44 THE STORY OF THE PLANTS. 



comes like argel. When 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 sun- 

 light. I do not mean the freedom of access of 

 the particular leaf or the particular plant, but the 

 average ancestral freedom of access in the kind 

 they belong to. Each kind has adapted itself, as 

 a rule, to certain situations for which it has spe- 

 cial advantages, and it has learnt by the teaching 

 of natural selection to produce such leaves as best 

 fit its chosen site and habits. Where access to 

 carbon and sunlight are easy, plants usually pro- 

 duce very full round leaves, with alT the inter- 

 stices 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 particu- 

 larly observable in submerged leaves, and in those 

 which grow in very crowded situations. 



The two types of rib-arrangement to which I 

 have already called attention exist for the most 

 part in one of the two great groups of flowering 

 plants about which I shall have more to say to 

 you hereafter. There is yet a third type, how- 

 ever, which occurs in the other great group (that 

 of the grasses and lilies), and it is known as the 

 parallel (Fig. 6). In this type, the ribs do not 

 form a radiating network at all, but run straight, 



