212 
MORPHOLOGY 
form, filling the egg with tissue (proembryo) more completely and per- 
manently than in any other known gymnosperm (fig. 472). The cells 
of the upper two thirds of this pro- 
embryo remain inactive ; while the cells 
of the lower third (which are much 
smaller) grow actively, forming a broad 
cylinder that invades the endosperm. 
This cylinder is really a massive sus- 
pensor, and at its tip the embryo is 
formed. This embryo, as in cycads, 
has two cotyledons, but sometimes three 
have been observed, and they also remain 
in the seed during germination. 
Conclusions. Ginkgo resembles the 
Cordaitales and the cycad line in the 
FIG. 47*- Proembryo of structure o f its ovules, and in its swim- 
Ginkgo, filling the egg with tissue. ..... . ., 
After STRASBURGER. mm g sperms ; but it is like the conifers 
in the habit of its sporophyte body and 
in its stem structure. Its origin from the Cordaitales seems clear, but 
the primitive reproductive characters which persist also distinguish it 
from Coniferales as a separate line. 
(6) CONIFERALES 
General character. This is the large group of living gymnosperms, 
comprising approximately 350 recognized species, included in forty 
genera. In contrast with the tropical distribution of the cycads, the 
conifers are characteristic of the north and south temperate zones. 
Two families are recognized : Taxaceae, in general with fleshy seeds and 
freely exposed ovules ; and Pinaceae, in general with dry seeds and ovules 
covered by scales. The Taxaceae comprise about eleven genera and 
100 species ; while the Pinaceae comprise about twenty-nine genera and 
250 species. The two families differ so much that they must be treated 
separately. 
(a) Taxaceae 
General character. The Taxaceae comprise two well-marked tribes 
or subfamilies : Podocarpineae (the podocarps) and Taxineae (the 
taxads). The podocarps in general are south temperate, Podocarpus 
being the largest genus (about sixty-five species), and as characteristic 
