126 



GYMNOSPERMS 



/3/ 



As in all seed plants, the megaspore, with its contained female 

 gametophyle, is never shed from the sporangium. The spore coats 

 of the heterosporous ferns, the ancient ancestors of the cycads, were 

 very thick. As the megaspore became retained, its spore coats be- 

 came thinner, but are easily recognized even in rock sections of the 

 extinct Cycadolilicales (figs. i6, 19, 20). In the Cycadales, the mega- 

 spore membrane stains brilliantly with safranin. Thomson''^^ found 



that the membrane consists of two 

 layers, which may be called the en- 

 dospore and exospore, the outer of 

 which is suberized, while the inner 

 consists of a substance closely related 

 to pectin. 



The inner layer is dense and homo- 

 geneous. The outer layer consists 

 of club-shaped bodies with enlarged 

 ends, so that under moderate magni- 

 fication there seem to be three layers, 

 a middle layer being the stalks of the 

 club-shaped bodies (figs. 130, 131). 

 The membrane, in Dioon cdulc, 

 is strongly developed as early as 

 November, and continues to increase 

 in thickness up to the germination of 

 the seed, when it reaches a thickness 

 of 10 microns. In fresh material, one 

 might mistake the jacket of the 

 female gametophyte for the mem- 

 brane, but the jacket is coarser and 

 can be stripped off entire with for- 

 ceps. The membrane is more deli- 

 cate, but pieces several millimeters in length can be stripped ofT. The 

 membrane is thicker at the middle of the gametophyte than at the 

 top or bottom. 



As the archegonial chamber develops, the membrane in that region 

 is ruptured, so that the pollen chamber and archegonial chamber 

 form a continuous cavity (figs. 132, 133; see also fig. 147). 



/30 



Fig. 130. — Cycas revolula: endo- 

 spore and e.xospore about equal in 

 thickness. Thickness of entire mem- 

 brane is 4.5 microns. — After R. B. 

 Thomson .^w 



Fig. 131. — Dioon cdulc: The 

 megaspore membrane; the inner 

 part is dense and homogeneous, 

 while the outer part consists of 

 ovoid, stalked bodies; X900. — After 



CHAMnKRLAIN.'"'' 



