portions, and assimilates the residue into vegetable matter for its
nourishment and growth.

119. But the fact is already familiar (10-30) that leaves occur under
other forms and serve for other uses,--for the storage of food already
assimilated, as in thickened seed-leaves and bulb-scales; for covering,
as in bud-scales; and still other uses are to be pointed out. Indeed,
sometimes they are of no service to the plant, being reduced to mere
scales or rudiments, such as those on the rootstocks of Peppermint (Fig.
97) or the tubers of Jerusalem Artichoke (Fig. 101). These may be said
to be of service only to the botanist, in explaining to him the plan
upon which a plant is constructed.

120. Accordingly, just as a rootstock, or a tuber, or a tendril is a
kind of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a
petal of a flower, is a kind of leaf. Even in respect to ordinary
leaves, it is natural to use the word either in a wider or in a narrower
sense; as when in one sense we say that a leaf consists of blade and
petiole or leaf-stalk, and in another sense say that a leaf is petioled,
or that the leaf of Hepatica is three-lobed. The connection should make
it plain whether by leaf we mean leaf-blade only, or the blade with any
other parts it may have. And the student will readily understand that by
leaf in its largest or _morphological_ sense, the botanist means the
organ which occupies the place of a leaf, whatever be its form or its
function.

Sec. 1. LEAVES AS FOLIAGE.

121. This is tautological; for foliage is simply leaves: but it is very
convenient to speak of typical leaves, or those which serve the plant
for assimilation, as foliage-leaves, or ordinary leaves. These may first
be considered.

122. =The Parts of a Leaf.= The ordinary leaf, complete in its parts,
consists of _blade_, _foot-stalk_, or _petiole_, and a pair of
_stipules_.

123. First the BLADE or LAMINA, which is the essential part of ordinary
leaves, that is, of such as serve the purpose of foliage. In structure
it consists of a softer part, the _green pulp_, called _parenchyma_,
which is traversed and supported by a fibrous frame, the parts of which
are called _ribs_ or _veins_, on account of a certain likeness in
arrangement to the veins of animals. The whole surface is covered by a
transparent skin, the _Epidermis_, not unlike that which covers the
surface of all fresh shoots.

124. Note that the leaf-blade expands horizontall