2IO 



MORPHOLOGY 



ing the mother cell, therefore, there are three distinct concentric zones 

 of tissue: (i) the invading digestive zone; (2) the invaded and dis- 

 organizing zone ; and (3) the storage zone, outside of the disorganizing 

 zone, and containing food surplus in the form of starch. After the 

 female gametophyte (endosperm) is organized, it in turn invades and 



destroys the digestive zone and all 

 the surrounding tissues of the nu- 

 cellus. The mother cell forms the 

 usual linear tetrad of megaspores, 

 only the innermost one enlarging 

 and functioning. 



Female gametophyte. — The fe- 

 male gametophyte develops as de- 

 scribed for Cycadales (see p. 196), 

 with free nuclear division (up to 

 256 free nuclei), parietal tissue (fig. 

 47O7 g)> ^-nd centripetal growth. 

 It is a remarkable fact that this 

 female gametophyte becomes green, 

 although enclosed within a three- 

 layered testa, one layer being thick 



Fig. 470. — Section of ovule of Ginkgo, 



showing thick integument (/), micropyle ^^'^ ^^shy and another Compact 



The gametophyte con- 

 tinues its growth until it destroys 



(m), nucellar beak with pollen chamber and Stony. 

 (/>), collar (c), and young female ga- 

 metophyte (g). — After Coulter and ,, , ,, . , , 



Chamberlain. 8-" '"^ nucellar tissues and reaches 



the testa. 



Archegonia. — The archegonia are usually two in number (sometimes 

 three), and develop as described for the cycads, including the organiza- 

 tion of the archegonial jacket (see p. 197). In cycads a ventral nucleus 

 is formed and speedily disorganizes, but in Ginkgo a cell wall is devel- 

 oped separating the ventral nucleus from the egg, so that there is a 

 ventral canal cell, a feature which persists in some of the conifers. In 

 Ginkgo the archegonial chamber is formed as usual, but between the 

 two archegonia the endosperm grows into a conspicuous central beak, 

 which reduces the archegonial chamber to a circular crevice (fig. 471). 

 This peculiar feature appears also in the Cordaitales, but is not known 

 in any other group of gymnosperms. 



Male gametophyte. — The male gametophyte develops two vegetative 

 cells (the first one lasting only until shedding, the second one persist- 



