GINKGOALES 



209 



Cell walls now begin to be formed perpendicular to this membrane, 

 and with their outer edges attached to it (fig. 225). As wall forma- 

 tion progresses, the inner side of the innermost cell has no wall; but 

 when the period of wall formation comes to a close a wall is formed on 

 this inner side, so that each of the inner cells has its own wall. 

 (Fig. 226). Consequently, the female gametophyte can be split 

 apart along this central region. 



During the early growth of the female gametophyte the spongy 

 tissue surrounding it encroaches upon the tissues outside it; but, 



Fig. 226. — Ginkgo biloba: vertical section of part of female gametophyte at line of 

 closure, showing independent end walls of opposite cells (August 21); X325. — After 

 Carothers.9" 



later, it is itself absorbed by the growing gametophyte and is re- 

 duced to a mass of collapsed and deeply staining cells. Most of the 

 upper part of the nucellus is also absorbed. 



Archegonium initials appear long before the wall formation in the 

 gametophyte has reached the center. By the middle of June the 

 two-celled neck and the central cell have been formed, and the cen- 

 tral cell is enlarging rapidly. Usually there are only two archegonia, 

 lying in such a plane that longitudinal sections of both can be se- 

 cured by cutting longitudinally and parallel with the flatter face of 

 the gametophyte. Occasionally there are three archegonia. In such 

 cases, the gametophyte and stony layer are triangular in transverse 

 section, and there are three bundles in the inner fleshy layer of the 

 ovule. 



