Ro\v 



EAT. 



the edge is not quite filled out, but irregularly 

 indented, we get forms like the oak leaf. 

 Finally, when the indentations go to the very 

 bottom of each vein, so as to reach the midrib, 

 we get a compound leaf like that of the vetch, 

 with a number of opposite and distinct leaflets. 



The reason why some leaves are thus more 

 filled out than others is simply this : it depends 

 upon the freedom of their access to air and 

 sunlight. I do not mean 

 the freedom of access of 

 the particular leaf or the 

 particular plant, but the 

 average ancestral free- 

 dom of access in the kind 

 they belong to. Each 

 kind has adapted itself, 



as a rule, to certain situa- FIQ . 6. i. Parallel veins, as 

 tions for which it has seen in one great group of 

 special advantages, and 

 it has learnt by the teach- 

 ing of natural selection 

 to produce such leaves 

 as best fit its chosen site 

 and habits. Where access to carbon and sunlight 

 is easy, plants usually produce very full round 

 leaves, with all the interstices between the ribs 

 filled amply in with cellular tissue ; but where 

 access is difficult, they usually produce rather 

 starved and unfilled leaves, which consist, as it 

 were, of scarcely covered skeletons (Figs. 4 and 

 5). This last condition is particularly observable 

 in submerged leaves, and in those which grow 

 in very crowded situations. 



plants, the lilies. II. 

 Branching veins, as seen 

 in another great group, 

 the trees and herbs of the 

 usual type. 



