to facilitate study of the data in these experimentsterm·mth, the different charactersterm·key of the seed plantterm·key will be indicated by A, B, C, those of the pollenterm·key plantterm·key by a, b, c, and the hybridterm·dom forms of the charactersterm·key by Aa, Bb, and Cc.

Expt. 1. — AB, seed parents A, form round B, albumen yellow ab, pollenterm·key parents a, form wrinkled b, albumen green The fertilized seeds appeared round and yellow like those of the seed parents. The plantsterm·key raised therefrom yielded seeds of four sorts, which frequently presented themselves in one pod. In all, 556 seeds were yielded by 15 plantsterm·key, and of these there were: round and yellow, 315 101 wrinkled and yellow, 108 round and green, 32 wrinkled and green.

All were sown the following year. Eleven of the round yellow seeds did not yield plantsterm·key, and three plantsterm·key did not form seeds. Among the rest: 38 had round yellow seeds 65 round yellow and green seeds 60 round yellow and wrinkled yellow seeds 138 round yellow and green, wrinkled yellow and green seeds AB ABb AaB AaBb From the wrinkled yellow seeds 96 resulting plantsterm·key bore seed, of which: CLASSICAL GENETICS GREGOR MENDELpersName (1865) 28 had only wrinkled yellow seeds 68 wrinkled yellow and green seeds aB aBb From 108 round green seeds 102 resulting plantsterm·key fruited, of which: 35 had only round green seeds 67 round and wrinkled green seeds Ab Aab The wrinkled green seeds yielded 30 plantsterm·key which bore seeds all of like characterterm·key; they remained constant ab.

The offspringterm·key of the hybridsterm·key appeared therefore under nine different forms, some of them in very unequal numbers. When these are collected and coordinated we find: 38 35 28 30 65 68 60 67 138 plantsterm·key with the sign " " " " " " " " " " " " " " " " " " " " " " " " AB Ab aB ab ABb aBb AaB Aab AaBb The whole of the forms may be classed into three essentially different groups. The first includes those with the signs AB, Ab, aB, and ab: they possess only constant charactersterm·key and do not vary again in the next generationterm·key. Each of these forms is represented on the average 33 times. The second group includes the signs ABb, aBb, AaB, Aab: these are constant in one characterterm·key and hybridterm·dom in another, and vary in the next generationterm·key only as regards the hybridterm·dom–characterterm·key. Each of these appears on any average 65 times. The form AaBb occurs 138 times: it is hybridterm·dom in both charactersterm·key, and behaves exactly as do the hybridsterm·key from which it is derived.

If the numbers in which the forms belonging to these classes appear be compared, the ratiosterm·key of 1:2:4 are unmistakably evident. The numbers 33, 65, 138 present very fair approximations to the ratioterm·key numbers of 33, 66, 132.

The development series consists, therefore, of nine classes, of which four appear therein always once and are constant in both charactersterm·key; the forms AB, ab, resemble the parental forms, the two others present combinations between the conjoined charactersterm·key A, a, B, b, which combinations are likewise possibly constant. Four classes appear always twice, and are constant in one characterterm·key and hybridterm·dom in the other. One class appears four times, and is hybridterm·dom in both ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden charactersterm·key. Consequently, the offspringterm·key of the hybridsterm·key, if two kinds of differentiating charactersterm·key are combined therein, are represented by the expression AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb This expression is indisputably a combination series in which the two expressions for the charactersterm·key A and a, B and b are combined. We arrive at the full number of the classes of the series by the combination of the expressions: A + 2Aa + a B + 2Bb + b Expt. 2. — ABC, seed parents A, form round B, albumen yellow C, seed coat grey–brown abc, pollenterm·key parents a, form wrinkled b, albumen green c, seed coat white This experimentterm·mth was made in precisely the same way as the previous one. Among all the experimentsterm·mth it demanded the most time and trouble. From 24 hybridsterm·key 687 seeds were obtained in all: these were all either spotted, gray–brown or gray–green, round or wrinkled. From these in the following year 639 plantsterm·key fruited, and as further investigation showed, there were among them: 8 plantsterm·key ABC Abc 14 AbC 9 Abc 11 aBC 8 aBc 10 abC 10 abc 7 " " " " " " " 22 plantsterm·key ABCc AbCc 17 aBCc 25 abCc 20 ABbC 15 Abbc 18 aBbC 19 aBbc 24 AaBC 14 AaBc 18 AabC 20 Aabc 16 " " " " " " " " " " " 45 plantsterm·key ABbCc aBbCc 36 AaBCc 38 AabCc 40 AaBbC 49 AaBbc 48 " " " " " AaBbCc The whole expression contains 27 terms. Of these eight are constant in all charactersterm·key, and each appears on the average ten times; twelve are constant in two charactersterm·key, and hybridterm·dom in the third; each appears on the average 19 times; six are constant in one characterterm·key and hybridterm·dom in the other two; each appears on the average 43 times. One form appears 78 times and is hybridterm·dom in all of the charactersterm·key. The ratiosterm·key 10:19:43:78 agree so closely with the ratiosterm·key 10:20:40:80, or 1:2:4:8 that this last undoubtedly represents the true value.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) The development of the hybridsterm·key when the original parents differ in three charactersterm·key results therefore according to the following expression: ABC + ABc + AbC + Abc + aBC + aBc + abC + abc + 2ABCc + 2AbCc + 2aBCc + 2abCc + 2ABbC + 2ABbc + 2aBbC + 2aBbc + 2AaBC + 2AaBc + 2AabC + 2Aabc + 4ABbCc + 4aBbCc + 4AaBCc + 4AabCc + 4AaBbC + 4AaBbc + 8AaBbCc.

Here also is involved a combination series in which the expressions for the charactersterm·key A and a, B and b, C and c, are united. The expressions: A + 2Aa + a B + 2Bb + b C + 2Cc + c give all the classes of the series. The constant combinations which occur therein agree with all combinations which are possible between the charactersterm·key A, B, C, a, b, c; two thereof, ABC and abc, resemble the two original parental stocks.

