38 THE STORY OF THE PLANTS. 



Leaves assume^ and what are their reasons for do- 

 ing so. 



The leaf has, as a rule, to be broad and flat, 

 in order to catch as much carbon as possible ; it 

 has also usually to be expanded horizontally to 

 the sunlight, so as to catch and fix it. For this 

 reason, most leaves that can raise themselves 

 freely to the sun and air are flat and horizontal. 

 But in very crowded and overgrown spots, like 

 thickets and hedgerows, the leaves have to fight 

 hard with one another for air and sunlight ; and 

 in such places particular kinds of plants have 

 been developed, with leaves of special forms 

 adapted to the situation. The fittest have sur- 

 vived, and have assumed such shapes as natural 

 selection dictated. 



Where the plants are large and grow freely 

 upward, with plenty of room, the leaves are usu- 

 ally broad and expanded, as in the tobacco-plant 

 and the sunflower. Where the plants grow thick 

 and close in meadows, the leaves are mostly long 

 and narrow, like grasses. In overgrown clumps 

 and hedgerows they are generally much subdi- 

 vided into numerous little leaflets, as is the case 

 with most ferns, and also with herb-Robert, chervil, 

 milfoil, and vetches. In these last cases, the plant 

 wants to get as much of the floating carbonic 

 acid, and of the sunlight, as it can ; and therefore 

 it makes its leaves into a sort of divided network, 

 so as to entrap the smallest passing atom of car- 

 bon, and to intercept such stray rays of broken 

 sunlight as have not been caught by the taller 

 plants above it. In almost all cases, too, the 

 leaves on the same plant are so arranged round 

 the stem and on the branches as to interfere with 

 one another as little as possible; they are placed 



