CYCADALES 



139 



I S3 



to the endosperm, contains a substance related to pectin, toward 

 the middle the pectin is replaced by cellulose, and the cellulose is 

 replaced in turn by suberin in the 

 outer part of the endospore, where 

 it borders upon the suberized exo- 

 spore. In structure the endospore 

 is rather homogeneous, while the 

 exospore is formed of little columns 

 or fibrillae, which give it a dis- 

 tinctly striated appearance (fig. 152). 

 The outer walls of the endosperm 

 cells bordering upon the endospore 

 become suberized, and might some- 

 times be mistaken for a part of 

 the endospore itself. The mem- 

 brane in Dioon edule is shown in figs. 

 136, 137, where it is in contact with 

 the endosperm above the arche- 

 gonia; and in fig. 153, which shows 

 the comparatively homogeneous 

 endospore and the exospore con- 

 sisting of club-shaped bodies with 

 an ovoidal outer portion connected 

 with the endospore by a stalk. 

 In sections more than i or 2 /^ in 

 thickness, the region of the stalks 

 appears as a nearly black line, so 

 that there seem to be three layers, 

 the extra layer being only an optical effect due to the stalk region, 



Figs. 152, 153. — Themegaspore 

 membrane of the cycads: fig. 152, 

 Cycas revoluta; three free nuclei of 

 the endosperm at the left, cells of 

 the endosperm jacket at the right, 

 and the megaspore membrane (4 . 5 /* 

 thick) between them; fig. 153, Dioon 

 edule; section of megaspore mem- 

 brane showing inner homogeneous 

 layer and outer layer of club-shaped 

 bodies; X 1,200. — Fig. 152, after 

 Thomson (45); fig. 153, after 

 Chamberlain (46). 



THE MALE GAMETOPHYTE 



The microspore is the first cell of the male gametophyte. The 

 peculiar thickening of the wall of the microspore mother cell, making 

 it resemble a heavy, foiur-celled case containing the four spores, has 

 been described by Jueanyi (5) for Ceratozamia longifolia, and by 

 Teexje (ii) for Zamia muricata (fig. 154). Jusanyi (5) gave also 

 a fairly full account of the germination of the microspore of Cera- 



