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 

 ^ *? G " J. 12 ' T Proemb 7°. of structure of its ovules, and in its swim- 



Gmkgo, filling the egg with tissue. . . . 



Af ter Strasburger. ming 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, Podocarpns 

 being the largest genus (about sixty-five species), and as characteristic 



