Take some seed--beans will do--and after soaking them, plant them in the
ground about two inches deep. In a week or ten days you will see the
earth cracked all about. This is not because the growing plant acts like
a wedge and splits the earth open, but because in growing the first
little leaves move round and round, boring their way out of the ground
very much as a corkscrew works its way into a cork. The first leaves of
most plants--a bean, for instance--do not come straight up out of the
seed; but when the seed coat bursts from the swelling of the inner part
a little arch projects, which raises itself up. This arch is the stem,
and after a while the leaves are pulled out of the sheath, and the arch
widens out, and finally straightens up. You have often seen a man who
had a heavy weight to lift bow himself over and receive the weight, and
then lift it by straightening himself, as the stem does to lift the
leaves (Fig. 1, first leaves). The root burrows into the earth in very
much the same way as the stem revolves, by going around and around as it
grows (Fig. 2). Take a morning-glory vine, and let it lie without any
wire or trellis to catch hold of. After a while you will find the stems
and tendrils coiled round each other in a tight twist (Fig. 3); you
could not begin to twist them so tightly yourself without breaking the
stem.
[Illustration: Fig. 3.--MORNING-GLORIES.]
The tips of all growing plants, like the first leaves that pierce the
ground, move around; they are forever weaving their magic circles in the
air; they take many hours sometimes to make a single turn, but they are
as regular as the hands of a clock, and never forget and go backward. I
have been watching some wistaria branches lately, and have been very
much interested to see the new shoots, as they grew rapidly in the soft
warm air, taking a slow turn around the wire placed to support them very
much as you might wrap your arm about a swing rope to take a better
hold. If there is a post or a wire near, you do not have to give your
vines the twist they need to climb; they do their own twisting as they
grow, and always in this quiet, deliberate way.
[Illustration: Fig. 4.--VIRGINIA CREEPER.]
[Illustration: Fig. 5.--PADS THROUGH THE MICROSCOPE.]
You have no doubt noticed that a Virginia creeper does not need a wire
to climb by; it grows beautifully up any wall which has little
unevennesses. Now look, if you can get hold of a new shoot, what the
creeper has to help it along. It sends out tendrils that branch into
many ends, and each one of these ends swells and becomes a sort of
sticky pad, which glues itself to the wall (Fig. 4). These little pads,
when they find no wall to fasten themselves upon, remain small, and
finally wither away. Those on the vine in Fig. 4, which was trailing
from a vine, are so, some small and some quite gone; but look at the
pads in Fig. 5, which were detached from a painted board, and see how
they look through the microscope. Very much like a boy's India rubber
sucker, are not they? Some of these have the paint from the board still
sticking fast on them. Others are all sparkling with the dried mucilage,
which makes them look as if they had been sprinkled with sugar.
These little many-armed suckers give the plant a firm hold, while its
head waves around until it touches some surface again, and again the
pads lay hold for another upward stretch.
There must be some curious arrangement by which plants, that can not
_feel_ and _will_ as animals do, can move. They have no brains to think
with, no nerves to feel with; it is strange to believe that they really
do move with a reason. Mr. Darwin has examined the subject so closely
that he has taken nearly six hundred good-sized pages to tell all he has
found out about it. His ways of finding out are many. One method is
this: he takes a small stiff bristle and glues it on the growing part of
a shoot. By watching this shoot and comparing it with other shoots which
had no bristle attached, he could not detect any difference in the
movements. Above the little branch with the bristle attached he placed a
piece of glass that had been smoked, so that the bristle, as it moved
with the movement of the tip, would travel over the glass. He did not
need to stand by and watch the branch; he could go away and attend to
anything he chose, and when he came back there on the glass was a
history of the travels the shoot had made, written by itself. He managed
to hang up a sprouting bean or pea so that the root recorded its own
movements in the same way. There were other ways which he used, all of
them being ingenious, and requiring the greatest attention to get a
correct map of their movements. He found that every plant in growing
moved around as well as upward, but that some moved far more than
others; the ones that grew tall and slender and needed support would
send out shoots that swayed round in bigger and bigger circles until
they could reach something to sustain themselves by, or else they would
fall in helpless heaps on the ground.
Mr. Darwin was not a man to be satisfied with finding that a thing is
so. He never rested until he found just how it came about. I do not mean
to say that he was the only man who studied these things, for there were
many others who did; but he wrote about what he had studied in such a
clear and simple and interesting way that anybody could understand him,
and so people who don't pretend to be very wise in such matters read Mr.
Darwin's account and nobody's else, and are apt to forget, though he is
always careful to mention their names and what they have done, that any
one else deserves any of the credit.
[Illustration: Fig. 6.--DIAGRAM OF STRAIGHT AND CURVED STEMS.
_a_, Stretched cells; _b_, crowded cells.]
By closely studying the little cells of which the leaf or stem is made
up, he found that when, for any reason, a plant needed to turn in a
certain direction, the water in the stem rushed from the inner to the
outer part of the curve, making the cells on the inner side of the stem
a little smaller and those on the outer a little larger than usual.
After a while the stretching of the outer cells makes them grow and stay
larger (see in the figure how it must be, Fig. 6), and so the curve
remains. You can not straighten a stem curved in this way without
breaking it.
Every movement of stems and leaves comes from the movement of the water
that fills their cells. But besides the water, there is something else
just as important, and that is the sun. The water is only a servant,
which obeys the light as its master. Many flowers turn their bright
faces always to the light. They follow the sun as he moves through the
heavens all the day long from his rising to his setting. This comes from
the effect the sun has on the water in the stem, and not because the
flower is beginning to "take notice," as the baby's bright eyes do of a
lamp when it is moved about a room, though it does remind one of it.
The movement of climbing plants is only one of many curious movements
that are made by stems and roots and leaves and flowers, though the
cause is the same in all cases.