166 EVOLUTION OF PLANTS 



they occur in great numbers. The oldest forms closely 

 resembled the existing genus Cycas, which has persisted 

 while many of the more specialized types have become 

 quite extinct. 



Perhaps allied to the Cycads, and like them also 

 a very old type, is the curious genus Gingko (Fig. 

 41, H), represented at present by a single species no 

 longer known in a wild state, but much planted about 

 temples in China and Japan, where gigantic trees, hun- 

 dreds of years old, are standing. From the fern-like 

 venation of the leaves, the tree is sometimes called the 

 maiden -hair tree, and this peculiarity of the leaves prob- 

 ably indicates a real affinity with the ferns. Many fossil 

 species, much like the existing one, are known, the oldest 

 ones from the Permian rocks, and therefore somewhat 

 more recent than the oldest Cycads. 



Gingko is usually referred to the Coniferse, but the 

 development of the gametophyte, especially th*e produc- 

 tion of multiciliate sperrnatozoids like those of Cycas, 

 as well as the fern-like character of the leaves, suggest 

 that its affinities are rather with the Cycads than with 

 the Conifers. 



THE CONIFERS 



Although the Cycads and Coniferse are usually asso- 

 ciated in a common group, Gymnospermse, it is at least 

 doubtful in view of the recent discoveries in regard to 

 the former, as well as because of other differences in 

 structure, whether these two orders are really related to 

 each other. 



The Coniferse are the familiar " evergreen " trees of 



