116 ‘ BOTANY. 
Leaves.—Leaves are most important organs of plants. They expose a 
great surface to the air, imbibe what the plant requires for its nourish- ~ 
ment, and give out what is no longer useful to it. They are of very great 
importance in the system of nature, the gases which the leaves give out 
being generally those most needful for the support of animal life, so that 
the nialeenile of leaves on the face of the earth keeps the air fit for the 
use of animals, whereas it would otherwise become such that animals 
could no longer breathe it and live. 
Leaves are curiously folded or rolled up in the bud before they are 
developed. In some plants, they are folded by the midrib, the two halves 
of the leaf lying together ; in others, they are folded in a fan-like manner; 
in some, they have their edges rolled inwards, in others, outwards; and 
in some, they are rolled together in a single coil. The different modes in 
which the leaf is formed in the bud are characteristic of different plants. 
The cherry and the plum are trees nearly allied, but the leaves of the 
cherry are folded together in the bud, while those of the plum are rolled 
up. 
The expanded part of a leaf generally faces the sun, the influence of 
light being necessary as well as that of air. This expanded part is called 
the blade of the leaf. The leaf, however, is often supported by a stalk, 
called the leaf-stalk or petiole,’ which in trees and shrubs is often 
woody, and in some, particularly in palms, becomes thick and hard 
like a branch. Leaves which have no leaf-stalk are called  sessile,? 
because they seem to sit upon the stem or branch. In many 
plants, the leaves which spring from the crown of the root differ very 
much from those which are produced higher on the stem. Thus, the 
common harebell has root-leaves nearly Sonuds whilst those of its stem 
are very narrow. The leaf-stalk divides into pone which form the 
ribs or veins of the leaf, and give it the necessary strength. In endogenous 
plants, the veins of the leaf are in general nearly parallel, running 
unbranched throughout the whole length of the leaf, as may be seen in 
grasses ; in exogenous plants they break into branches, which spread in 
various directions, and are again and again branched, so as to form a kind 
of network, as may be seen in the leaf of the elm or the primrose. This 
difference in the leaves characterises these two classes of plants as much 
as that in the structure of the stem. The leaves of acrogenous plants have 
generally forked veins. 
In some plants, the leaf-stalk is tie and the blade of the leaf at its 
extremity is very small, or scarcely exists, as in many species of Acacia. 
In this case the leaf-stalk serves the purposes which are ordinarily served 
1‘ The little foot,’ from Latin pes, pedis, the foot. 
2 From Latin sessilis, sitting, from sedeo, sessum, to sit. 
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