Part 8

We meet with an _essential difference_ in those hybrids which remain constant in their progeny and propagate themselves as truly as the pure species. According to Gärtner, to this class belong the _remarkably fertile hybrids_ _Aquilegia atropurpurea canadensis_, _Lavatera pseudolbia thuringiaca_, _Geum urbano-rivale_, and some _Dianthus_ hybrids; and, according to Wichura, the hybrids of the Willow species. For the history of the evolution of plants this circumstance is of special importance, since constant hybrids acquire the status of new species. The correctness of this is evidenced by most excellent observers, and cannot be doubted. Gärtner had opportunity to follow up _Dianthus Armeria deltoides_ to the tenth generation, since it regularly propagated itself in the garden.

With _Pisum_ it was shown by experiment that the hybrids form egg and pollen cells of _different_ kinds, and that herein lies the reason of the variability of their offspring. In other hybrids, likewise, whose offspring behave similarly we may assume a like cause; for those, on the other hand, which remain constant the assumption appears justifiable that their fertilising cells are all alike and agree with the foundation-cell [fertilised ovum] of the hybrid. In the opinion of renowned physiologists, for the purpose of propagation one pollen cell and one egg cell unite in Phanerogams[46] into a single cell, which is capable by 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 [fertilised ovum] of the mother plant, then the development of the new individual will follow the same law which rules the mother plant. If it chance that an egg cell unites with a _dissimilar_ pollen cell, we must then assume that between those elements of both cells, which determine the mutual differences, some sort of compromise is effected. The resulting compound cell becomes the foundation of the hybrid 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 hybrid embryo is formed from cells of like kind, in which the differences are _entirely and permanently accommodated_ together, the further result follows that the hybrids, like any other stable plant species, remain true to themselves in their offspring. The reproductive cells which are formed in their seed vessels and anthers are of one kind, and agree with the fundamental compound cell [fertilised ovum].

[46] In _Pisum_ it is placed beyond doubt that for the formation of the new embryo a perfect union of the elements of both fertilising cells must take place. How could we otherwise explain that among the offspring of the hybrids both original types reappear in equal numbers and with all their peculiarities? If the influence of the egg cell upon the pollen cell were only external, if it fulfilled the _rôle_ of a nurse only, then the result of each artificial fertilisation could be no other than that the developed hybrid should exactly resemble the pollen parent, or at any rate do so very closely. This the experiments so far have in no wise 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 hybrid, which of the original species is the seed parent or which the pollen parent.

With regard to those hybrids whose progeny is _variable_ we may perhaps assume that between the differentiating elements of the egg and pollen cells there also occurs a compromise, in so far that the formation of a cell as foundation of the hybrid becomes possible; but, nevertheless, the arrangement between the conflicting elements is only temporary and does not endure throughout the life of the hybrid plant. Since in the habit of the plant 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 fertilising cells are developed. In the formation of these cells all existing elements participate in an entirely free and equal arrangement, in 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 pollen cells as there are combinations possible of the formative elements.

The attribution attempted here of the essential difference in the development of hybrids 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 field. Some justification of the opinion expressed lies in the evidence afforded by _Pisum_ that the behaviour of each pair of differentiating characters in hybrid union is independent of the other differences between the two original plants, and, further, that the hybrid produces just so many kinds of egg and pollen cells as there are possible constant combination forms. The differentiating characters of two plants can finally, however, only depend upon differences in the composition and grouping of the elements which exist in the foundation-cells [fertilised ova] of the same in vital interaction[47].

[47] “_Welche in den Grundzellen derselben in lebendiger Wechselwirkung stehen._”

Even the validity of the law formulated for _Pisum_ requires still to be confirmed, and a repetition of the more important experiments is consequently much to be desired, that, for instance, relating to the composition of the hybrid fertilising cells. A differential [element] may easily escape the single observer[48], which although at the outset may appear to be unimportant, may yet accumulate to such an extent that it must not be ignored in the total result. Whether the variable hybrids of other plant species observe an entire agreement must also be first decided experimentally. In the meantime we may assume that in material points a difference in principle can scarcely occur, since the unity in the developmental plan of organic life is beyond question.