In addition, further experimentsterm·mth were made with a smaller number of experimental plantsterm·key in which the remaining charactersterm·key by twos and threes were united as hybridsterm·key: all yielded approximately the same results. There is therefore no doubt that for the whole of the charactersterm·key involved in the experimentsterm·mth the principle applies that the offspringterm·key of the hybridsterm·key in which several essentially different charactersterm·key are combined exhibit the terms of a series of combinations, in which the developmental series for each pair of differentiating charactersterm·key are united. It is demonstrated at the same time that the relation of each pair of different charactersterm·key in hybridterm·dom union is independent of the other differences in the two original parental stocks.

If n represent the number of the differentiating charactersterm·key in the two original stocks, 3n gives the number of terms of the combination series, 4n the number of individuals which belong to the series, and 2n the number of unions which remain constant. The series therefore contains, if the original stocks differ in four charactersterm·key, 34 = 81 classes, 44 = 256 individuals, and 24 = 16 constant forms: or, which is the same, among each 256 offspringterm·key of the hybridsterm·key are 81 different combinations, 16 of which are constant.

All constant combinations which in Peas are possible by the combination of the said seven differentiating charactersterm·key were actually obtained by repeated crossing. Their number is given by 27 = 128.

ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden Thereby is simultaneously given the practical proof that the constant charactersterm·key which appear in the several varieties of a group of plantsterm·key may be obtained in all the associations which are possible according to laws of combination, by means of repeated artificial the fertilization.

As regards the flowering time of the hybridsterm·key, the experimentsterm·mth are not yet concluded. It can, however, already be stated that the time stands almost exactly between those of the seed and pollenterm·key parents, and that the constitution of the hybridsterm·key with respect to this characterterm·key probably follows the rule ascertained in the case of the other charactersterm·key. The forms which are selected for experimentsterm·mth of this class must have a difference of at least 20 days from the middle flowering period of one to that of the other; furthermore, the seeds when sown must all be placed at the same depth in the earth, so that they may germinate simultaneously. Also, during the whole flowering period, the more important variations in temperature must be taken into account, and the partial hastening or delaying of the flowering which may result there from. It is clear that this experimentterm·mth presents many difficulties to be overcome and necessitates great attention.

If we endeavor to collate in a brief form the results arrived at, we find that those differentiating charactersterm·key, which admit of easy and certain recognition in the experimental plantsterm·key, all behave exactly alike in their hybridterm·dom associations. The offspringterm·key of the hybridsterm·key of each pair of differentiating charactersterm·key are, one–half, hybridterm·dom again, while the other half are constant in equal proportions having the charactersterm·key of the seed and pollenterm·key parents respectively. If several differentiating charactersterm·key are combined by cross–fertilization in a hybridterm·dom, the resulting offspringterm·key form the terms of a combination series in which the combination series for each pair of differentiating charactersterm·key are united.

The uniformity of behavior shown by the whole of the charactersterm·key submitted to experimentterm·mth permits, and fully justifies, the acceptance of the principle that a similar relation exists in the other charactersterm·key which appear less sharply defined in plantsterm·key, and therefore could not be included in the separate experimentsterm·mth. An experimentterm·mth with peduncles of different lengths gave on the whole a fairly satisfactory results, although the differentiation and serial arrangement of the forms could not be effected with that certainty which is indispensable for correct experimentterm·mth.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) THE REPRODUCTIVE CELLS OF THE HYBRIDSterm·key The results of the previously described experimentsterm·mth led to further experimentsterm·mth, the results of which appear fitted to afford some conclusions as regards the composition of the egg and pollenterm·key cells of hybridsterm·key. An important clue is afforded in Pisum by the circumstance that among the progeny of the hybridsterm·key constant forms appear, and that this occurs, too, in respect of all combinations of the associated charactersterm·key. So far as experience goes, we find it in every case confirmed that constant progeny can only be formed when the egg cells and the fertilizing pollenterm·key are of like characterterm·key, so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species. We must therefore regard it as certain that exactly similar factors must be at work also in the production of the constant forms in the hybridterm·dom plantsterm·key. Since the various constant forms are produced in one plantterm·key, or even in one flower of a plantterm·key, the conclusion appears logical that in the ovaries of the hybridsterm·key there are formed as many sorts of egg cells, and in the anthers as many sorts of pollenterm·key cells, as there are possible constant combination forms, and that these egg and pollenterm·key cells agree in their internal compositions with those of the separate forms.

In point of fact it is possible to demonstrate theoretically that this hypothesis would fully suffice to account for the development of the hybridsterm·key in the separate generationsterm·key, if we might at the same time assume that the various kinds of egg and pollenterm·key cells were formed in the hybridsterm·key on the average in equal numbers. In order to bring these assumptions to an experimental proof, the following experimentsterm·mth were designed. Two forms which were constantly different in the form of the seed and the color of the albumen were united by fertilization.

If the differentiating charactersterm·key are again indicated as A, B, a, b, we have: AB, Seed parents A, form round B, albumen yellow ab, Pollenterm·key parents a, form wrinkled b, albumen green The artificially fertilized seeds were sown together with several seeds of both original stocks, and the most vigorous examples were chosen for the reciprocal crossing. There were fertilized: 1. The hybridsterm·key with the pollenterm·key of AB 2. The hybridsterm·key with the pollenterm·key of ab 3. AB with the pollenterm·key of the hybridsterm·key.

ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden 4. ab with the pollenterm·key of the hybridsterm·key.

For each of these four experimentsterm·mth the whole of the flowers on three plantsterm·key were fertilized. If the above theory be correct, there must be developed on the hybridsterm·key egg and pollenterm·key cells of the forms AB, Ab, aB, ab, and there would be combined: 1. The egg cells AB, Ab, aB, ab with the pollenterm·key cells AB. 2. The egg cells AB, Ab, aB, ab with the pollenterm·key cells ab. 3. The egg cells AB with the pollenterm·key cells AB, Ab, aB, and ab.

4. The egg cells ab with the pollenterm·key cells AB, Ab, aB, and ab.

From each of these experimentsterm·mth there could then result only the following forms: 1. AB, ABb, AaB, AaBb 2. AaBb, Aab, aBb, ab 3. AB, ABb, AaB, AaBb 4. AaBb, Aab, aBb, ab If, furthermore, the several forms of the egg and pollenterm·key cells of the hybridsterm·key were produced on an average in equal numbers, then in each experimentterm·mth the said four combinations should stand in the same ratioterm·key to each other. A perfect agreement in the numerical relations was, however, not to be expected since in each fertilization, even in normal cases, some egg cells remain undeveloped or subsequently die, and many even of the well–formed seeds fail to germinate when sown. The above assumption is also limited in so far that while it demands the formation of an equal number of the various sorts of egg and pollenterm·key cells, it does not require that this should apply to each separate hybridterm·dom with mathematical exactness.

