204 



GYMNOSPERMS 



In the very abnormal cases in which several ovules are borne on a 

 branching axis, only one ovule terminates each branch, and its 

 peduncle has only two bundles, still further supporting the theory 

 that the collar is a modified leaf blade-sporophyll, bearing an ovule 



(fig. 221). 



Fig. 219. — Ginkgo biloba: longitudinal section of an ovule shortly after pollination; 

 o, outer fleshy layer of integument; s, stony layer of integument; /, inner fleshy layer of 

 integument; c, collar; (lower), abortive ovule. The inner fleshy layer and nucellus are 

 shaded with lines; X2. — From Coulter and Ch.^mberlain, Morphology of Cymno- 

 i perms'^ (University of Chicago Press). 



Fig. 220. — Ginkgo biloba: longitudinal section of ovule, after the stony layer has be- 

 come hard and the inner fleshy layer has become dry and papery; X 2. — From Coulter 

 and Chamberlain, Morphology of Gymnos perms'^* (University of Chicago Press). 



THE GAMETOPHYTES 



The gametophytes, especially the male, bear some resemblance 

 to those of the cycads, but have such definite characteristics that the 

 Ginkgoales and Cycadalcs could be identified by their gametophytes. 



The mule gamctophyic. — The microspore is the first cell of the 

 male gametophyte. As in the cycads, it germinates while still en- 

 closed in the microsporangium. At the first mitosis, two very un- 

 equal cells are formed, the smaller one, a prothallial cell. The inner, 