[48] “_Dem einzelnen Beobachter kann leicht ein Differenziale entgehen._”

In conclusion, the experiments carried out by Kölreuter, Gärtner, and others with respect to _the transformation of one species into another by artificial fertilisation_ merit special mention. A special importance has been attached to these experiments, and Gärtner reckons them among “the most difficult of all in hybridisation.”

If a species _A_ is to be transformed into a species _B_, both must be united by fertilisation and the resulting hybrids then be fertilised with the pollen of _B_; then, out of the various offspring resulting, that form would be selected which stood in nearest relation to _B_ and once more be fertilised with _B_ pollen, 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 thirty such experiments with plants of genera _Aquilegia_, _Dianthus_, _Geum_, _Lavatera_, _Lychnis_, _Malva_, _Nicotiana_, and _Œnothera_. The period of transformation was not alike for all species. While with some a triple fertilisation sufficed, with others this had to be repeated five or six times, and even in the same species fluctuations were observed in various experiments. Gärtner ascribes this difference to the circumstance that “the specific [_typische_] force by which a species, during reproduction, effects the change and transformation of the maternal type varies considerably in different plants, and that, consequently, the periods within which the one species is changed into the other must also vary, as also the number of generations, so that the transformation in some species is perfected in more, and in others in fewer generations.” Further, the same observer remarks “that in these transformation experiments a good deal depends upon which type and which individual be chosen for further transformation.”

If it may be assumed that in these experiments 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 hybrid forms as many kinds of egg cells as there are constant combinations possible of the characters conjoined therein, and one of these is always of the same kind as the fertilising pollen cells. Consequently there always exists the possibility with all such experiments that even from the second fertilisation there may result a constant form identical with that of the pollen parent. Whether this really be obtained depends in each separate case upon the number of the experimental plants, as well as upon the number of differentiating characters which are united by the fertilisation. Let us, for instance, assume that the plants selected for experiment differed in three characters, and the species _ABC_ is to be transformed into the other species _abc_ by repeated fertilisation with the pollen of the latter; the hybrids resulting from the first cross form eight different kinds of egg cells, viz.:

_ABC_, _ABc_, _AbC_, _aBC_, _Abc_, _aBc_, _abC_, _abc_.

These in the second year of experiment are united again with the pollen 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 components, it is consequently little likely that it would be missing among the experimental plants, even were these raised in a smaller number, and the transformation would be perfected already by a second fertilisation. If by chance it did not appear, then the fertilisation must be repeated with one of those forms nearest akin, _Aabc_, _aBbc_, _abCc_. It is perceived that such an experiment must extend the farther _the smaller the number of experimental plants and the larger the number of differentiating characters_ in the two original species; and that, furthermore, in the same species there can easily occur a delay of one or even of two generations such as Gärtner observed. The transformation of widely divergent species could generally only be completed in five or six years of experiment, since the number of different egg cells which are formed in the hybrid increases in square ratio with the number of differentiating characters.

Gärtner found by repeated experiments that the respective period of transformation varies in many species, so that frequently a species _A_ can be transformed into a species _B_ a generation sooner than can species _B_ into species _A_. He deduces therefrom that Kölreuter’s opinion can hardly be maintained that “the two natures in hybrids are perfectly in equilibrium.” It appears, however, that Kölreuter does not merit this criticism, but that Gärtner rather has overlooked a material point, to which he himself elsewhere draws attention, viz. that “it depends which individual is chosen for further transformation.” Experiments 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 fertilisation it may make a great difference which of two species is transformed into the other. The two experimental plants differed in five characters, while at the same time those of species _A_ were all dominant and those of species _B_ all recessive. For mutual transformation _A_ was fertilised with pollen of _B_, and _B_ with pollen of _A_, and this was repeated with both hybrids the following year. With the first experiment _B_/_A_ there were eighty-seven plants available in the third year of experiment for the selections of individuals for further crossing, and these were of the possible thirty-two forms; with the second experiment _A_/_B_ seventy-three plants resulted, which _agreed throughout perfectly in habit with the pollen parent_; in their internal composition, however, they must have been just as varied as the forms of the other experiment. A definite selection was consequently only possible with the first experiment; with the second some plants selected at random had to be excluded. Of the latter only a portion of the flowers were crossed with the _A_ pollen, the others were left to fertilise themselves. Among each five plants which were selected in both experiments for fertilisation there agreed, as the following year’s culture showed, with the pollen parent:--

1st Experiment. 2nd Experiment. 2 plants -- in all characters 3 " -- " 4 " -- 2 plants " 3 " -- 2 " " 2 " -- 1 plant " 1 character

In the first experiment, therefore, the transformation was completed; in the second, which was not continued further, two more fertilisations would probably have been required.