The first and second experimentsterm·mth had primarily the object of proving the composition of the hybridterm·dom egg cells, while the third and fourth experimentsterm·mth were to decide that of the pollenterm·key cells. As is shown by the above demonstration the first and third experimentsterm·mth and the second and fourth experimentsterm·mth should produce precisely the same combinations, and even in the second year the result should be partially visible in the form and color of the artificially fertilized seed. In the first and third experimentsterm·mth the dominant charactersterm·def of form and color, A and B, appear in each union, and are also partly constant and CLASSICAL GENETICS GREGOR MENDELpersName (1865) partly in hybridterm·dom union with the recessive charactersterm·def a and b, for which reason they must impress their peculiarity upon the whole of the seeds. All seeds should therefore appear round and yellow, if the theory be justified. In the second and fourth experimentsterm·mth, on the other hand, one union is hybridterm·dom in form and in color, and consequently the seeds are round and yellow; another is hybridterm·dom in form, but constant in the recessive characterterm·def of color, whence the seeds are round and green; the third is constant in the recessive characterterm·def of form but hybridterm·dom in color, consequently the seeds are wrinkled and yellow; the fourth is constant in both recessive charactersterm·def, so that the seeds are wrinkled and green. there were consequently four sorts of seed to be expected; namely, round and yellow, round and green, wrinkled and yellow, wrinkled and green.

these experimentsterm·mth In both The crop fulfilled these expectations perfectly. There were obtained in the 1st Experimentterm·mth, 98 exclusively round yellow seeds; 3rd " 94 exclusively round yellow seeds In the 2nd Experimentterm·mth, 31 round and yellow, 26 round and green, 22 wrinkled and yellow, 26 wrinkled and green seeds.

In the 4th Experimentterm·mth, 24 round and yellow, 25 round and green, 22 wrinkled and yellow, 27 wrinkled and green.

There could scarcely be now any doubt of the success of the experimentterm·mth; the next generationterm·key must afford the final proof. From the seed sown there resulted for the first experimentterm·mth 90 plantsterm·key, and for the third 87 plantsterm·key which fruited: these yielded for the 1st Exp. 3rd Exp.

20 23 25 22 25 19 22 21 round yellow seeds round yellow and green seeds round and wrinkled yellow seeds round and wrinkled green and yellow seeds AB ABb AaB AaBb In the second and fourth experimentsterm·mth the round and yellow seeds yielded plantsterm·key with round and wrinkled yellow and green seeds, AaBb.

From the round green seeds plantsterm·key resulted with round and wrinkled green seeds, Aab.

The wrinkled yellow seeds gave plantsterm·key with wrinkled yellow and green seeds, aBb.

From the wrinkled green seeds plantsterm·key were raised which yielded again only wrinkled and green seeds, ab.

ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden Although in these two experimentsterm·mth likewise some seeds did not germinate, the figures arrived at already in the previous year were not affected thereby, since each kind of seed gave plantsterm·key which, as regards their seed, were like each other and different from the others. There resulted therefore from the 2nd. Expt. 4th Expt. 31 24 plantsterm·key of the form AaBb 26 25 plantsterm·key of the form Aab 27 22 plantsterm·key of the form aBb 26 27 plantsterm·key of the form ab In all the experimentsterm·mth, therefore, there appeared all the forms which the proposed theory demands, and they came in nearly equal numbers.

In a further experimentterm·mth the charactersterm·key of flower–color and length of stem were experimented upon, and selection was so made that in the third year of the experimentterm·mth each characterterm·key ought to appear in half of all the plantsterm·key if the above theory were correct. A, B, a, b serve again as indicating the various charactersterm·key.

A, violet–red flowers. B, axis long.

a, white flowers. b, axis short.

The form Ab was fertilized with ab, which produced the hybridterm·dom Aab. Furthermore, aB was also fertilized with ab, whence the hybridterm·dom aBb. In the second year, for further fertilization, the hybridterm·dom Aab was used as seed parent, and hybridterm·dom aBb as pollenterm·key parent.

Seed parent, Aab. Possible egg cells, Ab, ab.

Pollenterm·key parent, aBb. Pollenterm·key cells, aB, ab.

From the fertilisation between the possible egg and pollenterm·key cells four combinations should result, namely: AaBb + aBb + Aab + ab From this it is perceived that, according to the above theory, in the third year of the experimentterm·mth out of all the plantsterm·key, Half should have violet–red flowers " " " white flowers " " " a long axis " " " a short axis (Aa), Classes 1, (a) " 2, (Bb) 1, " (b) " 3, 3 4 2 4 From 45 fertilizations of the second year 187 seeds resulted, of which only 166 reached the flowering stage in the third year. Among these the separate classes appeared in the numbers following: CLASSICAL GENETICS GREGOR MENDELpersName (1865) Class 1 2 3 4 Flower Color violet–red white violet–red white Stem long long short short 47 times 40 times 38 times 41 times There subsequently appeared, The " " " violet–red flower color white flower–color long stem short stem (Aa) (a) (Bb) (b) in 85 plantsterm·key in 81 plantsterm·key in 87 plantsterm·key in 79 plantsterm·key The theory adduced is therefore satisfactorily confirmed in this experimentterm·mth also.

For the charactersterm·key of form of pod, color of pod, and position of flowers, experimentsterm·mth were also made on a small scale and results obtained in perfect agreement. All combinations, which were possible through the union of the differentiating charactersterm·key duly appeared, and in nearly equal numbers.

Experimentally, therefore, the theory is confirmed that the pea hybridsterm·key form egg and pollenterm·key cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the charactersterm·key united in fertilization.

