122 Seward and Gowan . — The Maidenhair 
and may be absent. In longitudinal section the spiral proton 
xylem-tracheids may be traced gradually into short and 
broad reticulated elements occupying a centripetal position. 
The centrifugal tracheids next the protoxylem are reticulately 
pitted, and these are succeeded by tracheids with alternate or 
crowded bordered pits. 
II. Flowers. 
a. Female Flowers. 
The ordinary type of female flower has the form of a long 
naked peduncle bearing a single ovule on either side of the 
apex (PL IX, Fig. 6), the base of each ovule being enclosed by 
a small collar-like rim, the nature of which has been variously 
interpreted by different writers. 
A young ovule of Ginkgo consists of a conical nucellus 
surrounded by a single integument (Fig. 47, z), terminating in 
the form of a two-lipped micropyle. A large pollen-chamber 
(/c, Fig. 47) 1 occupies the apex of the nucellus, and im¬ 
mediately below this two or more archegonia are developed 
at the summit of the embryo-sac, each of which consists of 
a large egg-cell surmounted by two neck-cells and a ventral 
canal-cell, which is cut off shortly before fertilization. In the 
young ovule shown in median longitudinal section in PL IX, 
Fig. 47, the embryo-sac is represented at e in an early stage 
of development. Pax 2 calls attention to the many-celled 
archesporium of Ginkgo as a character resembling the Cycads. 
At a later stage, after the pollen-grains have entered the 
pollen-chamber, the opening of the latter becomes closed, the 
chamber being roofed over by a blunt protuberance of nucellar 
tissue, and at the beginning of June it increases in size and 
forms a large irregularly shaped cavity. During the growth 
of the embryo-sac the nucellus is gradually destroyed, and 
early in August a vertical outgrowth is formed from the top 
Bertrand ( 78 ). 
3 Pax (’ 90 ), p. 274. 
