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PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
stomata of the leaves. This effect is made apparent if the 
plant is growing under a bell glass, for then the moisture 
which has been given off settles on the inside of the glass 
in the form of visible vapour. It is thus evident that 
there is a constant though gradual current of water pass¬ 
ing through the living plant from the rootlets which absorb 
it from the soil to the leaves which give it off into the 
atmosphere. When a plant droops during dry weather in 
summer, it is because the moisture is given off from the 
leaves in greater quantity than it is taken up by the roots. 
But counterbalancing this result, which would soon prove 
fatal to the plant, is a provision which causes the stomata 
to close when necessary, and thus to regulate the amount 
of evaporation. In moist weather the stomata-cells are in¬ 
flated with water, but in dry weather they lose this water 
and so collapse and close the openings. The leaves then 
perform also the work which, in the animal, is performed 
by the skin. 
The next work of the leaf—namely, that of elaborating 
the sap, and preparing it for the nourishment of the plant 
—is closely connected with the two processes which I have 
already described. For the sake of simplicity, we shall take 
as an example one of our forest trees, such as the oak, and 
try to trace the course of the sap in it. In spring, we find 
it ascending into the leaves through the vessels contained 
in the outer or newest layers of wood in the form of a thin 
sweetish liquid. When it is in the leaf, it is exposed to a 
variety of influences, which affeet a material change in its 
chemical constitution. The carbon—which, as we have 
seen, is always being absorbed by the leaf—combines with 
the other substances drawn up with the sap from the soil, 
and thus starch and other organic compounds necessary for 
the nutrition of the plant are formed. Then, bj T the tran¬ 
spiration which takes place from the leaf, the sap loses a 
considerable portion of water, and so is rendered denser and 
more concentrated. By these means, when it returns from 
the leaf, it has beeome a thick viscid liquid, and in this 
form it starts on its course thronghout the plant, to fulfil 
its mission of nourishing and building up its tissues. Thus 
it comes about that, in autumn, we find the sap slowly de¬ 
scending the stem of our oak tree through the vessels con¬ 
tained in the innermost layer of the bark, and, as it 
proceeds, building up, on the one hand, a new layer of 
wood, and, on the other, a layer of bark, or, in other 
words, adding another to the “ rings ” which we have all 
observed in the exposed section of a felled tree, It is this 
descending sap which the American backwoodsman collects 
when, in the fall of the year, he taps the trunk of the 
maple tree for its valuable sugar. If we wish to prove the 
wood-forming property of this elaborated sap, we may do 
so by a simple experiment. If we bind a hoop of iron very 
tightly round the stem of a growing tree in the beginning 
of autumn, we shall presently observe a bulge or growth 
beginning to be formed above-this band, showing that the 
sap, unable to descend to the lower part of the stem, is ex¬ 
erting its energy, so to speak, in forming new woody tissue 
at the point where its course is arrested. The work which 
the leaves do in elaborating the sap is analagous to the 
process of digestion, so that they may be said to constitute 
also the stomach of the plant. It will now be understood 
that this process of elaboration is simply the two processes 
first described looked at in a different aspect. 
It is perhaps carrying the analogy of the animal a little 
too far to say that the leaves constitute the heart of the 
plant, and yet when we come to investigate the physical 
laws which cause the sap to ascend the stem of a tree 
against the force of gravity, we find that the idea is not so 
far-fetched. For what does a heart, or at least the left 
side of it, do? It propels the blood or nutritious fluid 
throughout the body of the animal, while the leaves per¬ 
form precisely the same work for the plant, though upon 
a very different mechanical principle. What this method 
is it would take too long to explain here, but I will merely 
ask you to bear in mind that the sap is not conducted 
through a continuous open tube, but through the walls of 
an infinite number of closed vessels which form the fibres 
of the veins, leaf-stalks, branches, and stem. 
To sum up the physiology of the leaf in its normal con¬ 
dition, it may be said to perform for the growing plant 
those functions which in the animal are performed by 
lungs, skin, stomach, and heart. This illustrates the fact, 
which it is well to remember, that one fundamental phy¬ 
siological difference between an animal and a plant is that 
in the former the vital processes are carried on by means 
of individual specialised organs, which are complete in 
themselves and distinct from the other organs; while in 
the plant these processes are carried on by a countless 
number of similar organs disseminated throughout the en¬ 
tire structure. Thus the oak we have been examining 
has not one pair of lungs, but breathes by the entire sur¬ 
face of every leaf—it has not one heart or one stomach, 
but as many, metaphorically speaking, as it has leaves. 
We might go farther, and point out how every one of the 
myriad green or chlorophyll-bearing cells in the leaf is a 
distinct organ of nutrition which acts independently of, 
and equally with, every other such cell. This distinction 
is applicable particularly to the more highly-developed 
branches of the two great divisions of the organic world, 
for in the lowest forms in both, the types approach much 
more closely to each other. 
