92 



THE LIVING CYCADS 



both eggs and sperms; in Selaginclla, the sporangium 

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

 only megaspores which give rise to prothallia bearing 

 eggs but no sperms. There is a corresponding "micro- 

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

 prothalUa bearing the sperms. Both kinds of spores 



are borne upon the same 

 plant but not in the same 

 sporangium. 



/ 



a 



Fig. 37. — Selaginclla: a mega- 

 sporangium, showing one mega- 

 spore mother-cell which has 

 divided, forming a tetrad (/) of 

 four spores, three of which are 

 sliown The rest of the spore 

 mother-cells (a) are abortive. 

 Highly magnified. 



THE FEMALE GAMETOPHYTE 



The ovule of a cycad is 

 strictly comparable with 

 the megasporangium of 

 Selaginclla. 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- 



nclla, without escaping 

 from the mcgaspt)rangium. 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 Zaniia pygmaca, less than an 

 eighth of an inch in length. 



The structure varies in details, hut the principal 

 features are rather uniform: there is one integument 



