194 MORPHOLOGY OF GYMNOSPERMS 



event, the structure emphasizes the interrelationships of Ginkgoales, 

 Cordaitales, and CycadofiHcales. 



The development of the microsporangium has been described by 

 Miss Starr (53). Early in July the strobili which are to mature 

 their pollen the following year can be recognized as small papillae in 

 the axils of bracts, and before winter checks the growth the oldest 

 sporangia at the base of the strobilus reach the mother cell stage 

 (fig. 219). Although the earliest stage was not seen, it was evident 

 that a single hypodermal cell divides anticlinally and then periclinal 

 divisions follow, or the order is reversed. The outer cells produce the 

 wall of four to seven layers, and the inner cells a large group of spo- 

 rogenous cells (fig. 220). The tapetum seems to be formed entirely 

 from the peripheral cells of the sporogenous group. In the mature 

 sporangium the hypodermal layer of cells (outermost wall layer) 

 and the layer beneath it develop thickening bands in the walls, and 

 in the upper part of the sporangium, near the mucilage cavity, the 

 walls of deeper cells are also thickened. Ginkgo seems to be the only 

 gymnosperm that develops an endothecium (32), in the sense that the 

 layer or layers of cells that show the structure and function of an endo- 

 thecium are hypodermal. The longitudinal slits of dehiscence of the 

 two sporangia face one another, and lie at such an angle that the 

 pollen is shed easily (32). 



THE MEGASPORANGIUM 



The ovulate strobili of Ginkgo are very much reduced, and 

 are borne in groups at the summit of the dwarf shoot (fig. 222). 

 The individual strobilus consists of a long stalk arising from the 

 axil of a bract and bearing at its apex mostly two ovules, one of 

 which usually aborts, although in favorable seasons it is by no means 

 uncommon to find both ovules fully developed (fig. 223). Occa- 

 sionally more than two ovules appear, and the indications are that 

 the strobilus of Ginkgo has been derived from one bearing several or 

 perhaps many ovules (fig. 224). 



About the base of the ovule there is a more or less conspicuous 

 cup or "collar," much more prominent in the young stages of the 

 ovule. This collar has been a prolific source of discussion, and a 

 brief resume of the views as to its nature and the nature of the strobilus 



