Feb., 1922 sexual evolution in the plant kingdom 107 



IV. HETEROSPOROUS SERIES WITH ANTITHETIC ALTER- 

 NATION OF GENERATIONS, THE GAMETOPHYTES 

 BEING UNISEXUAL AND THE SPOROPHYTE INDIVID- 

 UALS BISPORANGIATE OR AlONOSPORANGIATE. 



In this series the sporophyte shows dimorphism at least in 

 the spores and sporangia. The gametophyte generation is appar- 

 ently always unisexual with distinct male and female individ- 

 uals highly differentiated both in morphological and physiolog- 

 ical characteristics. Normally, the sexual state of the gameto- 

 phytes is irreversible and determined even before the spores 

 are produced from which they originate. 



15. The lowest stage of this series is represented by plants 

 whose bisporangiate sporophytes have only a slight dimorphism; 

 only the spores and sporangia are differentiated and show male 

 and female expression. The two kinds of sporangia are devel- 

 oped side by side on the same leaflet and sorus. The sexual 

 expression is controllable to a slight extent by artificial means. 



Example: Marsilea. 



Note: It must be remembered that this condition is evolv- 

 ing from a sporophyte that shows absolutely no sexuality in 

 morphological expression in any part of its body but that its 

 cells are, nevertheless, potentially sexual, the gametophytes 

 possessing a high degree of sexual dimorphism. 



16. The first advance on the preceding condition is slight 

 but important. This stage is represented by bisporangiate 

 sporophytes with distinct microsporangiate and megasporan- 

 giate sori which develop into sporocarps but on the same leaf 

 and which may be side by side. The sexual state is determined 

 considerably earlier than the incepts of the sporangia in the 

 group of cells or in the tissues from which the entire sorus is 

 developed. The given sexual state must, however, be rather 

 weak at first since vestiges of the opposite organs appear. 



Examples: AzoUa caroliniana and Salvinia natans. 



Note: The megasporangiate sorus of Azolla contains ves- 

 tigial microsporangia and the microsporangiate sorus contains 

 a vestigial megasporangium, plainly showing that the mono- 

 sporangiate condition of the sorus was derived from an original 

 bisporangiate sorus as in Marsilea, the beginning of the sexual 

 state being thrown back to some extent. The change from one 

 sexual state to the other in the incipent sporangia inhibits the 

 full development. 



