SIGNS OF LIFE
[Illustration: Fig. 1. Jar (A) with well-fitting cork, in which young
bean plants are growing. The tube leading from the jar dips into dish
of water (~s~) which has risen to levels marked in the course of three
days. (~b~) Small tube of caustic potash.]
If you were asked to give =the signs of life in an animal=, it is
likely that you would think at once of =its power of breathing, eating,
growing, and moving=. Now when we ask the same question about plants
the answer does not appear to be quite so easy to find, because at
first sight plants do not seem to do any of these things except the
growing. However, ~the same answer would be quite correct for plants~,
as well as animals, for they are really able to breathe, eat, grow, and
move; all you have to do is to watch them in the right way to see that
this is the case.
We are not in the habit of treating dry seeds as though they were
alive; beans are stored away in sacks all the winter and may be left
for months in dry cellars, and the precious seeds which will give us
our beautiful flowers in the summer are put away in boxes through the
winter. Yet you know that if you place seeds in the earth and keep
them warm and moist, little plants will come up and will grow. What
gives them the power of growth which is not possessed by the stones and
earth around them? Warmth and moisture alone could not put this power
into the seeds when we planted them. This power, which only belongs
to living things, was there all the time, but was lying asleep, shut
in and protected so that it was not easily disturbed till suitable
conditions made it time for it to wake.
You know when you are asleep that you do not eat or run about, but
simply lie still and breathe. This is what the seed was doing before
the baby plant began to break through its protecting coat and show
itself to the world as a living thing.
Let us watch some of these young plants just waking up to activity, and
see if we can find in them the four signs we take as being the tests of
animal life.
First let us see if we can show that they =breathe=.
You know that when you breathe you take air into your lungs, use some
of it, and give the rest out. You can show that plants also use up some
part of the air. If you would actually prove this to yourself or anyone
else, take some peas or beans, soak them in water, and leave them in
damp sawdust for a day or two till the tiny plant has just begun to
show. Then put them on wet blotting-paper in a jar which has a very
well-fitting cork with no leakage, and through which a fine bent glass
tube is fitted. Place a small tube of caustic potash in the jar. Then
place the end of the bent tube in a dish of water, which acts better if
you have dissolved some caustic potash in it (~see~ fig. 1). Once it
has begun to rise in the tube, mark the level of the water with a small
label. If then you mark it daily the labels will show how much water
has risen each day, and the amount of water rising in the tube shows us
the amount of air which has been absorbed by the growing beans.
This tells us, therefore, that ~air is absorbed by plants~ in the
course of their growth. But there is another thing we must notice about
breathing which is equally important.
You will find that you yourself, as well as all animals, not only use
up a part of the air, but also give out a waste product which we call
carbonic acid gas. You can see one of the characters of this gas from
your lungs if you take a jar of lime water and breathe into it for
some time. Compare this with a similar jar of lime-water through which
ordinary air has been pumped at about the same rate for the same time,
and you will see that the one you have breathed into has gone very much
more cloudy-white than the other (~see~ fig. 2). The cloudiness in jar
A is caused by the waste gas (carbonic acid gas) which you breathe out,
and which combines with the lime in the lime-water to make solid grains
of chalk. Fine white chalk grains always form in lime-water when this
gas is present, so that a jar of clear lime-water is a very good test
for the presence of the gas.
[Illustration: Fig. 2. Jar A contains lime-water through which human
breath has passed. Jar B, lime-water through which ordinary air has
been pumped for the same time. Note how much greater is the milky
deposit in A than in B.]
The giving out of carbonic acid gas is one of the most characteristic
things about animal breathing, and we can show that plants in breathing
give out this gas too.
To prove this, take another jar with a well-fitting cork, and put some
beans and peas, which are just beginning to grow, into it, with a
little damp blotting-paper to keep them sufficiently moist. Leave the
jar closed for a day or two and then open it and quickly and gently
pour in some lime-water. Put the lid on again at once and shake it up.
You will find that the lime-water turns quite milky, showing that ~the
same waste gas is given out by the plants as was given out in your own
breath~.
These experiments show us that plants breathe in a part of the air,
and also breathe out some of the same waste gas which is given off by
animals in breathing. So that we have found that ~plants do breathe~.
Now to go to the other signs of life. I think you will hardly need
to do any special experiment to show that seedlings =grow= into big
plants, you must have seen it so often for yourself in the woods and
fields and gardens.
We have still to show that plants eat and move, but before we can do
this properly, we must learn a little more about the parts of the
bodies of the plants themselves, for they have quite a different set
of organs to those we are accustomed to in animals, and their way of
eating is so different from that of animals that we cannot understand
it immediately.