The difference of the forms among the progeny of the hybridsterm·key, as well as the respective ratiosterm·key of the numbers in which they are observed, find a sufficient explanation in the principle above deduced. The simplest case is afforded by the developmental series of each pair of differentiating charactersterm·key. This series is represented by the expression A+2Aa+a, in which A and a signify the forms with constant differentiating charactersterm·key, and Aa the hybridterm·dom form of both. It includes in three different classes four individuals. In the formation of these, pollenterm·key and egg cells of the form A and a take part on the average equally in the fertilization; hence each form [occurs] twice, since four individuals are formed. There participate consequently in the fertilization The pollenterm·key cells: A + A + a + a, A + A + a + a. The egg cells: It remains, therefore, purely a matter of chance which of the two sorts of pollenterm·key will become united with each separate egg cell. According, however, to the law of probability, it will always happen, on the average ofterm·stat many cases, that each pollenterm·key form A and a will unite ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden equally often with each egg cell form A and a, consequently one of the two pollenterm·key cells A in the fertilization will meet with the egg cell A and the other with the egg cell a, and so likewise one pollenterm·key cell a will unite with an egg cell A, and the other with the egg cell a.

Pollenterm·key cells Egg cells The result of the fertilization may be made clear by putting the signs for the conjoined egg and pollenterm·key cells in the form of fractions, those for the pollenterm·key cells above and those for the egg cells below the line. We then have A +— A A +— a a +— A a .— a In the first and fourth term the egg and pollenterm·key cells are of like kind, consequently the product of their union must be constant, namely A and a; in the second and third, on the other hand, there again results a union of the two differentiating charactersterm·key of the stocks, consequently the forms resulting from these fertilizations are identical with those of the hybridterm·dom from which they sprang. There occurs accordingly a repeated hybridization. This explains the striking fact that the hybridsterm·key are able to produce, besides the two parental forms, offspringterm·key which are like themselves A/a and a/A both give the same union Aa, since, as already remarked above, it makes no difference in the result of fertilization to which of the two charactersterm·key the pollenterm·key or egg cells belong. We may write then A +— A A +— a a +— A a =— a 2Aa This represents the average result of the self–fertilization of the hybridsterm·key when two differentiating charactersterm·key are united in them. In individual flowers and in individual plantsterm·key, however, the ratiosterm·key in which the forms of the series are produced may suffer not inconsiderable fluctuations. Apart from the fact that the numbers in which both sorts of egg cells occur in the seed vessels can only be regarded as equal on the average, it remains purely a matter of chance which of the two sorts of pollenterm·key may fertilize each separate egg cell. For this reason the separate values must necessarily be subject to CLASSICAL GENETICS GREGOR MENDELpersName (1865) fluctuations, and there are even extreme cases possible, as were described earlier in connection with the experimentsterm·mth on the forms of the seed and the color of the albumen. The true ratiosterm·key of the numbers can only be ascertained by an average deduced from the sum of as many single values as possible; the greater the number the more are merely chance effects eliminated.

The developmental series for hybridsterm·key in which two kinds of differentiating charactersterm·key are united contains among 16 individuals nine different forms, viz.: AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb.

Between the differentiating charactersterm·key of the original stocks Aa and Bb, four constant combinations are possible, and consequently the hybridsterm·key produce the corresponding four forms of egg and pollenterm·key cells: AB, Ab, aB, ab, and each of these will on the average figure four times in the fertilization, since 16 individuals are included in the series. Therefore, the participators in the fertilization are Pollenterm·key cells: AB + AB + AB + AB + Ab + Ab + Ab + Ab + aB + aB + aB + aB + ab + ab + ab + ab.

Egg cells: AB + AB + AB + AB + Ab + Ab + Ab + Ab + aB + aB + aB + aB + ab + ab + ab + ab.

In the process of fertilization each pollenterm·key form unites on an average equally often with each egg cell form, so that each of the four pollenterm·key cells AB unites once with one of the forms of egg cell AB, Ab, aB, ab. In precisely the same way the rest of the pollenterm·key cells of the forms Ab, aB, ab, unite with all the other egg cells. We obtain therefore: AB AB aB AB AB Ab aB Ab AB aB aB aB AB ab aB ab Ab AB ab AB Ab Ab ab Ab Ab aB ab aB Ab ab ab ab AB + ABb + AaB + AaBb + ABb + Ab + AaBb + Aab + AaB + AaBb + aB + aBb + AaBb + Aab + aBb + ab = AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden In precisely similar fashion is the developmental series of hybridsterm·key exhibited when three kinds of differentiating charactersterm·key are conjoined in them. The hybridsterm·key form eight various kinds of egg and pollenterm·key cells: ABC, ABc, AbC, Abc, aBC, aBc, abC, abc, and each pollenterm·key form unites itself again on the average once with each form of egg cell.

The law of combination of different charactersterm·key which governs the development of the hybridsterm·key finds therefore its foundation and explanation in the principle enunciated, that the hybridsterm·key produce egg cells and pollenterm·key cells which in equal numbers represent all constant forms which result from the combinations of the charactersterm·key brought together in fertilization.

EXPERIMENTSterm·mth WITH HYBRIDSterm·key OF OTHER SPECIES OF PLANTSterm·key It must be the object of further experimentsterm·mth to ascertain whether the law of development discovered for Pisum applies also to the hybridsterm·key of other plantsterm·key. To this end several experimentsterm·mth were recently commenced. Two minor experimentsterm·mth with species of Phaseolus have been completed, and may be here mentioned.

An experimentterm·mth with Phaseolus vulgaris and Phaseolus nanus gave results in perfect agreement. Ph. nanus had together with the dwarf axis, simply inflated, green pods. Ph. vulgaris had, on the other hand, an axis 10 ft. to 12 ft. high, and yellow colored pods, constricted when ripe. The ratiosterm·key of the numbers in which the different forms appeared in the separate generationsterm·key were the same as with Pisum. Also the development of the constant combinations resulted according to the law of simple combination of charactersterm·key, exactly as in the case of Pisum. There were obtained: Constant Combinations 1 2 3 4 5 6 7 8 Axis long " " " short " " " Color of the unripe pods green " yellow " green " yellow " Form of the unripe pods inflated constricted inflated constricted inflated constricted inflated constricted CLASSICAL GENETICS GREGOR MENDELpersName (1865) The green color of the pod, the inflated forms, and the long axis were, as in Pisum, dominant charactersterm·def.

Another experimentterm·mth with two very different species of Phaseolus had only a partial result. Phaseolus nanus, L., served as seed parent, a perfectly constant species, with white flowers in short recemes and small white seeds in straight, inflated, smooth pods; as pollenterm·key parent was used Ph. multiflorus, W., with tall winding stem, purple–red flowers in very long racemes, rough, sickle–shaped crooked pods, and large seeds which bore black flecks and splashes on a peach–blood– red ground.

