GROWTH IN SEEDLINGS
When once the young plants start growing under suitable conditions they
steadily get bigger. At first sight they appear to grow equally all
over, stretching out in each direction as indiarubber does when it is
pulled. Let us try to find out whether this is actually the case.
[Illustration: Fig. 28. A Bean seedling: A, with divisions marked on
root and stem; B, after further growth, showing where most of the
stretching has taken place.]
Take a well-grown straight seedling and measure off along its stem and
along its root, beginning from the tip, distances 1 or 2 mm. apart,
marking them with a fine brush and waterproof ink. Take care not to
injure the plant, and also not to make the mark blurred or too big.
Draw the plant showing the marks on it as accurately as you can, and
make the drawing exactly life-size. Grow it in damp, but very loose
sawdust, so as not to rub off the marks, and after one or two days
take it out and compare it with your original drawing.
You will find that the whole plant is bigger than when you first drew
it. Look carefully at the marks on root and stem, and you will find
that they are not all the same distance apart, as they should be if the
plant had grown equally all over. The marks which are widest apart are
those just behind the tip of the root and below the top of the stem,
thus showing that there has been much more growth in these two regions
than in the rest of the stem or root (~see~ fig. 28). If you repeat
this often with many plants you will find that these are the actively
growing parts of the stems and roots; the individual leaves, of course,
are also growing. Thus we see that ~growth is not a simple stretching
of the whole, but that there are two definite regions~ where it is
specially active. That of the stem and first root carry on the growth
in opposite directions, as we noticed before (~see~ p. 11), the normal
stem growing up into the air and the root down into the soil.
[Illustration: Fig. 29. A, Bean seedling planted upside down. The root
has bent right over and is growing vertically down. B, later stage of
the same. The shoot has bent up.]
You can see ~how very determined the directions of growth are~ by
planting upside down a bean which is just beginning to sprout, so that
its root points up into the air. As it grows you will see the root
bending over till it points vertically downwards, while the stem bends
up and grows straight into the air (~see~ fig. 29). The same thing
happens if you plant a seedling on its side, and even if you take quite
a big seedling, which has grown in the usual way, and then place it
upside down in moist air, you will see the root and shoot bending in
order to get into their right positions. This very determined growth
on the part of roots and stems seems to show us that they must have
some means of “perceiving” and regulating their position. It is not an
accident that they always grow in these very definite directions. Let
us find out what we can about this question.
Take a seedling and mark its root as you marked the roots for the
experiment on the region of growth (~see~ fig. 28), lay this seedling
on its side on soft, damp sawdust, so that the root can easily bend
into it. Next day you should find that the end of the root has bent,
and that the bend is in just about the same region as that which showed
the most active growth.
Is this actively growing and bending region therefore the part of the
root which “realizes” that the whole is in a wrong position, and which
therefore bends to put it right?
To answer this question quite fully would require a great deal of work,
but there are three simple experiments which you can do, and which will
tell you the most important facts about it.
(1)[5] Take a seedling with a fairly long root which has been growing
straight down, then very quickly and with a sharp knife or razor, cut
off the last 2 mm. of the tip of the root. Lay the seedling on its side
on damp sawdust and examine it next day. ~It will not have bent~, even
though it has grown in length (~see~ fig. 30, A).
[Illustration: Fig. 30. Experiments on the bending of the root tips in
Beans. (~See~ description in text.)]
[5] It is better to have half a dozen examples for each experiment, for
the seedlings do not always act quite quickly and correctly, and from
half a dozen you can see the average result.
(2) Take another like it and leave it lying on its side for an hour,
and then cut off the tip in the same way as in number one, placing it
on its side once more. Next day you will find that ~it has bent in the
same way as one which had not been cut~ (~see~ fig. 30, B).
(3) Take a third, as like the other two as possible, and lay it on its
side all night; do not cut it till next day, when it has definitely
begun to bend (~see~ fig. 30, C), then quickly cut off the tip,
and place it in the upright position (C^1). You will find that ~it
continues to grow in the bent form~, the root tip going on to one side.
It does not seem to know that it is growing along instead of down. If
you keep it in this position for a few days it will then get a new tip
and begin to grow downwards in the usual way (~see~ fig. 30, D).
Think over the results of these three experiments, and you will see
that it is only when the tip of the root is not cut off that the plant
seems to “realize” that it is not in the right position. When the tip
is removed it does not bend down even when the whole plant is lying
horizontally, and in the other case (fig. 30, C^1, D) it will keep on
bending even after it has been put in its right position.
We noticed that it is not the very tip itself which bends, so that we
see that ~the very tip is the part which “feels” what is happening,
while the part just behind it grows and bends~ according to the need of
the plant.
This is a somewhat similar case to what happens when you realize with
your brain that you are in danger on the road, and your feet hurry you
across.
When we come to consider ~why~ the root should grow downwards in this
persistent way, we find that there is an outside influence at work on
the plant. You know when a stone is left without any support that it
always falls to the ground, and we say that it is attracted toward the
centre of the earth by the force of gravitation. It has been proved
that the strong tendency of roots to grow down into the soil is largely
the result of the same attraction, while the stem is not attracted by
it but driven away, and therefore grows away from the centre of the
earth. To prove this, however, requires more complicated apparatus than
you are likely to be able to use at present.
From the experiments which we have done already we see that plants,
as well as animals, are affected by their circumstances, and can in
some measure realize them, and move to alter themselves in accordance
with them. Later on we shall find that plants have a similar power
in relation to light, supply of water, and other things. Have we not
already observed in plants nearly all the signs of life we set out to
look for? (~see~ p. 4).
There is one very important point about the growth of plants which is
strikingly different from the growth of animals. A young kitten has
four legs, a head, and a tail, and as it grows to be a cat these only
alter a little in shape and get larger and stronger; the number of
its legs remains the same. A baby plant, on the other hand, has its
little root and shoot with a few tiny leaves, but as it gets older
these increase very much in number, till it may have many branches
and thousands of leaves. In fact, the number of its parts is much
more indefinite than those of an animal; its body is built on quite a
different plan. ~Yet both plants and animals show the same important
thing in their growth, that is the increase of their living body, which
they build up out of their non-living food.~