EXPERIMENTS IN PLANT HYBRIDISATION. 



29 



in their seed vessels and anthers are of one kind, and agree with the 

 fundamental compound cell. 



With regard to those hybrids whose progeny is variable we may per- 

 haps 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 Avhen 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 i)crmanent or temporarTj 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 wdde 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 

 fundamental 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 experiments is conse- 

 quently much to be desired, that, for instance, relating to the composition 

 of the hybrid fertilising cells. A differential [factor] may easily escape 

 the single observer, 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 experi- 

 mentally. In the meantime we may assume that in material points a 

 difference in principle can scarcely occur, since the unity in the develop- 

 mental plan of organic life is beyond question. 



In conclusion, the experiments carried out by Kolreuter, Giirtner, 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 Gartner reckons them among " the 

 most difficult of all in hybridisation." 



Should a species A be transformed into a specres B, both would 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 was arrived 

 at which was like B and constant in its progeny. By this process the 



