3<D THE VEGETATIVE FUNCTIONS OF PLANTS 
Leaves may be so altered as to cease to be foliage leaves, 
and serve, for example, as bud-scales (Fig. 19), or other 
organs. In some bud-scales both petiole and blade may 
be wanting, the leaf-base alone serving as the scale (Fig, 
20). Again, the bud-scale may consist chiefly of petiole. 
FIG. 22. Various types of leaf-blade. 
or of blade, or (as in the tulip-tree) of stipules only (Fig. 
21). Variations of leaves are illustrated in Fig. 22. 
It is not essential here to endeavor to frame a definition 
of a leaf, though this would be a profitable exercise for 
the reader. The main thing is to emphasize the fact that 
the leaf is an exceedingly plastic organ, that is, appearing 
