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 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 BBLAJEFF. 



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 