The hybridsterm·key had the greatest similarity to the pollenterm·key parent, but the flowers appeared less intensely colored. Their fertility was very limited; from 17 plantsterm·key, which together developed many hundreds of flowers, only 49 seeds in all were obtained. These were of medium size, and were flecked and splashed similarly to those of Ph. multiflorus, while the ground color was not materially different. The next year 44 plantsterm·key were raised from these seeds, of which only 31 reached the flowering stage. The charactersterm·key of Ph. nanus, which had been altogether in various combinations; their ratioterm·key, however, with relation to the dominantterm·dom plantsterm·key was necessarily very fluctuating owing to the small number of trial plantsterm·key. With certain charactersterm·key, as in those of the axis and the form of pod, it was, however, as in the case of Pisum, almost exactly 1:3.

the hybridsterm·key, reappeared latent Insignificant as the results of this experimentterm·mth may be as regards the determination of the relative numbers in which the various forms appeared, it presents, on the other hand, the phenomenon of a remarkable change of color in the flowers and seed of the hybridsterm·key. In Pisum it is known that the charactersterm·key of the flower– and seed–color present themselves unchanged in the first and further generationsterm·key, and that the offspringterm·key of the hybridsterm·key display exclusively the one or the other of the charactersterm·key of the original stocks. It is otherwise in the experimentterm·mth we are considering. The white flowers and the seed–color of Ph. nanus appeared, it is true, at once in the first generationterm·key in one fairly fertile example, but the remaining 30 plantsterm·key developed flower– colors which were of various grades of purple–red to pale violet. The coloring of the seed–coat was no less varied than that of the flowers. No plantterm·key could rank as fully fertile; many produced no fruit at all; others only yielded fruits from the flowers last produced, which did not ripen. From 15 plantsterm·key only were well–developed seeds obtained. The greatest disposition to infertility was seen in the forms with preponderantly red flowers, since out of 16 of these only four yielded ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden ripe seeds. Three of these had a similar seed pattern to Ph. multiflorus, but with a more or less pale ground color; the fourth plantterm·key yielded only one seed of plain brown tint. The forms with preponderantly violet–colored flowers had dark brown, black–brown, and quite black seeds.

The experimentterm·mth was continued through two more generationsterm·key under similar unfavorable circumstances, since even among the offspringterm·key of fairly fertile plantsterm·key there came again some which were less fertile and even quite sterile. Other flower–and seed–colors than those cited did not subsequently present themselves. The forms which in the first generationterm·key contained one or more of the recessive charactersterm·def remained, as regards these, constant without exception. Also of those plantsterm·key which possessed violet flowers and brown or black seed, some did not vary again in these respects in the next generationterm·key; the majority, however, yielded together with offspringterm·key exactly like themselves, some which displayed white flowers and white seed–coats. The red flowering plantsterm·key remained so slightly fertile that nothing can be said with certainty as regards their further development.

Despite the many disturbing factors with which the observations had to contend, it is nevertheless seen by this experimentterm·mth that the development of the hybridsterm·key, with regard to those charactersterm·key which concern the form of the plantsterm·key, follows the same laws as in Pisum. With regard to the color charactersterm·key, it certainly appears difficult to perceive a substantial agreement. Apart from the fact that from the union of a white and a purple–red coloring a whole series of colors results, from purple to pale violet and white, the circumstance is a striking one that among 31 flowering plantsterm·key only one received the recessive characterterm·def of the white color, while in Pisum this occurs on the average in every fourth plantterm·key.

Even these enigmatic results, however, might probably be explained by the law governing Pisum if we might assume that the color of the flowers and seeds of Ph. multiflorus is a combination of two or more entirely independent colors, which individually act like any other constant characterterm·key in the plantterm·key. If the flower–color A were a combination of the individual charactersterm·key A1+ A2 + ..... which produce the total impression of a purple coloration, then by fertilization with the differentiating characterterm·key, white color, a, there would be produced the hybridterm·dom unions A1a + A2a +..... and so would it be with the corresponding coloring of the seed–coats. According to the above assumptions, each of these hybridterm·dom color unions would be independent, CLASSICAL GENETICS GREGOR MENDELpersName (1865) and would consequently develop quite independently from the others. It is then easily seen that from the combination of the separate developmental series a complete color–series must result. If, for instance, A = A1 + A2, then the hybridsterm·key A1a and A2a form the developmental series: A1 + 2 A1a + a A2 + 2 A2a + a The members of this series can enter into nine different combinations, and each of these denotes another color: 1 A1A2 2 A1A2a 1 A1a 2 A1aA2 4 A1aA2a 2 A1aa 1 A2a 2 A2aa 1 aa The figures prescribed for the separate combinations also indicate how many plantsterm·key with the corresponding coloring belong to the series. Since the total is 16, the whole of the colors are on the average distributed over each 16 plantsterm·key, but, as the series itself indicated, in unequal proportions.

Should the color development really happen in this way, we could offer an explanation of the case above described, namely that of the white flowers and seed–coat color only appeared once among 31 plantsterm·key of the first generationterm·key. This coloring appears only once in the series, and could therefore also only be developed once in the average in each 16, and with three color charactersterm·key only once even in 64 plantsterm·key.

It must, nevertheless, not be forgotten that the explanation here attempted is based on a mere hypothesis, only supported by the very imperfect result of the experimentterm·mth just described. It would, however, be well worth while to follow up the development of color in hybridsterm·key by similar experimentsterm·mth, since it is probable that in this way we might learn the significance of the extraordinary variety in the coloring of our ornamental flowers.

