THE LEAF 27 
of stipules, which are scalelike or, rarely, leaflike outgrowths 
from the base. Frequently the stipules are temporary structures 
and drop off as the leaf matures (Fig. 15). 
Leaves are distinguished from stems by their origin, the leaf 
being the first lateral outgrowth from the stem (Fig. 92), while 
the branches grow in the axils of the leaves, that is, just above 
where the leaves are joined 
to the stem. Leaves are 
further distinguished from 
stems by the fact that the 
erowth of a leaf is usually 
limited, while that of most 
stems is not; that is, leaves 
reach a certain size and then 
cease to grow, while stems 
continue to grow in length 
as long as the plant lives. 
The blades of most leaves 
contain a narrow, thickened 
structure which is a contin- 
uation of the petiole and Fic. 16. Venation of leaves 
extends from the petiole Left, leaf of eggplant, showing midrib and 
throu gh the center of the netted veins; right, canna leaf with par- 
; allel veins 
blade to the opposite end 
(Fig. 16). This is the médrib. On both sides of the midrib there 
are lines which either are parallel with each other or form a 
network. These are veins. The midrib and veins contain con- 
ducting cells, through which water, coming from the roots, 1s 
earried to all parts of the leaf, and by means of which food, 
manufactured in the leaf, is conducted to the petiole on its way 
to other parts of the plant. The midrib and veins are also im- 
portant in giving stiffness to the leaves. Some leaves do not 
have a single midrib but, instead, have several large veins which 
serve the same purpose (Figs. 54, 58). 
Venation. The leaves of flowering plants show two very dis- 
tinct types of venation, that is, arrangement of the veins. In one 

