204 LEAVES 



water vapor being the gases principally concerned in these 

 exchanges. You know that the cells of the mesophyll 

 contain chloroplasts, and that the chloroplasts contain 

 chlorophyll, and that chlorophyll is the substance which 

 makes leaves green. You know that the chloroplasts are 

 the organs of photosynthesis. (See page 74.) 



55. External Characteristics. — In this section we shall 

 consider characteristics of leaves which are visible to the 

 naked eye. Flatness and greenness appear to be the 

 principal qualities which leaves have in common. In 

 features of their shape other than flatness we find a great 

 deal of variety among leaves, more than we find in the 

 shapes of stems and roots. Since the shapes of leaves 

 depend very largely upon the ways in which their veins are 

 arranged, we shall consider vein arrangements first. 



A . Venation. — Compare a blade of grass with the 

 broad leaf of any tree and you will notice a great difference 

 in the way the veins are arranged. The visible veins of 

 the grass blade are almost parallel, while the veins of the 

 broad leaf are arranged somewhat like the meshes of a 

 fine-meshed net. The arrangement of the veins is called 

 venation. 



There are two principal types of venation and these are 

 the two which have just been described. Leaves are either 

 net-veined or parallel-veined. Nearly all dicotyledons have 

 net-veined leaves and nearly all monocotyledons have 

 parallel-veined leaves. (See Figure 68.) Thus, you ob- 

 serve, net veins are associated with the cylindrical arrange- 

 ment of vascular bundles, and parallel veins with the 

 scattered arrangement. The veins are, of course, the 

 extensions of the vascular bundles into the leaves. It 



