AN ATOM \^ OF LEAVES. 
59 
" No tree in all the grove but has its charms, 
Though each its hue peculiar ; paler some, 
And of a wannish gray ; the willovv such, 
And poplar, that with silver lines his leaf ; 
And ash far stretching his umbrageous arm; 
Of deeper green the elm ; and deeper still. 
Lord of the woods, the long surviving oak."* 
The contrast between their shades, in forests, where different fam- 
ilies of trees are grouped together, has a fine effect, when obser"ved 
at such a distance as to give a view of the whole as forming one 
mass. 
A small quantity of iron, united to oxygen in the vegetable sub- 
stance, and acted upon by rays of light, is said to give rise to the 
various colours of plants.j If this theory is correct, the different 
shades of colour in plants, must be owing to the different proportion 
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 AND PHYSIOLOGY OF LEAVES — THEIR USE IN THE VEGETABLE SYS- 
TEM APPENDAGES TO PLANTS. 
Leaves are compared to the lungs of animals ; they are organs for 
respiring^ perspiring; and absorbing. When leaves are wanting, as 
in the Prickly Pear, (Cactus,) the green surface of the stem appears 
to perform their office. If you will observe a dead leaf which has for 
Eome time been exposed to the action of the atmosphere, you may 
see its skeleton, or frame-work ; this consists of various fibres, mi- 
nutely subdivided, which originate from the petiole. This skeleton of 
the leaf may be examined to advantage, after boiling the leaves 
slightly, or rubbing them in water ; the cuticle, or skin, easily separ- 
ates, and the pulp, or cellular texture, 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 
divisions and subdivisions ; these form what is called the vascular 
system. (See Fig. 61.) 
Though in extern.*! 
Fisr. 61. appearance, the or- 
gans which compose 
the vascular system of 
plants, are analogous 
to the bones which 
constitute the founda- 
tion of the animal sys- 
tem, yet they are ra- 
ther considered as 
performing the office 
of veins and arteries. 
They are found to be 
* Cowper 
t This idea coincides with the supposition, that the green colour of leaves is changed 
.0 brown by the loss of an acid principle; that the petals of flowers change from pur- 
A' to red by an increase of acid. The base of this acid is oxygen. 
What is the cause of these different shades of colour? —the use of leaves in the veg 
etable economy —Skeleton of the leaf— Vascular system. 
