166 



PLANT STRUCTURES 



leaf (Fig. 139). This is important as showing that sporan- 

 gia may be produced by stems as well as by leaves, those 

 being produced by leaves being called foliar, and those by 

 stem cauline. 



The most important f fact in connection with Selaginella, 

 however, is that it is heterosporous. Megasporangia, each 

 usually containing but four megaspores, are found in the 

 axils of a few of the lower leaves of the strobilus, and more 

 numerous microsporangia occur in the upper axils, con- 

 taining very many microspores (Fig. 139). The character 

 of the gametophytes of heterosporous Pteridophytes may 

 be well illustrated by those of Selaginella. 



The microspore germinates and forms a male gameto- 

 phyte so small that it is entirely included within the spore 



FIG. 140. Male gametophyte of Selaginella: in each case p is the prothallial cell, 

 the wall cells of the antheridium, s the sperm tissue: F, the biciliate sperms. 

 After BELAJEFF. 



wall (Fig. 140). A single small cell is all that represents 

 the ordinary cells of the prothallium, while all the rest is 

 an antheridium, consisting of a wall of a few cells sur- 

 rounding numerous sperm mother cells. In the presence 



