ORGANIC EVOLUTION 



139 



are very different from those we have studied hitherto. 

 The microspore develops, beginning while still within the 

 microsporangium, into a male prothallium of microscopic 

 size. It consists when mature of but few cells, one of these 

 representing the body of the prothallium and the others 

 the antheridial wall, inclosing a considerable number of 



Fig. 88. Selaginella. a and b, tip and base of a fruiting spray; c, diagram 

 of a fruiting spike, showing macro and micro-sporangia in the axils of 

 scales; tf, microsporangium ; e, macrosporangium ; /, the male prothal- 

 lium, spore mother cells dotted; g, as single sperm cell, h, the fe- 

 male prothallium, rupturing the wall (w) of the macrospore; m, a new 

 sporophyte embryo ; n, its suspensor, and p its growing point ; o, the 

 remains of an old archegonium; r, rhizoids. 



Sperm cells. There are no rhizoids or other nutritive 

 organs developed, and the prothallium is short lived After 

 a few cell divisions it differentiates the sperm cells, which are 

 liberated in the water by the dissolution of the surround- 

 ing cells. 



