12 
OUTLINE OF STRUCTURAL BOTANY 
THE LEAF 
In general the leaf is the digestive organ of the higher plants, 
for the principal food of plants is carbonic acid which, within the 
Y 
Fic. 16 
leaf, is converted into starch with elements which are 
supplied from the roots. That it contributes to the 
beauty and interest of the plant is an obvious fact. 
Of its color and of its physiology we need not here 
speak, but of the forms which the leaf assumes and of 
the arrangement of the leaves it is necessary to take 
note, since on these forms and on these arrangements 
we must base many of the comparisons between dif- 
ferent groups of plants. 
In most of the orders of plants usually known as 
the lower orders leaves are not found, and even in a 
few of the plants which come under our observation 
as flowering plants, the leaves may be only rudimen- 
tary, yet we may regard the leaf as an essential organ 
of the class of plants which is to be described in this work. 
We may take the leaf of the buttercup as a study of the leaf 
structure (Fig. 17). 
We find then, first, the broad expanse, 
the blade; second, the slender, yet 
thicker, somewhat cylindric organ, the 
petiole, or leaf-stalk, and third, and 
finally, the base of the petiole, which, in 
case of the buttercup, nearly or quite 
surrounds the stem to which it is closely 
attached. 
These three parts constitute the typi- 
cal leaf, but not all leaves are typical. 
In the case of many leaves, for example, 
the broadened base of the leaf stem is 
wanting and the petiole is attached to 
the stem by a narrow base, which may or 
may not be somewhat more expanded 
than the column of the petiole. Again, 
the petiole may be almost completely 
absent and the leaf blade is apparently 
attached directly to the stem without the 
intervention of the petiole. The leaf is 
then said to be sessile, as flowers are 
Fic. 17—Leaf of Ranuncu- 
Jus acris—a, the blade; b, the 
petiole or leaf-stalk; c, the 
base, 
said to be sessile when there is no pedicle or flower stem. 
