388 RELATION- TO ENVIRONMENT. 



matters of minute detail in form until it becomes necessary for 

 those in the future who pursue taxonomic work. It is evident 

 that a simple leaf, except those of minute size, possesses advantages 

 over a divided leaf in the amount of surface it exposes to the 

 light. But in other respects it is at a disadvantage, especially 

 as it increases in size, since it casts a deeper shade and does 

 not admit of such a free circulation of air. It will be found, 

 however, in our study of the relation of leaves to light and air 

 that the balance between the leaf and its environment is ob- 

 tained in the relation of the leaves to each other. 



756. Venation of leaves. A very prominent character of the 

 leaf is its "venation. " This is indicated by the presence of numer- 

 ous " veins," indicated usually by narrow depressed lines on the 

 upper surface, and by more or less distinct elevated lines on the 

 under surface. There are two general types: (i) In the corn, 

 Smilacina, Solomon's seal, etc., the veins extend lengthwise of the 

 leaf and are nearly parallel. Such leaves are said to be parallel- 

 veined. It is generally, though not always, a character of mono- 

 cotyledenous plants. (2) In the elm, rose, hawthorn, maple, oak, 

 etc., the veins are not all parallel. The larger ones either diverge 

 from the base of the blade (palmate leaf, maple), or the mid- 

 vein extends through the middle line of the leaf, while other 

 prominent ones branch off from this and extend, nearly parallel, 

 toward the edge of the leaf (pinnate venation). The smaller 

 intermediate veins which are also very distinct extend irregularly 

 and branch and anastomose in such a fashion as to give the figure 

 of a net with very fine meshes. These are netted-veined leaves. 

 These are characteristic of most of the dicotyledenous plants. 

 It is evident from what has been said of the examples cited that 

 there are two types of netted-veined leaves, the palmate and pinnate. 



NOTE. As we have already learned in Chapter V the veins contain the 

 vascular bundles of the leaf. Through them the water and food solutions 

 are distributed to all parts of the leaf, and the return current of food ma- 

 terial elaborated in the leaf moves back through the bast portion into the 

 shoot. The veins also possess a small amount of mechanical tissue. This 

 forms the framework of the leaf and aids in giving rigidity to the leaf and 



