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. 
470, g), and 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, . n . . . 
showing thick integument (0, micropyle and flesh y and another Compact 
(m), nucellar beak with pollen chamber and Stony. 
(/>), collar (0, and young female ga- tinueg j ts 
metophyte (g). After COULTER and , ,, , , 
CHAMBERLAIN. a " tne 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- 
The gametophyte COn- 
until it destroys 
