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A TEXTBOOK OF THEORETICAL BOTANY 



Against this, however, is the observation that when extra sporangia are 

 developed they are on the under side of the sporophyll and that in such cases 

 the hump is still present. The interest of the idea lies in this, that in Baiera, 

 a Mesozoic fossil closely related to Gifikgo, the numerous microsporangia 

 are arranged radially on the sporophyll, as in Taxiis, and it was supposed 

 that the hump on the upper side in Ginkgo might be a relic of this arrange- 

 ment. However, Baiera also has a hump, at the end of the sporophyll, and 

 when supernumerary sporangia occur in Ginkgo they are not radially arranged 

 but are pendant, which somewhat damages the theory. The hump is in fact 

 nothing more than a protective thickening of the sporophyll which covers 

 the young sporangia during their early development. 



The whole strobilus looks very much like a small catkin. Its axis contains 



Fig. 741. — Ginkgo biloba. A male sporophyll showing on the left, the two 

 sporangia and the sterile hump, and on the right the position assumed by the 

 sporangia on dehiscence. {After Pilger.) 



a circle of nine or ten vascular bundles which gives off a double trace, of 

 small collateral bundles, to each microsporophyll. 



The sporangium wall opens by a longitudinal slit. Its structure is notable 

 in that it is the middle layer of the wall, the endothecium, which bears 

 cuticularized thickenings and is active in causing dehiscence. In this Ginkgo 

 agrees with the Angiosperms and differs from other Gymnosperms, in which 

 the outer layer or exothecium of the wall is the thickened, active layer. 



The pollen grains are very like those of Cycas and the division of the 

 contents to form prothallial cells begins, as in the latter, before the sporangium 

 opens. Each grain has two little " ears " like miniatures of the wings on a 

 Pinus spore. 



Two prothallial cells are formed, the first of which aborts and later only 

 shows as a thickening of the spore coat, as in Pinus. The second one persists. 

 The remaining third nucleus divides again producing an antheridial cell, in 

 contact with the prothallial cell, and a large tube-cell, which does not divide 

 again. In this four-celled condition the pollen is shed. 



The Ovule 



Ovuliferous shoots appear in the axils of the leaves or scales on the spurs 

 of the female tree. A simple peduncle, about i in. long, bifurcates at its 

 apex, each branch bearing one terminal, sessile ovule, the base of which is 

 surrounded by a fleshy cup, the ring or collar (Figs. 742 and 743). 



