THE EVIDENCE 77 



seed, the development of the embryo going straight 

 on into that of the seedling. There is much diver- 

 sity, however, in this respect. 



The embryo usually has two cotyledons, which 

 are often fused together. In Ceratozamia one of 

 the cotyledons is, as a rule, abortive. Oh germina- 

 tion they remain in the seed, serving as suckers to 

 absorb food from the endosperm. It will be re- 

 membered that in most Conifers (though not in 

 all) the cotyledons expand as green leaves. 



I have dwelt at some length on the modern 

 Cycads because a knowledge of this family is 

 essential, not only in order to understand the 

 Mesozoic Flora, which immediately preceded the 

 advent of the Flowering Plants, but also for com- 

 parison with the Palaeozoic Seed-plants (see 

 Chapter IV). We will now see what the Cycads 

 were like in Mesozoic times, and inquire how far 

 they may help us towards a solution of the great 

 problem of the evolution of the Flowering Plants. 



At the present day the Cycads scarcely amount 

 to one hi a thousand of the vascular plants; in 

 the Secondary Floras about one plant in every 

 three was a Cycad. The little family now living 

 represents what was then the dominant class of 

 plants. But it must be understood from the 

 outset that in speaking of Mesozoic Cycads we 

 use the name for a far wider group than the sur- 

 viving family; the whole class may bear the name 



