68 
LEAVES. 
properties : one familiar with the principles of chemistry, would 
first speak of the materials which compose salt ; he would de- 
scribe it as a compound substance, consisting of chlorine and so- 
dium ; and then might enumerate its properties. In the first 
definition, given without any reference to scientific principles, 
there is nothing so definite as to afford a certain mark of distinc- 
tion between salt and other substances ; in the chemical defini- 
tion, we have a test for salt in a knowledge of its composition, 
which distinguishes it from all other substances. 
In botanical definitions, we do not include the constituent ele- 
ments of the vegetable substance ; this belongs to the depart- 
ment of chemistry, but we consider the external forms and uses 
of the various parts of the plant. 
Leaves proceed from buds and seeds ; they are so various in 
appearance, that it is very difficult to give any one description 
which may apply to all. 
The leaf is generally a thin, flat organ, consisting of an ex- 
pansion of the fibres of the bark connected by a substance which 
is called the cellular tissue ; the whole is covered with a green 
coat or skin called the cuticle. Leaves are furnished with pores* 
for exhaling and inhaling gases ; and as they present to the air 
a more extended surface than all other parts of a plant, they are 
of great utility to the vegetable, by imbibing suitable nourish- 
ment, and throwing off such gases as would be useless or in- 
jurious. 
We have seen how the bud is formed, and by what curious 
means the principle of vegetable life which it contains is pre- 
served and protected through the cold and dampnesss of winter. 
In the spring, when the sun having performed his journey to 
the southern tropic, has turned his course towards the north, re- 
crossed the equator, and is advancing towards the tropic of Can- 
cer, the vegetable world, which had for months lain dormant, 
begins to awaken ; the leaf buds expand, and soon bursting their 
envelopes, the green leaves come forth. The manner in which 
the leaf, before it expands, 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/ 
In the buds of grasses the leaves are said to be rolled together; 
in the rose, one side of the leaf is doubled upon the stem ; in 
the maple, the leaves are plaited^ or folded up like a fan ; in the 
blue bell, imbricated, or laid over one another like tiles on the 
* Technically called stomas. 
Origin of leaves — General description of a leaf — Utility of leaves to the 
whole plant — The period at which leaves appear — Foliation. 
