HOW PLANTS EAT. 



41 



Similarly, the ribs themselves are usually called 

 veins — a very bad name again, as they are much 

 more like the bones of a wing or hand ; they are 

 mainly there for support, as a bony or wooden 

 framework, though they also act for the convey- 

 ance of sap or water. 



And now we are in a position to begin to 

 understand the various shapes of leaves as we 

 see them in nature. They depend most of all 

 upon certain inherited types of ribs or so-called 

 veins, and these types are usually pretty constant 

 in great groups of plants closely related by de- 

 scent to one another. The immense difference in 

 their external shape (which often varies enor- 

 mously even on the same stem) is mainly due to 

 the relative extent to which the framework is 

 filled out or not with living cell-stuff, or, as it is 

 technically called, cellular tissue. 



There are two chief ways of arranging the ribs 

 or veins in a leaf, which may be distinguished as 



Fig. 2. — Finger-veined leaves. The veins are the same in the three 

 leaves, but they differ in the amount to which they are filled in. 



\.\\Q finger-like and th.^ feather-like methods (in tech- 

 nical language, /(^//;/^?/t' ^.nd pin?iafe). In the ^;/- 

 ger-like plan the ribs all diverge from a common 

 point, more or less radially. In the feather-like 



