THE CIRCULATION OF WATER
As we have already found out, water is one of the things which are
necessary for the well-being of plants. Seedlings can begin to sprout
only when they are well supplied with it, and in the growing plant it
is the water in the cells which keeps it firm and fresh. Directly the
plant is deprived of some of its water it becomes limp and flabby, and
“withers.” We noticed in Chapter IV. that the rootlets absorb the water
(with its salts contained in solution) from the soil, and from them
it travels all over the plant. The salts dissolved in water, however,
are in very weak solution, and to provide the plant with sufficient of
them for its growth ~it is necessary that a continuous stream of water
should enter the plant~. How is this stream kept up?
[Illustration: Fig. 17. Experiment to show that leaves give off
water. Notice the drops collecting in the tube, which is closed with
cotton-wool.]
The leaves play a very important part in the water circulation, their
thin expanded surfaces giving a large area from which the evaporation
of water can take place. The water which comes off from them is not
generally visible to us, because it comes off as vapour. However, you
can easily make experiments which will show you that it actually does
come off from the leaves.
Take a large test tube or a small glass flask, and place it over a
good-sized fresh green leaf, which you leave attached to a healthy
plant or a branch in water. Round the leaf-stalk wrap cotton wool till
it fits like a cork in the neck of the flask, so that it shuts the leaf
into the vessel, leaving no communication with the outer air, and at
the same time does not injure it in any way (~see~ fig. 17). Very soon,
even after an hour or two, you will find a misty appearance inside the
glass, and this will settle gradually in the form of drops of water
which collect together and run down the sides of the flask. You do not
see all this water coming off from the leaf under ordinary conditions
because it goes into the air as invisible vapour, but when it is given
off continually into a closed space the air soon gets saturated with
all it can hold, and the rest must form liquid drops which we can see.
If you keep a record of the time of your experiment, and also measure
the amount of water collected in the flask, and then measure the size
of the leaf, it only needs a little simple arithmetic to give you a
rough idea of the quantities of water which must be given off every day
by a single leaf. From that you can imagine the amount passing away
from a whole plant or a great tree; and I think you will be surprised
to find how much it is.
Another simple experiment shows us that the leaves play an important
part in giving off water. Take three flasks with long, thin necks, and
of as nearly equal sizes as possible. In one place a branch to which a
number of fresh, green leaves are attached, in another a branch of the
same size with only small buds (cut off the leaves if necessary), and
leave the third as a check to show how much water has simply evaporated
away. Fill all the flasks up to the same level with water, and mark
this in all three when you start. Leave them for a day or two and then
mark the level of the water, some of which will now have evaporated
(~see~ fig. 18). This will show clearly that more water has gone from
the one with the branch than from the empty flask, and that a great
deal more water has gone from the one in which was the branch with big
leaves attached.
[Illustration: Fig. 18. Experiment to show that leaves give off water.
The flasks were all filled to the same level I., and left for the same
time. The one with the leaves in it lose far more than the others.]
You can see roughly the ~rate~ at which the water goes off from the
leaves by completely filling with water an apparatus like that in fig.
19. As the leafy branch (which is firmly fastened in the cork with no
air leakage) uses up the water, it must be drawn along the narrow tube,
which is graduated so as to show the quantity lost.
[Illustration: Fig. 19. Experiment to measure the amount of water given
off by leaves in a given time. At first the tube is full of water,
which is drawn back to points 1, 2, etc., as the leaves use it.]
From these experiments we find that even although we do not actually
see it coming off, yet ~the leaves of the plant give off a great deal
of water in the form of vapour~. By this process large quantities of
water are drawn through the plant, and the salts in weak solution in it
are kept and used by the plant as they are needed for building up its
structure.
Now you may think that the loss is simply the result of evaporation
from the leaves, because the surface of the leaves is great, and
they would therefore naturally lose a considerable amount of water
by evaporation. But this view is only partly correct, because the
giving off of water by leaves or “transpiration,” as it is called, is
regulated by a number of little pores in the skin of the leaf, which
can open and close. You can see the importance of these pores as water
regulators in plants which have them only on one side of the leaf,
because practically all the water escapes from the side on which they
are situated.
[Illustration: Fig. 20. Leaf A greased on the ~lower~ side, leaf B on
the ~upper~ side, and C not at all. B withers as fast as C.]
