I02 



THE LIVING CYCADS 



■'^ 



47 



primitive cycads, showed an average of more than 700 

 microsporangia to the sporophyll, Encephalartos cafer 



has about 500, Dioon cdule 

 about 300, Ceratozamia 

 ^^^i ]^^^ about 200, Stangcria 150, 



^^^P' — -\.^|^^ Bowenia 50, and Zamia 25. 



^HB? .^sy The structure of the 



.3^^ microsporangium is inter- 



esting. In the case of the 

 megasporangium, or ovule, 

 the structure has been so 

 profoundly modified that 



§^^|^ its relation to the sporan- 

 ^^^^^ gium of its remote fern 

 ^^^^H ancestors is established 

 ^^^V only by the details of 



development and by the 

 evidence of comparative 

 morphology; but the 

 microsporangium bears 

 such a close resemblance 

 to its fern prototype that 

 the novice may have great 

 difficulty in distinguishing 

 them. The similarity will 

 be appreciated by com- 

 paring the sporangia of 

 Dioon and Angiopteris, a 

 primitive fern (Figs. 50 

 and 51). In both there is 

 an outer layer of thick-walled cells, then several layers 

 of thin-walled cells, and beyond these a layer of modified 



48 



Figs. 47-49. — Ceratozamia vtcxi- 

 cana: male sporophylls with micro- 

 sporangia: Fig. 47, the sporangia 

 not yet opened; Fig. 48, the 

 sporangia on the upper half of the 

 sporophyll opened, but the pollen 

 not yet shed; Fig. 49, the 

 sporangia, having shed their pollen. 

 The arrangement ot sporangia in 

 three's and four's is easily seen. 

 The two "horns" at the top of the 

 six)roi)hyll give the name to the 

 genus; about i\ times natural size. 



