THE PLUM IN KANSAS. 99 



I have shown, present a number of peculiarities of morphology and 

 function, unusually interesting and significant and at the same time 

 unusually intelligible, nor are these peculiarities exhibited to the 

 same degree by any other group of plants. A study of these numer- 

 ous adaptations to insure cross-fertilization must necessarily end in 

 the conclusion that our species is gradually approaching a state of 

 dioeciousness, and, fortunately for our discussion, there appears to be 

 no missing link in the chain. With these numerous adaptations and 

 structures to prevent self-fertilization and to insure cross-fertilization 

 in view, we are prepared to understand" why, in the several cases, self- 

 and cross-fertilization are possible, and why impossible ; why cross- 

 fertilization is possible in a certain direction, while the reciprocal 

 cross may be sterile ; and, finally, by what means our si3ecie8 is gradu- 

 ally becoming dioecious. 



The wedge of variation, having gained a hold of our hermaphrodite 

 form, still in existence and capable of self-fertilization, and forced by 

 long-continued self- and occasional cross-fertilization, produced the 

 earliest types of our dichogamous group which the better insured 

 cross-fertilization. In the i^roterogynous form, the pistil, protruding 

 through the still undeveloped petals and stamens and receiving the 

 advantage of sunlight, air, and warmth, was encouraged to greater 

 development, the stamens being correspondingly retarded. In the 

 IDroterandrous form, the stamens received the benefit and the pistil 

 was retarded. Through successive generations, the influence of the 

 law of balancement has been at work, the evolution towards a separa- 

 tion of the sexual organs has fairly started, and we have the founda- 

 tion in the proterandrous and proterogynous forms for the pistillate 

 and staminate forms of a future dioecious species. Simultaneously 

 with the development of the pistil and retardation of the stamens, 

 and vice versa, came the further adaptation of difference in time of 

 maturity of the reproductive elements, with an additional protection 

 against self-fertilization. 



The development of the reproductive organs, aided by the law of. 

 balancement, continued, and we have developed the heterostyled groujj.. 

 What we before accomplished to quite an extent by purely mechanical 

 adaptations is now accomplished by a differentiation in the reproduc- 

 tive elements. So great a differentiation in the reproductive organs, 

 surely caused a differentiation in the sexual elements. Our species has 

 now become divided against itself. The differentiation of the repro- 

 ductive elements was followed by still further development and re- 

 tardation of the reproductive organs, and we have nearly reached the 

 bisexual stage, not only in morphology but in function. 



We now have only a step further to the complete separation of the 



