306 PLANT MORPHOLOGY 



groups of living pteridophytes, as well as of a number of extinct forms. 

 Its appearance is important because it is a feature carried on into the 



spermatophytes. 



In the Lycopodiinae and Filicinae the sporangia are borne in connection 

 with some or all of the leaves. In the Lycopodiinae they are solitary and 

 adaxial, in the Filicinae numerous and mainly abaxial. In the Psilo- 

 phytinae the sporangia are terminal, either on the main stem (Psilophy- 

 tales) or on a very short lateral branch (Psilotales). In the Equisetinae 

 the sporangia are borne on sporangiophores. The sporangium of the 

 pteridophytes is always epidermal in origin and may arise from a single 

 cell or a small group of cells. In its development, all pteridophytes are 

 eusporangiate except two orders of ferns, the Filicales and Hydropter- 

 idales, which are leptosporangiate. With the exception of the Psilotales, 

 a tapetum is present in all living pteridophytes. In the Lycopodiinae the 

 tapetum is persistent, but in the Equisetinae and Filicinae it soon breaks 



down. 



Heterospory. The appearance of heterospory represents a third great 

 forward step in evolution introduced by the pteridophytes. Among 

 modern representatives it occurs only in Selaginella, Isoetes, and the five 

 genera of the Marsileaceae and Salviniaceae. Heterospory was devel- 

 oped, however, in many extinct forms. It introduces into the life history 

 two kinds of spores, two kinds of gametophytes, and a great reduction in 

 the gametophyte generation. Heterospory, introduced by the pterido- 

 phytes, is an established feature of the spermatophytes. In fact, it 

 makes seed formation possible. 



The gametophytes of homosporous pteridophytes may be tuberous and 

 subterranean, as in the Psilotales, Lycopodiales, and Ophioglossales, 

 where the absence of chlorophyll is associated with a saprophytic mode of 

 nutrition; or they may be flat, green, and aerial, as in the Equisetales, 

 Marat tiales, and Filicales. In practically all homosporous pteridophytes 

 both kinds of sex organs are borne in comparatively large numbers on the 

 same gametophyte. In the heterosporous forms, however, the gameto- 

 phytes are always dioecious and reduced, both kinds developing largely or 

 entirely within the spore wall. The male gametophyte produces only one 

 or two prothallial cells and one or two antheridia. The female gameto- 

 phyte usually has more vegetative tissue, but generally only one to several 

 archegonia. 



The gametophyte of the homosporous pteridophytes, with much veg- 

 etative tissue, must not only make its own food, but also enough for the 

 embryo sporophyte that is dependent upon it. The development of such 

 a gametophyte requires a considerable period of favorable external condi- 

 tions. This handicap is largely avoided by the heterosporous pterido- 

 phytes. Their gametophytes are formed inside the spores that produce 



