# Mendel's Principles of Heredity; a Defence/Chapter 3

ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION
By G. Mendel.
{Communicated to the Meeting 9 June, 1869.)
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.
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 under- standing 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 pro- genitors.
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 trans- mutation 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 ♀ × 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 ♀ × 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 ♀ × H. aurantiacum ♂ constitute approximately intermediate forms which do not agree in certain characters. On the contrary in H. Auricula ♀ × H. aurantiacum ♂ and in H. Auricula ♀ × 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.
2. With a single exception the hybrids in question form seeds capable of germination. H. echioides ♀ × H. aurantiacum ♂ may be described as fully fertile; H. præaltum ♀ × H. flagellare ♂ as fertile; H. præultum ♀ × H. aurantiacum ♂ and H. Auricula ♀ × H. pratense ♂ as ​Page:Mendel's principles of heredity; a defence.pdf/121 ​Page:Mendel's principles of heredity; a defence.pdf/122 ​In Pisum the hybrids, ebtained from the immediate crossing of two forms, have in all cases the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In Hieracium according to the present experiments the exactly opposite phenomenon seems to be exhibited. Already in describing the Pisum experiments it was remarked that there are also hybrids whose posterity do not vary, and that, for example, according to Wichura the hybrids of Salix reproduce themselves like pure species. In Hieracium we may take it we have a similar case. Whether from this circumstance we may venture to draw the conelusion that the polymorphism of the genera Salix and Hieracium is connected with the special condition of their hybrids is still an open question, which may well be raised but not as yet answered.
* ↑ [Published in Verh. naturf. Ver. Brünn, Abhandlungen, viii. 1869, p. 26, which appeared in 1870.]
* ↑ 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.