ANATOMY OF LEAVES. 
A small quantity of iron, united to oxygen in the vegetable 
substance, and acted upon by rays of light, is said to give rise 
to the various colours of plants.'- If this theory is correct, the 
different shades of colour in plants, must be owing to the differ- 
ent proportions in which the iron and oxygen are combined. 
To quote the words of a celebrated Chemist, “ When Nature 
takes her pencil, iron is the colouring she uses.” 
LECTURE X. 
Anatomy of Leaves . — Their' use in the Vegetable System. — Ap- 
pendages to 'plants. 
Leaves are compared to the lungs of animals ; they are or- 
gans for respiring, perspiring, and absorbing. W hen leaves 
are wanting, as in the Prickly Pear, (Cactus,) the green surface 
of the stem appears to perform the office of leaves. If you will 
observe a dead leaf which has for some time been exposed to the 
action of the atmosphere, you may see the skeleton, or frame 
work of the leaf; (see plate IX.) this consists of various fibres, 
minutely subdivided, which originate from the petiole. This skel- 
eton of the leaf may be examined to advantage, after boiling the 
leaves slightly, or rubbing them in water; the cuticle or skin easily 
separates, and the pulp may then be washed out from between the 
meshes of the veined net-work ; thus the most minute cords of 
the different vessels become perceptible, with their various divi- 
sions and subdivisions ; this forms what is called the vascular 
system. 
It would be extending our researches beyond our proper lim- 
its, to attempt to follow the writers who have remarked upon 
the peculiar offices of these organs. In external appearance, 
they are analogous to the bones which constitute the foundation 
of the animal system, but they are considered as performing the 
office of veins or arteries. They are found to be tubes ; in some 
cases, this is ascertained by the naked eye ; in others, beauti- 
fully illustrated by immersing the fibres of the leaf in some 
coloured liquid ; on taking it out, they are found to contain in- 
ternally a portion of the liquid ; this experiment proves them to 
be both tubular and transparent. 
* This idea coincides with the supposition, that the green colour of leaves 
is changed to brown by the loss of an acid principle ; that when the petals 
of flowers turn from purple to red, they have an increase of an acid. The 
base of this acid is oxygen. 
Leaves, their use — Skeleton of the leaf — Vascular system — Found to be 
tubular and transparent. 
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