ALLEN: EVOLUTION OF SEXUAL CHARACTERS 7 



But it would not be surprising to discover that this type of similarity presents 

 an additional instance of parallel evolution. 



The history of angiosperms begins at a level at which the greatly reduced 

 members of the gamete-bearing generation had long been sharply separated 

 as female and male individuals. Sexual differentiation also had been projected 

 backward to effect a distinction in the parental generation between female 

 and male spore-bearing structures. These structures — macro- and micro- 

 sporangia — were now borne upon or within likewise sexually differentiated 

 organs later to be designated pistils and stamens. 



Less confident must be any statement as to the distribution of pistils and 

 stamens in primitive angiosperms. Three conditions are conceivable. Either 

 the original angiosperm flower was bisexual (bisporangiate), the plant bearing 

 it being hermaphroditic ; or there were separate pistillate and staminate 

 flowers, borne either on the same plant (a condition of monoecism) or on 

 distinct plants (a condition of dioecism). 



Attempts to choose between these possibilities were based first upon 

 comparative morphology ; then, as fossil evidence accumulated, the assistance 

 of paleobotany was sought. The latter source has as yet contributed little 

 to the problem here involved. It has shown that the equivalent of a bisporan- 

 giate flower was developed by Cretaceous times in the Bennettitales ; and that 

 the equivalent of a unisporangiate flower was present in the Caytoniales as 

 early as the Triassic. But it is agreed that neither Bennettitales nor Cay- 

 toniales were ancestral to modern angiosperms. Probably the great majority 

 of those who have discussed the question have concluded that primitive angio- 

 sperms had bisexual flowers. But unanimity upon this point is not reached ; 

 and the possibility of a polyphyletic origin, some lines starting with herma- 

 phroditism, others with monoecism or dioecism, is not wholly excluded. 



Since the sharp distinction between female and male gametophytes was 

 established at a pre-angiosperm level, a discussion of the evolution of sexual 

 characters in angiosperms can deal only with developments within the spore- 

 bearing generation. It may be asked, first, what if any genetic evidence is 

 there as to the type of distribution of sexual structures in primitive angio- 

 sperms? Second, what appears to have been the most probable course or 

 courses of evolution of sexual characters since the dawn of angiosperm history ^ 



Two general sets of facts, long recognized and both to be referred to 

 later, suggest the derivation of unisexual from bisexual flowers. One of these 

 concerns the presence in the majority of monoecious and dioecious species 

 of pistil-rudiments in staminate flowers and of stamen-rudiments in pistillate 

 flowers. The stage to which these rudimentary structures develop varies from 

 that of a small hump of undifferentiated tissue to that of the reflexed stamens 

 of the functionally female flowers of the grape, which produce non-viable or 



