388 RELATION- TO ENVIKONMENT. 



matters of minute detail in form until it becomes necessary for 

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

 that a simiile leaf, except thoseof 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 relatimi <if 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 tvpes: (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 general!}-, 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 Ijlade (palmate leaf, maijle), or the mid- 

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

 prominent ones branch off from this and extend, nearl\' parallel, 

 toward the edge of the leaf (piiuiate \'enation). The smaller 

 intermediate ^X'ins which are also \-ery tlistinct extend irregularlv 

 and branch and anast(jmose in such a fashion as togi\-elhe figure 

 of a net with \er\ tine meshes. These are uelted-reined leaves. 

 These are characteristic of most of the clicotvledenous plants. 

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

 there are two types of netted-veined Iea\-es, the palmate and pinnate. 



Note. ,\s we have alrcad>' Irarned in Chajitcr \- the veins eontain the 

 vascular bundles of the leaf. Throuf^h them the water and food solutions 

 are distributed to all jiarts of the leaf, and the return eurrent of food ma- 

 terial elaborated in the li'af moves back throii}^h the bast i»)rtion into the 

 shocit. 'I'he veins also |iossess a small aniounl .if me. hanic .d tissue. This 

 forms the framcw.irk of the leaf an.l ai.ls in gi\ing rigidity to the leaf and 



