LOSS OF WATER 27 
32. External Anatomy of a Leaf. We have seen 
(Chapter II) that the three main parts of a leaf are the 
blade, petiole and leaf-base. These parts may manifest 
every conceivable variation as to shape and size, and bear 
every relation to each other as to proportion. The petiole 
FIG. 17. Leaf of Hercules club (Aralia spinosa), partly thrice compound. 
The leaf-blade measured 15 in. wide at the base, and 12 in. long. 
may be more or less shortened, or it may be entirely wanting 
so as to make the leaf-blade sessile (seated) on the stem (Figs. 
14 and 15). The blade may be greatly enlarged (Fig. 16), 
or more or less branched (Fig. 17), or it may be greatly re- 
duced, or even entirely wanting. In the latter case, the 
