196 



GYMNOSPERMS 



THE SPOROPHYTE — REPRODUCTIVE 



ITie reproductive structures of Ginkgo are the most primitive in 

 living seed plants, except the Cycadales. 



The male slrobilus. — The male strobilus is strikingly like that of 

 the extinct Baiera, the principal difference being that Baiera has as 

 many as six microsporangia on a sporophyll, while Ginkgo almost 

 always has only two. The number two, however, is not entirely 



Fig. 210. — Ginkgo biloba: section of a young leaf cut at a right angle to the vein. 

 The spiral proto.vylem has lignified but the walls of the rest of the xylem have 

 scarcely begun to thicken. A large mucilage cavity is shown at the right; X180. 



rigid, for material grown in the United States often shows three or 

 four sporangia, and Sprecher"^ cites a case with seven. 



The slender sporophyll is surmounted by a hump and bears two 

 pendant microsporangia (figs. 211, 212). The stalk has two small 

 collateral endarch bundles. 



The development of the microsporangium follows the usual 

 eusporangiate type, an archesporial cell giving rise to a primary wall 

 cell and a primary sporogenous cell, the latter giving rise to the spo- 

 rogenous tissue and, later, to the spores. 



Miss Anna Starr"^ investigated the hump at the top of the sporo- 

 phyll and found that its large mucilage cavity developed like a 



