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A TEXTBOOK OF THEORETICAL BOTANY 



the prothallus some rather vague walls extend downwards into the rest of 

 the protoplasm, but without dividing it completely into closed cells. The 

 bottom of the prothallus itself becomes thickened, and it is thus clearly 

 separated from the material below. The thickened layer is called the 

 diaphragm (Fig. 596). 



Certain cells now begin to enlarge to form archegonia, and at this stage 



Fig. 597. — Selaginello kroussimia. Dehiscent megaspore showing 

 exposed prothallial tissue with rhizoids and scattered 

 archegonia. 



the spore opens along the ridge, exposing the prothallus. Three little pro- 

 trusions are formed on its surface, which develop rhizoids (Fig. 597), but 

 they are purely vestigial, as the prothallus is never independent. Numerous 

 archegonia are formed. They are of a simple type, the initial cell dividing 

 only into two, not three as in Lycopodium. The upper cell forms a short 

 neck, two cells high, and the lower cell divides into the oosphere, the 

 ventral canal cell and a single neck-canal cell which penetrates into the neck. 



