100 THE PLUM IN KANSAS. 



sexual organs. In the earlier stages of the dichogamous group 

 self-fertilization was possible, but mainly prevented by mechanical 

 adaptations to insure cross-fertilization. In the heterostyled group, 

 self-fertilization is prevented by the great differentiation in the re- 

 productive elements, and the sexual affinity destroyed. There can be 

 no doubt that the differentiation into sjjecies was accomplished by 

 variation of form, foliage, cell structure, etc., without a corresponding 

 differentiation of the sexual elements. We know this from the fact 

 that distinct species will sometimes cross, but not freely, in a recipro- 

 cal direction. 



I have myself, within the past ten years, produced hybrids between 

 P. angustifolia and P. americana ; between P. domestica and P. 

 americana; between P. hesseyi Bailey {P. pumila Lin.) and P. 

 hortulana B.; between Cerasus avium var. and P. hesseyi B. I made 

 several hundred crosses to produce hybrids between our Sand cherry 

 {P. hesseyi) and horticultural varieties of Cerasus avium. Pollen 

 of C. avium var. on P. hesseyi invariably proved sterile ; reciprocal 

 crosses set fruit, but they failed to germinate, the seed containing 

 only a trace of the aborted ovule. When I finally used the pollen of 

 a proterandrous form of P. hesseyi on a short-styled form of C. avium 

 fertilization was effected and developed a normal fruit, the seed of 

 which germinated and produced an undoubted hybrid. The recipro- 

 cal crosses of the same varieties failed to fertilize a single ovule out 

 of over fifty crosses made. I had applied the same principle in the 

 production of hybrids between P. hortulana and P. hesseyi with fair 

 success. The successful crosses just mentioned were made with pollen 

 which had not been too greatly differentiated, on a pistil which, in 

 accordance with the theory advanced for the evolution of the differ- 

 ent forms, had been retarded. The unsuccessful crosses were made 

 with differentiated pollen on a pistil not sufficiently differentiated. 



Finally, we must conclude that the means by which the bisexual 

 forms have been produced, though gradually and necessarily very 

 slow, are identical with the forces that produced the different species. 

 In the crossing of the different species, we find that, by applying the 

 same rule for cross-fertilization, we can trace the genealogy back to 

 the forms wherein the differentiation of the sexual elements had not 

 destroyed their affinity. From these experiments we deduce the fol- 

 lowing : 



CONCLUSIONS. 



Self-sterility of P. americana in the heterostyled and bisexual 

 forms is caused by the great differentiation of the sexual elements. 

 Pollination by wind and insects cannot be controlled to any extent. 

 Mixed planting, therefore, unless it be done with respect to the nat- 



