92 



THE LIVING CYCADS 



both eggs and sperms; in Selaginella, the sporangium 

 shown in the figure is a ''megasporangium," producing 

 only megaspores which give rise to prothaUia bearing 

 eggs but no sperms. There is a corresponding '^ micro- 

 sporangium," with '' microspores" which give rise to 

 prothaUia bearing the sperms. Both kinds of spores 



are borne upon the same 

 ^ plant but not in the same 



sporangium. 



THE FEMALE GAMETOPHYTE 



The ovule of a cycad is 

 strictly comparable with 

 the megasporangium of 

 Selaginella. At an early 

 stage of development it 

 contains four megaspores, 

 three of which begin to 

 abort as soon as they are 

 formed, while the other 

 germinates, as in Selagi- 

 nella, without escaping 

 from the megasporangium. To the uninitiated this 

 plant — variously called the "endosperm" or prothallium, 

 or female gametophyte — looks like a part of the tissue 

 of the ovule itself (Figs. 39 and 40). 



Ovules vary in size from that of Cycas circinalis, 

 which sometimes reaches a length of more than two 

 inches, down to that of Zamia pygmaea, less than an 

 eighth of an inch in length. 



The structure varies in details, but the principal 

 features are rather uniform: there is one integument 



Fig. 37. — Selaginella: a mega- 

 sporangium, "showing one mega- 

 spore mother-cell which has 

 divided, forming a tetrad (/) of 

 four spores, three of which are 

 shown The rest of the spore 

 mother-cells (a) are abortive. 

 Highly magnified. 



