158 



GYMNOSPERMS 



beginning to be retained within the sporangium, sometimes falling 

 out — ^whcn the plant must be classed as an heterosporous fern — and 

 sometimes remaining inside the sporangium — when the plant must 

 be classed as a seed plant. When some sporangia on a plant shed the 



Fig. 171. — Reduction of the megasporophyll in Cycadales: A, theoretical ancestor 

 of Cycas\ B, Cycas revolula; C, Cycas circinalis; D, occasionally in Cycas media, and 

 usually in Cycas normanbyana; E, Dioon edule; F, Macrozamia; G, Ceralozamia; H, 

 Zamia. 



Fig. 172. — Reduction of the megasporophyll in Bcnncttitales: A, B,C, stages in the 

 reduction in some hypothetical ancestor; I), the usual condition found in fossils; E, the 

 sporophyll has become entirely sterile; /*', fertile and sterile sporophylls with about the 

 arrangement found in Bennellites gibsonuinus. 



megaspore, while others on the same plant retained it, as sometimes 



happens in Sclaginclla apiis, only a taxonomist, by carefully selecting 

 small portions of the frond, could get satisfactory specimens for his 

 herbarium. 



The reduction of the megasporophyll in the Cycadofilicales has 