So far, little at present is known with certainty beyond the fact that the color of the flowers in most ornamental plantsterm·key is an extremely variable characterterm·key. The opinion has often been expressed that the stability of the species is greatly disturbed or entirely upset by cultivation, and consequently there is an inclination to regard the development of cultivated forms as a matter of chance devoid of rules; the coloring of ornamental plantsterm·key is indeed usually cited as an example of great instability. It is, however, not clear why the simple ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden transference into garden soil should result in such a thorough and persistent revolution in the plantterm·key organism. No one will seriously maintain that in the open country the development of plantsterm·key is ruled by other laws than in the garden bed. Here, as there, changes of type must take place if the conditions of life be altered, and the species possesses the capacity of fitting itself to its new environment. It is willingly granted that by cultivation the origination of new varieties is favored, and that by man’s labor many varieties are acquired which, under natural conditions, would be lost; but nothing justifies the assumption that the tendency to formation of varieties is so extraordinarily increased that the species speedily lose all stability, and their offspringterm·key diverge into an endless series of extremely variable forms. Were the change in the conditions the sole cause of variability we might expect that those cultivated plantsterm·key which are grown for centuries under almost identical conditions would again attain constancy. This, as is well known, is not the case since it is precisely under such circumstances that not only the most varied but also the most variable forms are found. It is only the Leguminosae, like Pisum, Phaseolus, Lens, whose organs of fertilization are protected by the keel, which constitute a noteworthy exception. Even here there have arisen numerous varieties during a cultural period of more than 1,000 years under most various conditions; these maintain, however, under unchanging environments a stability as great as that of species growing wild.

It is more than probable that as regards the variability of cultivated plantsterm·key there exists a factor which so far has received little attention. Various experimentsterm·mth force us to the conclusion that our cultivated plantsterm·key, with few exceptions, are members of various hybridterm·dom series, whose further development in conformity with law is varied and interrupted by frequent crossings inter se. The circumstance must not be overlooked that cultivated plantsterm·key are mostly grown in great numbers and close together, affording the most favorable conditions for reciprocal fertilization between the varieties present and species itself. The probability of this is supported by the fact that among the great array of variable forms solitary examples are always found, which in one characterterm·key or another remain constant, if only foreign influence be carefully excluded. These forms behave precisely as do those which are known to be members of the compound hybridterm·dom series. Also with the most susceptible of all charactersterm·key, that of color, it cannot escape the careful observer that in the separate forms the inclination to vary is displayed in very different degrees. Among plantsterm·key which arise from one spontaneous fertilization there are often CLASSICAL GENETICS GREGOR MENDELpersName (1865) some who offspringterm·key vary widely in the constitution and arrangement of the colors, while that of others shows little deviation, and among a greater number solitary examples occur which transmit the color of the flowers unchanged to their offspringterm·key. The cultivated species of Dianthus afford an instructive example of this. A white–flowered example of Dianthus caryophyllus, which itself was derived from a white–flowered variety, was shut up during its blooming period in a greenhouse; the numerous seeds obtained therefrom yielded plantsterm·key entirely white–flowered like itself. A similar result was obtained from a sub–species, with red flowers somewhat flushed with violet, and one with flowers white, striped with red. Many others, on the other hand, which were similarly protected, yielded progeny which were more or less variously colored and marked.

Whoever studies the coloration which results in ornamental plantsterm·key from similar fertilization can hardly escape the conviction that here also the development follows a definite law which possibly finds its expression independent color charactersterm·key.

the combination of several CONCLUDING REMARKS It can hardly fail to be of interest to compare the observations made regarding Pisum with the results arrived at by the two authorities in this branch of knowledge, Kölreuter and Gärtner, in their investigations. According to the opinion of both, the hybridsterm·key in outward appearance present either a form intermediate between the original species, or they closely resemble either the one or the other type, and sometimes can hardly be discriminated from it. From their seeds usually arise, if the fertilization was effected by their own pollenterm·key, various forms which differ from the normal type. As a rule, the majority of individuals obtained by one fertilization maintain the hybridterm·dom form, while some few others come more like the seed parent, and one or other individual approaches the pollenterm·key parent. This, however, is not the case with hybridsterm·key without exception. Sometimes the offspringterm·key have more nearly approached, some the one and some the other of the two original stocks, or they all incline more to one or the other side; while in other cases they remain perfectly like the hybridterm·dom and continue constant in their offspringterm·key. The hybridsterm·key of varieties behave like hybridsterm·key of species, but they possess greater ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden variability of form and more pronounced tendency to revert to the original types.

in Pisum the observations made With regard to the form of the hybridsterm·key and their development, as a rule an agreement with is unmistakable. It is otherwise with the exceptional cases cited. Gärtner confesses even that the exact determination whether a form bears a greater resemblance to one or to the other of the two original species the often involved great difficulty, so much depending upon subjective point of view of the observer. Another circumstance could, however, contribute to render the results fluctuating and uncertain, despite the most careful observation and differentiation. For the experimentsterm·mth, plantsterm·key were mostly used which rank as good species and are differentiated by a large number of charactersterm·key. In addition to the sharply defined charactersterm·key, where it is a question of greatly or less similarity, those charactersterm·key must also be taken into account which are often difficult to define in words, but yet suffice, as every plantterm·key specialist knows, to give the forms a peculiar appearance. If it be accepted that the development of hybridsterm·key follows the law which is valid for Pisum, the series in each separate experimentterm·mth must contain very many forms, since the number of terms, as is known, increases with the number of the differentiating charactersterm·key as the powers of three. With a relatively small number of experimental plantsterm·key the results therefore could only be approximately right, and in single cases might fluctuate considerably. If, for instance, the two original stocks differ in seven charactersterm·key, and 100 – 200 plantsterm·key were raised from the seeds of their hybridsterm·key to determine the grade of relationship of the offspringterm·key, we can easily see how uncertain the decision must become since for seven differentiating charactersterm·key the combination series contains 16,384 individuals under 2,187 various forms; now one and then another relationship could assert its predominance, just according as chance presented this or that form to the observer in a majority of cases.

then the hybridsterm·key, If, furthermore, there appear among the differentiating charactersterm·key at the same time dominant charactersterm·def, which are transmitted entire or nearly unchanged the developmental series that one of the two original parents which possesses the majority of dominant charactersterm·def must always be predominant. In the experimentterm·mth described relative to Pisum, in which three kinds of differentiating charactersterm·key were concerned, all the dominant charactersterm·def belonged to the seed parent. Although the terms of the series in their internal composition approach both original parents equally, yet in this experimentterm·mth the type of the seed parent terms of the CLASSICAL GENETICS GREGOR MENDELpersName (1865) obtained so great a preponderance that out of each 64 plantsterm·key of the first generationterm·key 54 exactly resembled it, or only differed in one characterterm·key. It is seen how rash it must be under such circumstances to draw from the external resemblances of hybridsterm·key conclusions as to their internal nature.

