52 THE EVOLUTION OF PLANTS 



which live for a number of years. The wood, it 

 will be remembered, serves chiefly, though not 

 solely, to conduct the water absorbed by the 

 roots up to the leaves, while the great function 

 of the bast is to convey the food substances 

 formed in the leaves to the places where they are 

 wanted, either for immediate use in growth, or 

 to be stored up for future purposes. In a Mono- 

 cotyledon the vascular strands, instead of being 

 ranged in a regular ring, are usually much more 

 scattered, forming a number of irregular con- 

 centric circles, and often extending quite to the 

 centre of the stem. Each has its wood and bast, 

 as in a Dicotyledon, but as a rule, there is no 

 cambium, so that no further increase can take 

 place when once the strand has been formed. 

 Further, the bundles remain separate, while in a 

 Dicotyledon they are usually joined up, so as to 

 form an uninterrupted zone of wood and bast. 

 In all these respects the Dicotyledons resemble 

 the Gymnosperms, while the Monocotyledons 

 are quite peculiar and unlike any other plants. 

 At one time it was thought that the Monocoty- 

 ledons were the more primitive class of plants, 

 but it is a curious fact that none of the older 

 fossil plants (i. e. those before the Cretaceous 

 epoch) have stems like Monocotyledons, while a 

 very great number, like the living Gymnosperms, 

 are of the same type with the Dicotyledons. 



