PROCEEDINGS OE THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
89 
They are also affected by the season—bright and sunny 
weather tending to intensify the tints. The autumn 
colouring of the deciduous trees is beautifully contrasted 
and relieved by the dark green heads of the pines which 
over-top the rounded summits of the former. It is not, 
however, to the trees alone that we have to look for bril¬ 
liant colours—the ground is carpeted with fallen leaves, 
where their tints, sombre and gay, are beautifully mingled 
together; and peeping through these are the plants, whose 
flowers in the by-gone summer made the woods bright with 
many colours, and whose leaves now serve the same end 
with more mellow tints. 
All these effects of colour we have observed and admired 
many times in our autumn rambles, but have we asked 
ourselves, What are they caused by ? How is it that with 
the first sign of approaching winter, the uniform green for¬ 
sakes the leaf and is replaced by red and yellow and 
brown ? And why, after a brief display of these brilliant 
hues, does it wither and drop off from the branch ? Before 
trying to answer these questions, it may be well to look 
briefly at what a leaf is, and at the work it has to do for 
the plant to which it belongs—in other words, at the 
anatomy and physiology of the leaf. 
Looked at with the naked eye, the first feature that 
strikes us is that the surface is traversed by a number 
of lines or veins. These are, in fact, the continua¬ 
tions of the fibres, or bundles of vessels, of which 
the leaf-stalk is composed, sub-divided in order to be 
distributed throughout the leaf. Their use we shall 
learn presently. If we now make a very thin tran- 
verse section of the leaf—that is, from the upper to the 
under surface—and examine it under a microscope, we 
shall find that it is entirely built up of minute closed 
sacs or cells arranged in a certain characteristic manner. 
Those on the two surfaces are more or less oblong in shape, 
small, and placed side by side quite close together so as to 
form the outer skin or epidermis. Next to these are layers 
of small rounded cells, packed tolerably closely together, 
while the central tissues of the leaf are made up of con¬ 
siderably larger cells, which are round and arranged very 
loosely, so as to leave air spaces between them. Imbedded 
in this spongy central mass are the bundles of vessels, as 
they are called, which constitute the veins. These vessels 
are modified cells, very much elongated, and placed side 
by side so as to form a continuous series. They, as well 
as the cells forming the thin outer skin, are perfectly 
colourless, but the other tissues appear as though coloured 
green on account of the minute particles of green matter, 
called chlorophyll, which they contain. If we now peel off 
carefully a very thin portion of the outer skin of a smooth 
leaf and place that under the microscope, we shall see the 
colourless empty surface cells under a different aspect, and 
scattered throughout these will be observed a number of 
minute openings or slits, each guarded by two crescent¬ 
shaped cells, specially adapted for the purpose. These 
little mouths, of which there are countless numbers on one 
or both surfaces of most leaves, are called stomata, and 
they play a most important part in the economy of the 
plant. 
Such is a very brief sketch of the structure of a leaf. 
We shall now glance at the various functions which it has 
to perform for the living plant. Of these, perhaps, the 
first in importance is respiration, or, as it is more correctly 
termed, assimilation. So long as a plant is exposed to the 
light, its leaves are constantly taking in carbonic acid gas 
from the atmosphere, and, by the aid of the green colour¬ 
ing matter contained in its cells, separating this gas into 
its two constituent elements—namely, carbon and oxygen. 
The carbon it appropriates to its own use, for the purpose 
of building up the tissues of the plant, which, as you are 
aware, chiefly consist of carbon iD combination with one or 
two other elemenst. The oxygen, on the other hand, which 
has been disengaged from the carbonic acid, is returned to 
the atmosphere. This process, it will be observed, is 
exactly the converse of what takes place in animal respira¬ 
tion; but, in the dark, certain parts of plants are also 
found to absorb oxygen and give off carbonic acid. How 
the leaf actually disengages the carbon from the carbonic 
acid we cannot tell. All we know is that two conditions 
are necessary to the process—namely, light, and the 
presence of green colouring matter or chlorophyll. In 
thus absorbing from the atmosphere the gases necessary 
for the life and growth of the plant, the leaves may be said 
to be the breathing organs or lungs of the plant. 
Another important function of the leaf is that of 
evaporation, or transpiration, as it is termed. This pro¬ 
cess corresponds to perspiration in the animal world, and 
is carried on by the agency of the little mouths or 
“ stomata,” which we observed in the skin under the 
microscope. It is evident from many observations that 
there is a constant circulation of water throughout a living 
plant, and that, without this, its life cannot be maintained. 
If a plant growing in a flower-pot is neglected to be 
watered, in the course of a few hours it begins to droop, 
and ultimately withers and dies. The reason, of course, 
is that its tissues have ceased to be distended by the 
moisture with which every growing part of a plant must 
be supplied. But if this be so, where has the moisture 
with which it was last supplied gone to ? The answer is, 
that it has evaporated or “transpired” through the 