Gärtner mentions that in those cases where the development was regular among the offspringterm·key of the hybridsterm·key the two original species were not reproduced, but only a few individuals which approached them. With very extended developmental series it could not in fact be otherwise. For seven differentiating charactersterm·key, for instance, among more than 16,000 individuals –– offspringterm·key of the hybridsterm·key –– each of the two original species would occur only once. It is therefore hardly possible that these should appear at all among a small number of experimental plantsterm·key; with some probability, however, we might reckon upon the appearance in the series of a few forms which approach them.

We meet with an essential difference in those hybridsterm·key which remain constant in their progeny and propagate themselves as truly as the pure species. According to Gärtner, to this class belong the fertile hybridsterm·key Aquilegia atropurpurea canadensis, remarkably Lavatera pseudolbia thuringiaca, Geum urbanorivale, and some Dianthus hybridsterm·key; and, according to Wichura, the hybridsterm·key of the Willow family. For the history of the evolution of plantsterm·key this circumstance is of special importance, since constant hybridsterm·key acquire the status of new species. The correctness of the facts is guaranteed by eminent observers, and cannot be doubted. Gärtner had an opportunity of following up Dianthus Armeria deltoides to the tenth generationterm·key, since it regularly propagated itself in the garden.

With Pisum it was shown by experimentterm·mth that the hybridsterm·key form egg and pollenterm·key cells of different kinds, and that herein lies the reason of the variability of their offspringterm·key. In other hybridsterm·key, likewise, whose offspringterm·key behave similarly we may assume a like cause; for those, on the other hand, which remain constant the assumption appears justifiable that their reproductive cells are all alike and agree with the foundation–cell of renowned physiologists, for the purpose of propagation one pollenterm·key cell and one egg cells unite in Phanerogams* into a single cell, which is capable by the opinion of the hybridterm·dom.

In Pisum it is placed beyond doubt that for the formation of the new embryo a perfect union of the elements of both reproductive cells must take place. How could we otherwise explain that among the offspringterm·key of the hybridsterm·key both original types reappear in equal numbers and with all their peculiarities? If the influence of the egg cell upon the pollenterm·key cell were only external, if it fulfilled the role of a nurse ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden assimilation and formation of new cells to become an independent organism. This development follows a constant law, which is founded on the material composition and arrangement of the elements which meet in the cell in a vivifying union. If the reproductive cells be of the same kind and agree with the foundation cell of the mother plantterm·key, then the development of the new individual will follow the same law which rules the mother plantterm·key. If it chance that an egg cell unites with a dissimilar pollenterm·key cell, we must then assume that between those elements of both cells, which determine opposite charactersterm·key some sort of compromise is effected. The resulting compound cell becomes the foundation of the hybridterm·dom organism the development of which necessarily follows a different scheme from that obtaining in each of the two original species. If the compromise be taken to be a complete one, in the sense, namely, that the hybridterm·dom embryo is formed from two similar cells, in which the differences are entirely and permanently accommodated together, the further result follows that the hybridsterm·key, like any other stable plantterm·key species, reproduce themselves truly in their offspringterm·key. The reproductive cells which are formed in their seed vessels and anthers are of one kind, and agree with the fundamental compound cell.

With regard to those hybridsterm·key whose progeny is variable we may perhaps assume that between the differentiating elements of the egg and pollenterm·key cells there also occurs a compromise, in so far that the formation of a cell as the foundation of the hybridterm·dom becomes possible; but, nevertheless, the arrangement between the conflicting elements is only temporary and does not endure throughout the life of the hybridterm·dom plantterm·key. Since in the habit of the plantterm·key no changes are perceptible during the whole period of vegetation, we must further assume that it is only possible for the differentiating elements to liberate themselves from the enforced union when the fertilizing cells are developed. In the formation of these cells all existing elements participate in an entirely free and equal arrangement, by which it is only the differentiating ones which mutually separate themselves. In this way the production would be rendered possible of as many sorts of egg and pollenterm·key cells as there are combinations possible of the formative elements.

only, then the result of each fertilization could be no other than that the developed hybridterm·dom should exactly resemble the pollenterm·key parent, or at any rate do so very closely. This the experimentsterm·mth so far have in nowise confirmed. An evident proof of the complete union of the contents of both cells is afforded by the experience gained on all sides that it is immaterial, as regards the form of the hybridterm·dom, which of the original species is the seed parent or which the pollenterm·key parent.

CLASSICAL GENETICS GREGOR MENDELpersName (1865) The attribution attempted here of the essential difference in the development of hybridsterm·key to a permanent or temporary union of the differing cell elements can, of course, only claim the value of an hypothesis for which the lack of definite data offers a wide scope. Some justification of the opinion expressed lies in the evidence afforded by Pisum that the behavior of each pair of differentiating charactersterm·key in hybridterm·dom union is independent of the other differences between the two original plantsterm·key, and, further, that the hybridterm·dom produces just so many kinds of egg and pollenterm·key cells as there are possible constant combination forms. The differentiating charactersterm·key of two plantsterm·key can finally, however, only depend upon differences in the composition and grouping of the elements which exist in the foundation–cells of the same in vital interaction.

Even the validity of the law formulated for Pisum requires still to be confirmed, and a repetition of the more important experimentsterm·mth is consequently must to be desired, that, for instance, relating to the composition of the hybridterm·dom fertilizing cells. A differential may easily escape the single observer, which although at the outset may appear to be unimportant, yet accumulate to such an extent that it must not be ignored in the total result. Whether the variable hybridsterm·key of other plantterm·key species observe an entire agreement must also be first decided experimentally. In the meantime we may assume that in material points an essential difference can scarcely occur, since the unity in the developmental plantterm·key of organic life is beyond question.

In conclusion, the experimentsterm·mth carried out by Kölreuter, Gärtner, and others with respect to the transformation of one species into another by artificial fertilization merit special mention. Particular importance has been attached to these experimentsterm·mth and Gärtner reckons them “among the most difficult of all in hybridization.”

