Feb., 1922 sexual evolution in the plant kingdom 103 



maphrodites. They must be either male or female although 

 they may be vegetative sister cells. So soon as a multicellular 

 condition is attained hermaphroditic individuals are possible, 

 and in the lower multicellular forms hermaphroditism is the 

 usual condition. If a progeny of unicellular individuals coming 

 from an ancestral cell by vegetative growth develops some as 

 male cells and some as female, we evidently have a condition 

 essentially like an ordinary multicellular hermaphrodite; for 

 if the cells remained associated after division instead of splitting 

 apart the combined units going into the sexual state at a certain 

 stage of their life history would represent genetical conditions 

 identical to a multicellular hermaphrodite. The further evolu- 

 tion from hermaphrodites is by gradual steps to unisexual, 

 multicellular individuals. In the higher plants with an anti- 

 thetic alternation of generations, the sporophyte is in the lower 

 levels entirely neutral and homosporous, showing no sexual 

 states; and from this condition the higher bisporangiate type 

 with heterospory is evolved. The heterosporous sporophytes 

 of the lowest types have bisporangiate leaves or bisporangiate 

 floral axes. This is the general condition from which both 

 monecious and diecious species are derived. In the mone- 

 cious individual the entire floral axis is monosporangiate or the 

 entire inflorescence, while in the extreme diecious condition the 

 entire individual produces but one kind of spores, or in other 

 words has but one kind of sexual expression. The highest 

 plants, therefore, have unisexual, dimorphic gametophytes and 

 diecious, dimorphic sporophytes. 



The detailed description of these evolutionary series is given 

 in outline below although no attempt has been made to catalog 

 the numerous intermediate and special cases. 



I. NONSEXUAL STAGE. 



1. The organization of the protoplast is apparently such 

 that the origin of sexual states or phases is impossible. This 

 may be true both morphologically and chemically. No sexual 

 state is shown at any stage of the life cycle although there is 

 considerable evolution and morphological differentiation dis- 

 played by the highest species of this type of plants. 



Apparent examples: Merismopedia, Micrococcus, Anki- 

 strodesmus, Rivularia. 



