EXPEKIMENTS IN PLANT HYBRIDISATION. 



17 



The Reproductive Cells of Hybrids. 



The results of the previously described experiments induced further 

 experiments, the results of which appear fitted to afford some conclusions 

 as regards the composition of the egg and pollen cells of hybrids. An 

 important basis for argument is afforded in Pisum by the circumstance 

 that among the progeny of the hybrids constant forms appear, and that 

 this occurs, too, in all combinations of the associated characters. 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 fertilising 

 pollen are of like character, so that both are provided with the material 

 for vitalising quite similar individuals, as is the case with the normal 

 fertilisation of pure species. We must therefore regard it as essential 

 that exactly similar factors are at work also in the production of the 

 constant forms in the hybrid plants. Since the various constant forms 

 are produced in one plant, or even in one flower of a plant, the conclusion 

 appears logical that in the ovaries of the hybrids there are formed as 

 many sorts of egg cells, and in the anthers as many sorts of pollen cells, 

 as there are possible constant combination forms, and that these egg 

 and pollen cells agree in their internal composition 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 hybrids 

 in the separate generations, if we might at the same time assume that 

 the various kinds of egg and pollen cells were formed in the hybrids on 

 the average in equal numbers. 



In order to bring these assumptions experimentally to the proof, the 

 following trials were selected. Two forms which were constantly dif- 

 ferent in the form of the seed and the colour of the albumen were united 

 by fertilisation. 



If the differentiating characters are again indicated A, B, a, b, we 

 have : 



AB, seed parent ; ab, pollen parent ; 



A, form round ; a, form angular ; 



B, albumen yellow. b, albumen green. 



The artificially fertilised seeds were sown together with several seeds 

 of both original stocks, and the most vigorous examples were chosen for 

 the reciprocal crossing. There were fertilised : — 



1. The hybrids with the pollen of AB. 



2. The hybrids „ „ ab. 



3. AB „ ,, the hybrids. 



4. ab „ „ the hybrids. 



For each of these four trials the whole of the flowers on three plants 

 were fertilised. If the above theory be correct, there must be developed 

 on the hybrids egg and pollen cells of the forms AB, Ab, aB, ab, and 

 there would be combined : — 



1. The egg cells AB, Ab, aB, ab with the pollen ce'ls AB. 



„ AB, Ab, aB, ab „ „ ab. 



3- „ AB ,, „ AB. Ab, aB, ab. 



4 . „ ab » j> ftl^> al^' 



c 



