ALTERNATING GENERATIONS 



that heterospory may occur without any essential change in the 

 sporophyte; for the plant of Selaginella is of the general Lycopod type, 

 with small-leaved, much-branched shoot rooted in the soil, showing 



FIG. 24. 



B, megasporangium of Selaginella. apus in median vertical section, showing three of the 

 four megaspores. X2i. A, a single megaspore, with prothallus and an archegonium, 

 more highly magnified. (After Miss Lyon.) 



continued apical growth and terminal strobili. These are constructed 

 essentially upon the Lycopodinous plan, but instead of the sporangia 

 being all alike, some contain numerous small microspores, others contain 



FIG. 25. 



Microspore of Selaginella apus, after 

 germination. (After Miss Lyon.) 



FIG. 26. 



Microspore of Selaginella apus, just before extru- 

 sion of the spermatozoids. (After Miss Lyon.) 



only four large megaspores. In both cases these result, like other spores, 

 from a tetrad division : the chief difference is in their size (Figs. 23, 24 B). 

 But though the sporophyte is essentially unaltered, the changes in the 

 gametophyte which accompany the heterosporous state are important. The 

 prothallus is no longer a free-growing, self-nourishing organism, but it 



