50 THE EVOLUTION OF PLANTS 



medullary rays, usually serves to distinguish 

 that class from Dicotyledons, though there are 

 exceptions on both sides. 



We have said very little so far about the 

 distinctions between the two great classes, 

 Dicotyledons and Monocotyledons, into which 

 Angiosperms are divided, for our object has 

 been to dwell on the points common to the 

 whole group. Dicotyledons and Monocotyle- 

 dons are as a rule, though with many exceptions, 

 very different in habit, that is to say in general 

 appearance; there is not often any difficulty in 

 telling, at a glance, to which class a plant be- 

 longs. The outward distinction depends partly 

 on the character of the leaves and partly on the 

 mode of growth. It is generally said that in 

 Dicotyledons the veins of the leaf form a net- 

 work, while in Monocotyledons they are parallel. 

 This is not strictly true, for in Monocotyledons 

 also there is generally a kind of network of 

 veins, consisting of a number of large longi- 

 tudinal strands joined at intervals by more 

 delicate transverse connections. The former 

 are much the more conspicuous and form the 

 "parallel venation" which is all that the casual 

 observer notices. In most Dicotyledonous leaves 

 the network is more complicated and more 

 noticeable, the bundles branching and re-uniting 

 in every direction. The different character of 