To see this, take three leaves of the indiarubber tree, which is grown
so often in rooms. Choose three which are as nearly as possible just
alike in size and shape. Of one of them carefully cover the whole of
the ~lower~ side, and the cut-end of the stalk, with vaseline or coco
butter; do the same to the ~upper~ side and the cut-stalk of the second
leaf, and leave the third untouched. Fasten all three separately on to
a string so that they all hang with both sides exposed to the air, and
leave them for some days. The leaf which was not greased will shrivel
up; as it gives up its water and can get no more, it “withers” and dies
completely. The leaf which was greased on upper side also withers at
about the same rate as the ungreased one, but the one which was greased
on the ~lower~ side remains fresh and green (~see~ fig. 20). This is
because all the pores are on the lower sides of these leaves, and in
the one greased on the lower side the vaseline had completely closed
them, and so prevented the water from passing away through them. The
upper surface is well protected against ordinary evaporation by a
thick skin which does not allow the water to pass through it. The leaf
greased on the upper side had all its pores left open, and so in this
way was withered as quickly as one not greased at all. Not all leaves
have their pores only on one side, but in nearly all plants the pores
can open and shut. These facts show that transpiration is more than
mere evaporation; it is a “life process,” that is, a physical process
which is regulated by the structure of the living plant.
Transpiration is very important for plants, for it helps to keep the
continual stream of water going through them, which brings with it the
necessary food salts. Some plants cannot afford to let much water pass
away, for they find it very hard indeed to get enough to keep them
fresh; such plants as live in deserts or on bare, sandy places, for
example, protect themselves from much transpiration by various devices
and special arrangements, which we will study in Chapter XVIII.
We have already observed the fact that water enters the plant at its
roots, and have just seen that it passes off as water-vapour from its
leaves. Let us now consider for a moment the manner of its entrance.
=How can water enter the roots of plants?=
Let us first look at a somewhat similar case in non-living things which
will, perhaps, help us to understand the process in living plants.
Take a small “thistle-funnel” and tie tightly over the wide opening a
piece of bladder; then pour some very strong solution of sugar into the
funnel and place it in a glass of pure water. Mark the level of the
sugar with a label (~see~ fig. 21, S). Leave this for a short time,
and you will find that the water has entered the funnel tube and run up
it for quite a long way.
[Illustration: Fig. 21. “Thistle funnel” covered with bladder B, filled
with sugar solution up to level S, and placed in a jar of water. After
a time the water is seen to have risen to W.]
You should take another similar tube and do everything in the same
way, except that you leave out the sugar solution. Then you will find
that the water remains inside the funnel at just the same level as in
the outer jar. This is the usual behaviour of water, and in the first
case, where the water rose inside the funnel, the rise was due to the
influence of the sugar, which has the power of drawing in water. Now we
can compare the skin of the root hairs (~see~ fig. 9) to the bladder
membrane covering the funnel, and it has been found that inside the
cells are substances which have the same power of attracting water as
we found was possessed by the sugar. So that ~the entrance of water
into the roots depends chiefly on the attraction of the substances
within its cells~.
That a large amount of water enters the root in this way you can see if
you cut off a quickly growing plant (a vine is very good if you can
get it) just near its base, and attach to the cut-end a long glass tube
in place of the shoot you have cut away. You must fasten this tube by a
very well-fitting indiarubber tube, which you bind tightly so that it
will allow no leakage, and support the glass. Pour a few drops of water
down the tube to keep the cut-end of the plant from drying up at the
beginning of the experiment. Then mark the level reached by the water,
and do this every day as it rises in the tube. You should find that for
some time it steadily rises day by day (~see~ fig. 22).
[Illustration: Fig. 22. Plant P, which has been cut off near the root,
is attached by the indiarubber tube I to a tall glass pipe, which is
supported by stand S. On the glass are marked the levels reached by the
water rising from the root.]
We see in this way that the roots take in a large and continual
supply of water, and this must get pressed up the stem even without
the influence of the transpiring leaves. This is called the “root
pressure,” and is a very important factor in supplying the plant with
water. In a plant which is growing under usual conditions, both the
transpiration of the leaves and the root pressure are at work, and
are both necessary to keep a good stream of water passing through
the plant. This stream of water provides it with its mineral food
materials, and also keeps it stiff and fresh, and is, as we have seen,
absolutely necessary for the growth of the plant.