LIGHT AND ITS INFLUENCES
When we were experimenting on the building of starchy food in leaves
(Chapter VI.) we saw how very important and even essential light is for
the activity of the plant, and it is therefore natural to expect that
light should influence its growth very considerably.
You may see the effect of light which comes only from one side on
plants grown in the windows of rooms. If they are left in one position
they grow in a one-sided manner with only the bare stalks toward the
darker side of the room and all their leaves turned towards the window
through which the light comes. If you want them to look pretty towards
the room side also, they must be turned round frequently, so that the
leaves are drawn in many directions instead of one only. The usual
effect of light is to make the leaves grow towards it. You may see this
still more clearly by placing a pot of seedlings in a blackened box
with a small hole on one side. Very soon they will bend over towards
the light entering by it (~see~ fig. 23).
[Illustration: Fig. 23. Grass seedlings growing in an earthenware dish
enclosed in a strong box blacked inside so that the light only enters
at ~a.~ (Note how the seedlings bend towards it.)]
Leaves can absorb most light when their upper surfaces are at right
angles to it, and you will find some leaf-stems will bend right round
in order to allow their leaves to get into this position. For example,
if you take a pot of nasturtiums growing in the usual way, and support
the pot on a stand, and cover it over with a bell jar which has been
blackened, or with a black box, so that all the light reaches the plant
from below, you will find that in a day or two the leaves will have
turned completely round on their stalks and are now facing the light,
so that they are upside down in their relation to the position of the
whole plant (~see~ fig. 24).
[Illustration: Fig. 24. Nasturtium covered over, so that the light only
enters from below. The leaf surfaces bend over to face it.]
[Illustration: Fig. 25. Spray of Maple showing stalks of leaves of the
same pair of very different lengths, so as to place the leaves well as
regards light.]
In a small plant, or one with only a few big leaves, this desire
for the light is easily arranged for, as there is room for each of
them. But if all the leaves of a great tree were turned in the same
direction, you will see that many of the under ones must be shaded by
the others. This is not so bad as one might expect, however, owing
to the wonderful way in which the leaves arrange themselves so as to
use every bit of space they can, and yet to overlap and screen each
other as little as possible. Particularly in plants which grow flat on
the ground or against walls, and which therefore get all their light
from one side, this is very well shown. In plants with the leaves in
opposite pairs you will often find one leaf of the pair big, and the
other one small, or that the leaf-stalks are of different lengths, and
if you examine this pair in relation to the rest of the branch, you
will see how it is developed in this way so as to use every bit of
space it can and get as much light as possible without overlapping its
neighbours (~see~ fig. 25). Although it is true in one way that each
leaf works as a separate individual, yet each separate leaf is only a
small part of the plant, and they all work together for the good of the
whole. Branches which have their leaves arranged in this way so that
they seem to fit into a pattern, form what is called “Leaf Mosaic.” You
may see this kind of arrangement among the leaves of very many plants
(~see~ figs. 25 and 26).
[Illustration: Fig. 26. Leaves of Ivy growing out from the stem so as
not to overlap each other.]
~If, as we have already seen, light is so very important for the plant,
what is the result of growing it in the dark?~ As you know, it will not
be able to build itself food, and so would finally starve and die. If,
however, we choose a plant which has already much food stored up and
can therefore grow for a time without making a new supply, then we can
study the effect of darkness on its growth.
Take some beans which are just beginning to sprout, plant them in a
pot, and place the pot in some quite dark place such as a cellar or a
dark room, or cover them with a well-made blackened box which shuts out
all the light. Also take a potato which is just beginning to sprout at
its “eyes,” and keep it in the dark. Both these plants have food in
reserve; the beans have much in their nurse-leaves, and the potato is
packed with starch, as you saw before. At the same time grow a potful
of beans and a potato plant in the light, so that you can compare the
growth of the plants under the two different conditions of light and
darkness.
You will find that those grown in the dark are very straggling and of
a sickly yellowish colour, and are a great contrast to the shorter
sturdier young green plants grown in the open air. The stems of those
grown in the dark are long and limp, and not able to support themselves
upright, while the distance between the leaves is very great, and the
leaves themselves are small and useless (~see~ fig. 27).
[Illustration: Fig. 27. Seedlings of Bean of the same age, A grown in
the light, B in the dark.]
Why should these plants have such a great length of stem? It shows us,
that when the plant is already supplied with food, ~darkness does not
prevent mere growth in length~. In fact it grows faster in ~length~ in
the dark, which is an effort on the part of the plant to grow away from
the darkness into the light. It economises in material and does not
form stiff, thick stems and big leaves which would be useless until it
reaches the light.
If you now make a small chink in the black box with which you cover
the plants, you will find that they grow towards it and through it into
the light. Once the tip of the stem is outside in the light, it will
form the usual leaves at the proper intervals from one another.
The power of rapid growth in length of a plant growing in darkness,
which economises the material generally used for strengthening the
plant, and its power of growing towards the light, combine to be of
practical use to a bulb or seed which is planted too deep in the earth.
You will find that the part underground has much the same character as
a plant grown in artificial darkness, until it reaches the surface.
These weak underground stems bring the growing part into the light, and
the plant does not waste material in forming large leaves and strong
stems underground where they would be useless.
Although light is so important, it does not follow that the stronger
the light, the better it is for the plant; just as it does not follow
that because we like to be warm, we like to be as hot as possible. It
has been found that plants bend away from the light when it is too
strong for them, as you may see in some plants near one of those very
brilliant electric lamps. The sun even is sometimes too brilliant
(English plants, however, do not suffer from that very much), and many
plants living in the tropics and regions of strong sunlight, protect
themselves from its direct rays by a number of different devices.