Although the case may not frequently occur that the dominant characters belong exclusively to one or the other of the original parent plants, it will always make a difference which of the two possesses the majority. If the pollen parent shows the majority, then the selection of forms for further crossing will afford a less degree of security than in the reverse case, which must imply a delay in the period of transformation, provided that the experiment is only considered as completed when a form is arrived at which not only exactly resembles the pollen plant in form, but also remains as constant in its progeny.

Gärtner, by the results of these transformation experiments, was led to oppose the opinion of those naturalists who dispute the stability of plant 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 within limits beyond which they cannot change. Although this opinion cannot be unconditionally accepted we find on the other hand in Gärtner’s experiments a noteworthy confirmation of that supposition regarding variability of cultivated plants which has already been expressed.

Among the experimental species there were cultivated plants, such as _Aquilegia atropurpurea_ and _canadensis_, _Dianthus caryophyllus_, _chinensis_, and _japonicus_, _Nicotiana rustica_ and _paniculata_, and hybrids between these species lost none of their stability after four or five generations[49].

[49] [The argument of these two last paragraphs appears to be that though the general mutability of natural species might be doubtful, yet among cultivated plants the transference of characters may be accomplished, and may occur by integral steps until one species is definitely “transformed” into the other.]

ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION

By G. Mendel.

(_Communicated to the Meeting 9 June, 1869[50]._)

[50] [Published in _Verh. naturf. Ver. Brünn, Abhandlungen_, VIII. 1869, p. 26, which appeared in 1870.]

Although I have already undertaken many experiments in fertilisation between species of _Hieracium_, I have only succeeded in obtaining the following 6 hybrids, and only from one to three specimens of them.

_H. Auricula_ ♀ × _H. aurantiacum_ ♂ _H. Auricula_ ♀ × _H. Pilosella_ ♂ _H. Auricula_ ♀ × _H. pratense_ ♂ _H. echioides_[51] ♀ × _H. aurantiacum_ ♂ _H. præaltum_ ♀ × _H. flagellare_ Rchb. ♂ _H. præaltum_ ♀ × _H. aurantiacum_ ♂

[51] The plant used in this experiment is not exactly the typical _H. echioides_. It appears to belong to the series transitional to _H. præaltum_, but approaches more nearly to _H. echioides_ and for this reason was reckoned as belonging to the latter.

The difficulty of obtaining a larger number of hybrids is due to the minuteness of the flowers and their peculiar structure. On account of this circumstance it was seldom possible to remove the anthers from the flowers chosen for fertilisation without either letting pollen get on to the stigma or injuring the pistil so that it withered away. As is well known, the anthers are united to form a tube, which closely embraces the pistil. As soon as the flower opens, the stigma, already covered with pollen, protrudes. In order to prevent self-fertilisation the anther-tube must be taken out before the flower opens, and for this purpose the bud must be slit up with a fine needle. If this operation is attempted at a time when the pollen is mature, which is the case two or three days before the flower opens, it is seldom possible to prevent self-fertilisation; for with every care it is not easily possible to prevent a few pollen grains getting scattered and communicated to the stigma. No better result has been obtained hitherto by removing the anthers at an earlier stage of development. Before the approach of maturity the tender pistil and stigma are exceedingly sensitive to injury, and even if they are not actually injured, they generally wither and dry up after a little time if deprived of their protecting investments. I hope to obviate this last misfortune by placing the plants after the operation for two or three days in the damp atmosphere of a greenhouse. An experiment lately made with _H. Auricula_ treated in this way gave a good result.

