FOOD MATERIALS OF THE OLDER PLANT
(2) IN THE AIR
The experiments you have just done show that plants absolutely require
the mineral salts dissolved in the water of the soil or of their food
solutions. Yet although these salts are so necessary, they do not use
a large quantity of them, as you may prove by taking the solution C,
which is left after the plant has grown in it, and slowly drying off
all the water (taking care not to destroy a part of the salt crystals)
by gentle heat, and then weighing this dry salt, and comparing its
weight with that of the salts you put into C. You will find that the
growing plant has only removed a small quantity of the salts. Yet the
plant should have grown to some considerable size. Of course, the
water itself goes into the plant tissues, but you can drive this off
by gentle heat. Before drying it, however, cut off a part of the plant
which is equal in weight to the weight of the young plant you put into
the food solution at first (~see~ p. 16), so that you have only to
deal with the amount of its growth while using the food solution. Then
if you weigh the fully dried plant, you get the weight of the solid
structure added to its body while it was growing in the food solution,
and you will find that this is ~much heavier than the amount of the
salts it used during its growth~.
=What is this extra substance?=
Now let us examine the dried plant more carefully. Heat it on an open
dish, and you will find that it goes black and chars, very like the
charred wood on a fire or specially prepared charcoal. The black
charcoal is well known to consist chiefly of ~carbon~, and so does this
black plant-ash. You know that charcoal can burn, and so will this
charred plant if you heat it more strongly. Although you can burn the
carbon (that is, you can make it combine with oxygen gas and go off
in an invisible form), yet you cannot absolutely destroy it. Like all
elements it is not to be made or destroyed by us, ~nor can the plant
make carbon for itself~.
If you examine the list of substances you put into the food solution
once more, you will find that carbon is not among them, nor is it
contained in any of them.
~Carbon, then, is the extra substance which makes the weight of the
plant greater than that of the salts used from its food solution.~
=Where does the plant find this carbon?=
You may know that there are three chief gases in the air: oxygen and
nitrogen, which are the important parts for our breathing, and a little
carbonic acid gas, which you may remember is breathed out by animals
and plants (~see~ p. 6), and is made of carbon joined with oxygen. As
there was no carbon in the food solution, and the plant was surrounded
by air containing carbon and oxygen in the invisible form of gas,
the idea is suggested that perhaps it is from the air that the plant
gets its carbon. Now let us see if this is true by trying the effect
of removing the carbonic acid gas from the air in which the plant is
growing.
To do this we must set up an apparatus which will allow only air freed
from carbonic acid gas to surround the plant. Such an apparatus is
shown in the figure 12. The plant is grown in the closed bell jar D,
which stands over the dish C filled with lime-water, which prevents
carbonic acid gas entering through the cracks between the foot of the
jar and the table. All the air which enters the jar D must come first
through jar A, which is filled with a solution of caustic potash that
has the power of absorbing the carbonic acid gas, and then through
jar B with lime-water. You can draw plenty of air through jar D for
the use of the plant by sucking at the indiarubber tube G, which must
be carefully shut with a clamp when you stop the current. The bell-jar
D will now be filled with air which is quite free from carbonic acid
gas, and the small quantity which is breathed out by the plant itself
will be absorbed by the lime-water in dish C. Place the whole in a
light or sunny position, and change the air every day or two in the way
you filled it, that is by drawing at G so that the fresh air comes in
through A and B, and is free from carbonic acid gas.
[Illustration: Fig. 12. Apparatus used to keep a plant without any
carbonic acid gas. A, jar of caustic potash, B, jar of lime water,
which absorb the carbonic acid gas, through which all the air entering
jar D must pass; C, basin of lime water to absorb any of the gas given
out by the plant growing in D; G, indiarubber tube which can be closed
or attached to a siphon to draw air through D.]
If you keep the plant growing under these conditions for some time you
will find, in comparison with another quite similar plant growing in
the open near it, that its growth is very slow. The leaves it forms are
smaller, and finally its growth almost ceases. Further, if you test
the leaves of the plant growing out in the air for starch (see pp. 24
and 25), you will find that they contain plenty, but that the leaves
on the plant in the bell-jar are empty of starch. Now all healthily
growing green leaves contain starch, so that this is a good proof that
something is seriously wrong with the plant, which has been deprived
of the supply of carbon in the air. This shows us that ~plants use the
carbonic acid gas in the air~ for their growth.
Carbonic acid gas is composed of a union of carbon and oxygen gas. If,
then, the carbon is used by the plant, what happens to the oxygen?
[Illustration: Fig. 13. Jar of Elodea in water, giving off bubbles of
oxygen gas in the sunlight.]
You must have noticed bubbles rising from the “pond scum” and
water-plants when they are in the sunlight, the little bubbles
sometimes coming up in a quick, regular succession from the leaves
and stems. Let us collect this gas and test it to find out what it
is. This is more easily done if the plants are living in glass jars,
where you can see them and get at them readily. A very good plant to
use is the common Canadian water-plant (~Elodea~), which you can buy
in aquarium shops if you cannot get it from the ponds for yourself.
Place a handful of this plant in a tall, glass jar filled with fresh
water, and cover it with a glass funnel, so as to collect the bubbles
as they rise. See that the funnel is well under the water and support
over it a test tube full of water, as in fig. 13. Place the jar in as
bright sunlight as possible, when you should see the bubbles beginning
to come off quite quickly. As the bubbles rise in the tube A, the water
is forced out till the whole vessel is filled with gas. Then place
your thumb over the mouth of the tube of gas, and remove it quickly
from the water. Test it by plunging into it a splinter of wood which
has been burning, but just blown out, so that it is still glowing. If
you plunge it quickly enough into the tube, it should catch fire and
burn brilliantly. Now this is the test for oxygen gas, so that we have
proved that the tube was full of oxygen. This oxygen is the part of the
carbonic acid gas which is given off by the plant as it uses the carbon
and frees the oxygen it does not need.
You will find that the gas bubbles are given off much more rapidly when
the plant is placed in bright sunshine than when it is shaded, and that
when the plant is in darkness the bubbles stop altogether. This seems
to show us that the sunshine must assist the plant to split up the
carbonic acid gas, and we will find out more about this later on (~see~
p. 25).
We have now found that carbon forms a large part of the plant body,
that plants cannot grow in air in which there is no carbonic acid gas,
and that in getting the carbon from the carbonic acid gas, they split
it up and give off the oxygen. So that we see that =plants use the
carbon in the air as well as the salts dissolved in the water of the
soil as raw materials, with which they finally build up their food=. We
must now try to find what food substance it is that they build up from
these raw materials.