Sometimes more water is drawn up into the leaves than
can be used, and then the leaf opens thousands of
little mouths in the skin of its under surface, which
let the drops out just as drops of perspiration ooze
through our skin when we are overheated. These
little mouths, which are called stomates (a, Fig.
42) are made of two flattened cells, fitting against
each other. When the air is damp and the plant
has too much water these lie open and let it out,
but when the air is dry, and the plant wants to keep
as much water as it can, then they are closely shut.
There are as many as a hundred thousand of these mouths
under one apple-leaf, so you may imagine how small
they often are.
Plants which only live one year, such as mignonette,
the sweet pea, and the poppy, take in just enough
food to supply their daily wants and to make the seeds
we shall speak of presently. Then, as soon as
their seeds are ripe their roots begin to shrivel,
and water is no longer carried up. The green
cells can no longer get food to digest, and they themselves
are broken up by the sunbeams and turn yellow, and
the plant dies.
But many plants are more industrious than the stock
and mignonette, and lay by store for another year,
and our primrose is one of these. Look at this
thick solid mass below the primrose leaves, out of
which the roots spring. (See the plant in the foreground
of the heading of the lecture.) This is really the
stem of the primrose hidden underground, and all the
starch, albuminoids, &c., which the plant can spare
as it grows, are sent down into this underground stem
and stored up there, to lie quietly in the ground
through the long winter, and then when the warm spring
comes this stem begins to send out leaves for a new
plant.
Week 21
We have now seen how a plant springs up, feeds itself,
grows, stores up food, withers, and dies; but we have
said nothing yet about its beautiful flowers or how
it forms its seeds. If we look down close to
the bottom of the leaves in a primrose root in spring-time,
we shall always find three or four little green buds
nestling in among the leaves, and day by day we may
see the stalk of these buds lengthening till they
reach up into the open sunshine, and then the flower
opens and shows its beautiful pale-yellow crown.
We all know that seeds are formed in the flower, and
that the seeds are necessary to grow into new plants.
But do we know the history of how they are formed,
or what is the use of the different parts of the bud?
Let us examine them all, and then I think you will
agree with me that this is not the least wonderful
part of the plant.
Remember that the seed is the one important thing
and then notice how the flower protects it. First,
look at the outside green covering, which we call
the calyx. See how closely it fits in the bud,
so that no insects can creep in to gnaw the flower,
nor any harm come to it from cold or blight.
Then, when the calyx opens, notice that the yellow
leaves which form the crown or corolla, are each alternate
with one of the calyx leaves, so that anything which
got past the first covering would be stopped by the
second. Lastly, when the delicate corolla has
opened out, look at those curious yellow bags just
at the top of the tube (b,2, Fig. 43). What is
their use?