To indicate the object with which these fertilisation experiments were undertaken, I venture to make some preliminary remarks respecting the genus _Hieracium_. This genus possesses such an extraordinary profusion of distinct forms that no other genus of plants can compare with it. Some of these forms are distinguished by special peculiarities and may be taken as type-forms of species, while all the rest represent intermediate and transitional forms by which the type-forms are connected together. The difficulty in the separation and delimitation of these forms has demanded the close attention of the experts. Regarding no other genus has so much been written or have so many and such fierce controversies arisen, without as yet coming to a definite conclusion. It is obvious that no general understanding can be arrived at, so long as the value and significance of the intermediate and transitional forms is unknown.

Regarding the question whether and to what extent hybridisation plays a part in the production of this wealth of forms, we find very various and conflicting views held by leading botanists. While some of them maintain that this phenomenon has a far-reaching influence, others, for example, Fries, will have nothing to do with hybrids in _Hieracia_. Others take up an intermediate position; and while granting that hybrids are not rarely formed between the species in a wild state, still maintain that no great importance is to be attached to the fact, on the ground that they are only of short duration. The [suggested] causes of this are partly their restricted fertility or complete sterility; partly also the knowledge, obtained by experiment, that in hybrids self-fertilisation is always prevented if pollen of one of the parent-forms reaches the stigma. On these grounds it is regarded as inconceivable that _Hieracium_ hybrids can constitute and maintain themselves as fully fertile and constant forms when growing near their progenitors.

The question of the origin of the numerous and constant intermediate forms has recently acquired no small interest since a famous _Hieracium_ specialist has, in the spirit of the Darwinian teaching, defended the view that these forms are to be regarded as [arising] from the transmutation of lost or still existing species.

From the nature of the subject it is clear that without an exact knowledge of the structure and fertility of the hybrids and the condition of their offspring through several generations no one can undertake to determine the possible influence exercised by hybridisation over the multiplicity of intermediate forms in _Hieracium_. The condition of the _Hieracium_ hybrids in the range we are concerned with must necessarily be determined by experiments; for we do not possess a complete theory of hybridisation, and we may be led into erroneous conclusions if we take rules deduced from observation of certain other hybrids to be Laws of hybridisation, and try to apply them to _Hieracium_ without further consideration. If by the experimental method we can obtain a sufficient insight into the phenomenon of hybridisation in _Hieracium_, then by the help of the experience which has been collected respecting the structural relations of the wild forms, a satisfactory judgment in regard to this question may become possible.

Thus we may express the object which was sought after in these experiments. I venture now to relate the very slight results which I have as yet obtained with reference to this object.

1. Respecting the structure of the hybrids, we have to record the striking phenomenon that the forms hitherto obtained by similar fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three individuals, while as to the remainder only one of each has been obtained.

If we compare the individual characters of the hybrids with the corresponding characters of the two parent types, we find that they sometimes present intermediate structures, but are sometimes so near to one of the parent characters that the [corresponding] character of the other has receded considerably or almost evades observation. So, for instance, we see in one of the two forms of _H. Auricula_ ♀ x _H. aurantiacum_ ♂ pure yellow disc-florets; only the petals of the marginal florets are on the outside tinged with red to a scarcely noticeable degree: in the other on the contrary the colour of these florets comes very near to _H. aurantiacum_, only in the centre of the disc the orange red passes into a deep golden-yellow. This difference is noteworthy, for the flower-colour in _Hieracium_ has the value of a constant character. Other similar cases are to be found in the leaves, the peduncles, &c.

If the hybrids are compared with the parent types as regards the sum total of their characters, then the two forms of _H. præaltum_ ♀ x _H. aurantiacum_ ♂ constitute approximately intermediate forms which do not agree in certain characters. On the contrary in _H. Auricula_ ♀ x _H. aurantiacum_ ♂ and in _H. Auricula_ ♀ x _H. pratense_ ♂ we see the forms widely divergent, so that one of them is nearer to the one and the other to the other parental type, while in the case of the last-named hybrid there is still a third which is almost precisely intermediate between them.

The conviction is then forced on us that we have here only single terms in an unknown series which may be formed by the direct action of the pollen of one species on the egg-cells of another.