506 



A TEXTBOOK OF BOTANY 



[Ch. XII 



out flat when watered, — presumably in the functional 

 class of the Tumbleweeds (page 359). 



Of special interest is the reproduction in Selaginella (Fig. 

 359), for its form of heterospory brings it very close to the 



Fig. 359. — The life cycle of Selaginella, the stages from various species. 



Left, a branch from a typical plant ; X i Next, above, a strobilus ; 

 X 5. Second line, part of a strobilus in median longitudinal section, show- 

 ing megasporangium with megaspores on left, and microsporangium with 

 microspores on right ; X 25. Next a microspore, X 250, and a germinated 

 microspore, in section, the single prothallial cell being below, X 500, and 

 two sperm cells, X 500. Lower line, a megaspore, X 75, and a germinated 

 megaspore, with archegonia and a fully formed embryo embedded in the 

 prothallus, X 150. Finally, a young plant developing from the embryo, 

 X 40. (In part after Dodel Port.) 



Seed plants. The sporophylls are collected in terminal 

 strobili (Fig. 223), and bear in their axils two kinds of spo- 

 rangia, containing two kinds of spores. Both kinds are 

 formed in fours, in correlation with the reduction division of 

 the chromosomes. They begin to germinate while still 



