When these tubes come into the leaves, they open into little spaces just
under the outside skin of the leaf. These spaces are like the hollow of
a mouth, and each one has generally two lips, that are sometimes open
and sometimes shut. Through these tiny mouths (Fig. 3) the plant
breathes. It draws in air, and it sends out, as you do, a mixture of air
and water. If you want to know how much water there is in your own
breath, try holding a piece of cold glass before your mouth.
Plants are not wasteful of the water so necessary to their lives. What
they do not use they give back to the air from which it was received, as
we make our thank-offerings to God of what He has given us. The roots
suck up the water, and each little cell takes a drink as the water
passes it, and hands on the rest to the cell just above it. And so the
water takes its course, supplying each thirsty cell with drink as it
passes, spreading through every part of the plant until it reaches the
little mouths. And there all that is left is breathed out in a fine
steam which you can not see until it touches some cold substances, and
is turned into water again.
Some one who wanted to know exactly how much water was given back to the
air by growing plants carefully examined a number of them, and found
that a single sunflower gave off in twelve hours a pound and a
quarter--enough to fill nearly to the brim three common table goblets.
Another plant, the wild cornel, was found to breathe but more than twice
its own weight of water in a day and a night.
[Illustration: FIG. 4.--WATER-CARRYING PLANTS.]
In order to find out what parts of the flowers were the principal
water-carriers, a deutzia, one of our most delicate and beautiful spring
flowers, which you probably know by sight if not by name, was put into
some very blue water, colored with a mixture of what is called aniline,
and in a little while every vein of the flower was a beautiful dark
blue. The poor little blossom was, however, poisoned with its dose, and
wilted away in a few minutes (Fig. 4).
The quantity of water that plants breathe off is so great that it makes
an entire change in the climate when forests are cut down. Plants, like
grasses and small weeds that grow on the surface, of course do not make
the same difference, for their roots only go down a little way. But
trees are very important: unless the air is kept damp by the sea or some
large body of water, it depends very much upon trees for its moisture.
Where there are no trees, the rain that does fall sinks into the earth,
and runs away in little under-ground currents, and is lost. There are no
deep roots to stop this waste, to suck up the water, and restore a large
part of it to the air.
[Illustration: FIG. 5.--CACTUS.]
In places where the rain-fall is frequent, and the air is always kept
soft, plants may be as lavish of their water as we are in the great
cities where the supply never fails. Plants growing in such places very
often keep their mouths open all the time. If this were the habit of
those which grow in very dry places, they would soon perish of thirst.
On the high Western plains beyond the Mississippi only a few things are
able to live. Among these are some kinds of cactus plants, which you
have probably seen in greenhouses or as window plants (Fig. 5). The
reason why they manage to grow such bulgy leaves and fat stems where
there is so little moisture, is because this plant is so very stingy of
its water. It hoards it up as the travellers over the great African
deserts do, knowing how hard it will be to get more. The roots of the
cactus suck up every drop of water they can find, and the leaves keep
their millions of little mouths tight shut so as to hold it all. Only
such plants can grow on these plains as are able to do with very little
water, or else are wise enough to hoard up all they can get. This water
we have been talking about is not sap--that is the blood of the
plant--but it is like the water we drink, and which not only helps to
make the blood, but keeps all of the parts soft and moist so that it may
live. The largest part of every living thing is water. It is not without
good reason that the Bible so often speaks of the _Water of Life_, for
without water no life could exist for a single hour.