272 GYMNOSPERMJE. 



to these, Taxacese, Abietaceae, and Taxodiaceaa appeared in the Carboniferous 

 period. The TAXACE;E appear to have attained their culmination in the 

 Jurassic and Cretaceous periods ; Ginkgo appears in the Rhaetic ; Torreya, in the 

 Cretaceous ; Taxus and Podocarpus in the Tertiary periods. The ABIETACE^: 

 also appear in the Carboniferous ; Pinus was first known with certainty in the 

 English Weald and in the Cretaceous ; almost all other contemporary genera are 

 represented in this latter period. The ABAUCABIACE^; first appear, with certainty, 

 in the Jurassic. The TAXODIACE^E may be traced back as far as the Carboni- 

 ferous (?) ; Sequoia is first found in the lowest Cretaceous, at tbat period it 

 spread throughout the entire Arctic zone, and being represented by a large 

 number of species, formed an essential part of the forest vegetation. Sequoia 

 played a similar part in the Tertiary period. The CUPRESSACE^E are first 

 known with certainty in the Jurassic, but they appeared more frequently and 

 numerously in the Tertiary period, in which most of the present living genera 

 were to be found. The GNETACEJE, according to a theory advanced by Renault 

 were represented in the Coal period by the genus Steplianospermum, which had 

 four ovules enclosed by an envelope. 



Recent investigations have established Ginkgoineee as a distinct Class inter- 

 mediate between the Cycadeae and the Conifers, Ginkgo biloba being the only 

 existing representative of this Class. It is more closely allied to the Cycadeaa. 



FIG. 275A. Ginkgo bilobo: 

 Motile Rpermatozoida in the 

 pollen-tube; n nucleus. 



The pollen-grains are spherical, and in these a prothallium is formed generally 

 composed of three cells, the most internal of which is the antheridium. This 

 contains two ovoid spermatozoids having at the anterior end a three-coiled 

 spiral groove bearing numerous cilia (Fig. 275A). 



