PRINCIPAL GROUPS OF PLANTS. 87 



gametophytes which give rise to archegonia, and hence called 

 female gametophytes. When a plant produces both microspores 

 and megaspores it is said to be HETEROSPOROUS, as in Selaginella 

 (Figs. 60, 62, and 63) ; while one that produces but one kind of 

 sporangium and one kind of asexual spores is said to be ISOSPOROUS. 

 In this connection attention should be called to the fact that the 

 spores from a single sporangium of an isosporous plant may give 

 rise to male and female gametophytes, which shows that a certain 

 degree of differentiation in the spores has already taken place. 

 The causes leading to the differentiation of the spores seem to be 



B 



FIG. 50. Male fern [Dryopteris (Aspidium or Nephrodium) Filix-mas], A, prothallus 

 of garpetophyte as seen from the under (ventral) side showing archegonia (ar), antheridia 

 (an), and rhizoids (rh); B, prothallus showing young plant (sporophyte) which has devel- 

 oped from an oospore and is still connected with the gametophyte, roots (w), and the first 

 leaf (b). After Schenck. 



connected with nutrition, those nuclei which are in more favorable 

 positions giving rise to larger and better nourished spores which 

 eventually lead to the formation of the megaspores, and those 

 which are less favorably placed leading to the microspores. 



The subject of heterospory is one of great interest, and when 

 it is pointed out that all of the higher plants are heterosporous 

 the subject has even more interest. 



FILICALES. 



General Characters. On germination the asexual spore in 

 the Filicales or Ferns gives rise to a thallus-like body known as 



