EXTERNAL CHARACTERISTICS 



207 



others, however, have leaves of such variable shape that 

 they are of little value as a means of recognition. 



The shape of a leaf depends a good deal, as has been 

 noted, upon the character of its venation. Parallel-veined 

 leaves tend to be long and narrow ; net-veined leaves tend 

 to be oval. Long and nar- 

 row leaves, like grass blades, 

 are called linear. Leaves 

 whose shape approaches that 

 of an oval are called ovate. 



As to the nature of their 

 margins, leaves are called 

 entire, or toothed, or lobed. 

 Entire leaves are those whose 

 margin is an even line, un- 

 broken by teeth or lobes; all 

 parallel-veined leaves are en- 

 tire. Net-veined leaves quite 

 usually have their margins 

 broken into teeth or lobes, or 

 even into deeper divisions 

 such as are shown by maple 

 leaves. The leaf of the white 

 oak is a deeply lobed leaf. 

 The leaf of the elm is a 

 toothed leaf. (See Figure 6g. ) 

 If the teeth resemble in their outline the teeth of a saw, 

 the leaf is called serrate. 



As to the extent to which their blades are divided or 

 branched; leaves are classed as simple or compound. Com- 

 pound leaves are those whose expanded part is in the form, 

 not of one, but of a number of separate blade-like parts 



Fig. 70. — Leaf of geranium showing 

 palmate type of venation. Note also 

 the stipules at the base of the petiole. 