If a species A is to be transformed into a species B, both must be united by fertilization and the resulting hybridsterm·key then be fertilized with the pollenterm·key of B; then, out of the various offspringterm·key resulting, that form would be selected which stood in nearest relation to B and once more be fertilized with B pollenterm·key, and so continuously until finally a form is arrived at which is like B and constant in its progeny. By this process the species A would change into the species B. Gärtner alone has effected 30 such experimentsterm·mth with plantsterm·key of genera Aquilegia, Dianthus, Geum, Lavatera, Lynchnis, Malva, Nicotiana, and Oenothera. The period of transformation was not alike for all species. While with some a triple fertilization sufficed, with others this had to be repeated five or six times, and even in the same species fluctuations were observed in various experimentsterm·mth. Gärtner ascribes ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden this difference to the circumstance that “the specific power by which a species, during reproduction, effects the change and transformation of the maternal type varies considerably in different plantsterm·key, and that, consequently, the periods with which the one species is changed into the other must also vary, as also the number of generationsterm·key, so that the transformation in some species is perfected in more, and in others in fewer generationsterm·key”. Further, the same observer remarks “that in these transformation experimentsterm·mth a good deal depends upon which type and which individual be chosen for further transformation”.

If it may be assumed that in these experimentsterm·mth the constitution of the forms resulted in a similar way to that of Pisum, the entire process of transformation would find a fairly simple explanation. The hybridterm·dom forms as many kinds of egg cells as there are constant combinations possible of the charactersterm·key conjoined therein, and one of these is always of the same kind as that of the fertilizing pollenterm·key cells. Consequently there always exists the possibility with all such experimentsterm·mth that even from the second fertilization there may result a constant form identical with that of the pollenterm·key parent. Whether this really be obtained depends in each separate case upon the number of the experimental plantsterm·key, as well as upon the number of differentiating charactersterm·key which are united by the fertilization. Let us, for instance, assume that the plantsterm·key selected for experimentterm·mth differed in three charactersterm·key, and the species ABC is to be transformed into the other species abc by repeated fertilization with the pollenterm·key of the latter; the hybridsterm·key resulting from the first cross form eight different kinds of egg cells, namely: ABC, ABc, AbC, aBC, Abc, aBc, abC, abc These in the second year of experimentterm·mth are united again with the pollenterm·key cells abc, and we obtain the series AaBbCc + AaBbc + AabCc + aBbCc + Aabc + aBbc + abCc + abc Since the form abc occurs once in the series of eight terms, it is consequently little likely that it would be missing among the experimental plantsterm·key, even were these raised in a smaller number, and the transformation would be perfected already by a second fertilization. If by chance it did not appear, then the fertilization must be repeated with one of those forms nearest akin, Aabc, aBbc, abCc. It is perceived that such an experimentterm·mth must extend the farther the smaller the number of experimental plantsterm·key and the larger the number of differentiating charactersterm·key in the two original species; and that, furthermore, in the same species there can easily occur a delay of one CLASSICAL GENETICS GREGOR MENDELpersName (1865) or even of two generationsterm·key such as Gärtner observed. The transformation of widely divergent species could generally only be completed in five or six years of experimentterm·mth, since the number of different egg cells which are formed in the hybridterm·dom increases as the powers of two with the number of differentiating charactersterm·key.

Gärtner found by repeated experimentsterm·mth that the respective period of transformation varies in many species, so that frequently a species A can be transformed into a species B a generationterm·key sooner than can species B into species A. He deduces therefrom that Kölreuter’s opinion can hardly be maintained that “the two natures in hybridsterm·key are perfectly in equilibrium”. Experimentsterm·mth which in this connection were carried out with two species of Pisum demonstrated that as regards the choice of the fittest individuals for the purpose of further fertilization it may make a great difference which of two species is transformed into the other. The two experimental plantsterm·key differed in five charactersterm·key, while at the same time those of species A were all dominantterm·dom and those of species B all recessiveterm·dom. For mutual transformation A was fertilized with pollenterm·key of B, and B with pollenterm·key of A, and this was repeated with both hybridsterm·key the following year. With the first experimentterm·mth, B/A, there were 87 plantsterm·key available in the third year of experimentterm·mth for selection of the individuals for further crossing, and these were of the possible 32 forms; with the second experimentterm·mth, A/B, 73 plantsterm·key resulted, which agreed throughout perfectly in habit with the pollenterm·key parent; in their internal composition, however, they must have been just as varied as the forms in the other experimentterm·mth. A definite selection was consequently only possible with the first experimentterm·mth; with the second the selection had to be made at random, merely. Of the latter only a portion of the flowers were crossed with the A pollenterm·key, the others were left to fertilize themselves. Among each five plantsterm·key which were selected in both experimentsterm·mth for fertilization there agreed, as the following year’s culture showed, with the pollenterm·key parent: 1st Experimentterm·mth 2 plantsterm·key 3 plantsterm·key —— —— —— 2nd Experimentterm·mth —— —— 2 plantsterm·key 2 plantsterm·key 1 plantterm·key in all charactersterm·key " 4 " 3 " 2 " 1 characterterm·key " " " the first experimentterm·mth, transformation was completed; in the second, which was not continued further, two more fertilizations would probably have been required.

therefore, the ESP FOUNDATIONS SERIES Versuche über Plflanzenhybriden Although the case may not frequently occur in which the dominant charactersterm·def belong exclusively to one or the other of the original parent plantsterm·key, it will always make a difference which of the two possesses the majority of dominantsterm·dom. If the pollenterm·key parent has the majority, then the selection of forms for further crossing will afford a less degree of certainty than in the reverse case, which must imply a delay in the period of transformation, provided that the experimentterm·mth is only considered as completed when a form is arrived at which not only exactly resembles the pollenterm·key parent in form, but also remains as constant in its progeny.

Gärtner, by the results of theses transformation experimentsterm·mth, was led to oppose the opinion of those naturalists who dispute the stability of plantterm·key species and believe in a continuous evolution of vegetation. He perceives in the complete transformation of one species into another an indubitable proof that species are fixed with limits beyond this opinion cannot be which unconditionally accepted we find on the other hand in Gärtner’s experimentsterm·mth a noteworthy confirmation of that supposition regarding variability of cultivated plantsterm·key which has already been expressed.

they cannot change. Although Among the experimental species there were cultivated plantsterm·key, canadensis, Dianthus such as Aquilegia atropurpurea and caryophyllus, chinensis, and japonicus, Nicotiana rustica and paniculata, and hybridsterm·key between these species lost none of their stability after four or five generationsterm·key.

CLASSICAL GENETICS