OF LEAVES. 
49 
a compound substance, consisting of chlorine and sodium. In the 
first definition, given without any reference to scientific principles, 
there is nothing so definite as to afford a certain mark of distinction 
between salt and other substances ; in the chemical definition, we 
have a test for salt, in a knowledge of its composition, which dis- 
tinguishes it from all other substances. 
In botanical definitions, we do not include the constituent elements 
of the vegetable substance ; this belongs to the department of chem- 
istry, but we consider the external forms and uses of the various 
parts of the plant. 
The leaf is an expansion of the fibres of the bark, connected by a 
substance, called the cellular tissue ; the whole is covered with a 
green coat, or skin, called the cuticle. Leaves are furnished with 
pores called stomas^ for exhaling and inhaling gases. They present 
to the air a more extended surface than all the other vegetable or- 
gans, and are of great importance by imbibing suitable nourishment, 
and throwing oft' such gases as would be useless or injurious to the 
plant. 
We have seen how the bud is formed, and by what wise means the 
principle of life which it contains, is protected through the cold and 
dampness of winter. In the spring, when the sun, having recrossed 
the equator, is advancing towards our hemisphere, the vegetable 
world, quickened by its influence, begins to awaken from a dormant 
state; the buds expand, and bursting their envelopes, the new 
branches, bearing leaves and flowers, come forth. 
The manner in which the leaf lies wrapped up in the scales of the 
bud, is called Foliation; this presents an interesting study, and is 
said to be sufficiently various, in different families of plants, to afford 
a mark of distinction between them. 
Fio-. 37. Figure 37, at a, shows a 
young leaf of the currant ; 
this is folded. At 6, is a 
young leaf of the Aconi- 
tum, (monk's-hood ;) this 
is inflected. At c, is the 
young leaf of a fern, (aspi- 
dium,) this is circinate, or 
rolled from the summit to- 
wards the base. 
Some plants are desti- 
tute of leaves; they are 
then called Aphyllous^ from 
the Greek, a, to want, phyl- 
lon, a leaf. 
In determining the species of plants, the leaves are much regarded. 
Specific names are often given from some circumstance of the leaf; 
the Hepatica triloba is that species of the Hepatica, which has leaves 
with three divisions, called lobes. The viola rotundifolia, is a spe- 
cies of violet with round leaves. 
A knowledge of the various appearances presented by leaves, is 
of great importance to the botanical student; in order to become ac- 
quainted with these, much practice in the analysis of plants is neces- 
sary. Engravings will assist you in understanding the definitions, 
but you must chiefly consult nature. / 
Definition of the leaf— Utility of leaves to the whole plant— The period at which 
leaves appear— FoHation— Aphyllous plants— Leaves furnish specific characters. 
