Seed-Plants 145 



present distribution indicates that they are probably 

 pretty old types. 



RESUME 



Seed-plants Not All of Common Origin. That 

 the seed habit developed a number of times in quite 

 unrelated groups of pteridophytes is amply shown 

 by the fossil remains of seed-bearing plants in the 

 Paleozoic. Some one, or perhaps more than one, of 

 the seed-bearing ferns were probably the progen- 

 itors of the existing cycads, and some of the 

 more specialized cycad-like forms of the Mesozoic 

 formations. It is also pretty certain that the pe- 

 culiar genus Ginkgo is also descended from some 

 fern-like Paleozoic type. The cycads of the pres- 

 ent time are much scattered, and seldom occur in 

 numbers to make them important constituents of a 

 flora. The existing types are descended from some 

 of the less specialized fossil ones. 



The conifers are decidedly the prevailing type of 

 gymnosperms at the present day. Although the 

 number of known species is less than four hun- 

 dred, they are nevertheless very important factors in 

 the existing flora of many parts of the world, as 

 they often form extensive forests, including the 

 largest of known trees. The order is an adaptable 

 one, and conifers grow under quite different condi- 

 tions; but there are no real aquatic forms, though 

 some, like the cypress and tamarack, are swamp 



