THE FOOD MANUFACTURED BY THE PLANT
You will remember that much of the food provided in the nurse leaves
consisted of starch, and that the baby plants use this food as they
grow.
In the full grown plant we also find much starch; in fact, nearly all
the parts of plants which we eat as food contain large quantities of
starch, as you can test with iodine in potatoes, turnips, radishes,
oatmeal, flour, and a host of our other vegetable foods. This is also
the case in many parts of plants which we do not generally use as
food, for example, in the lily and tulip bulbs, underground stems of
Solomon’s Seal, and the stems and leaves of most plants. So that we
find that ~the food grains of starch are developed in grown plants, and
are not only provided for the young ones~.
=What is starch made of?= Try heating a piece of laundry starch on an
iron plate or the bars of the grate, and you will see that it blackens,
and finally, if you put a light to it, may burn. If you simply heat it
without quite burning it, you will find that it chars and goes black
like a piece of charcoal. ~The solid element of starch is carbon.~ Now
you may remember that in the plant growing under the bell-jar from
which we shut out all the carbonic acid gas, we found that the leaves
did not show any starch (~see~ p. 20). The plant had not been able to
build up starch without the carbon obtained from the air.
The leaves of a plant are spread out in the sunshine and air, and it
is in the leaves that we get the starch first formed. The leaves,
in fact, are the food factories of the plant. You should study the
appearance of starch in the leaves. As their green colour hides the
iodine colouration, it is better first to remove it from any leaves you
are studying in the following manner. So soon after picking them as
possible, throw them on to some very hot or boiling water for a moment.
This kills them quickly and makes them soft; then put them in a jar or
tube of alcohol,[4] and leave them in it overnight. By next day the
green colour should be gone, having been absorbed out of the tissues by
the alcohol, and the leaves left yellowish or white. Then put them to
soak in water till the stiffness caused by the alcohol has gone, when
you should add the iodine. If you examine ordinary leaves in this way
you will find that they go violet or brownish blue, showing that they
contain starch.
[4] Ordinary methylated spirit is rather impure alcohol, which will do
if you cannot get any better.
Now do leaves always contain starch? You will remember that the oxygen
bubbles were given off much more quickly from the plants in the
sunlight than from those in the dark (~see~ p. 21). This shows that the
leaves in the sunlight split up the carbonic acid gas more quickly than
the others, which would give them more carbon to work on, and therefore
it seems that they should be able to build up more starch in the light
than in dull weather or darkness. You can see if this is true by doing
a simple experiment.
[Illustration: Fig. 14. Leaf partly covered with cork sheets, A, and
place in sunny position (compare Fig. 15).]
If you take a leaf growing in the sunlight, and cover a portion of it,
leaving the rest exposed, you will be able to see the effect of light
and darkness on the starch-building powers of this particular leaf. To
do this use two flat pieces of cork or thick cardboard, covered with
silver paper or tin foil about 1 in. to 1-1/2 in. big, and of the same
size and shape. Place a part of a healthy leaf between them and bind
them tightly together, as in fig. 14. If the weight of the cork makes
the leaf bend down out of the full sunlight, then support it so that
it lies in a position where it is well lighted. Leave it untouched for
three days, and then in the middle of a bright day cut the leaf from
the tree, remove the cork when you get into the house, and immediately
treat it as described above for the iodine test. You will find that
the part of the leaf which was exposed shows a good violet colour,
proving that starch is present there, while the part which was covered
is only yellowish, showing that starch has not been developed in this
portion (see fig. 15). This proves that ~the covered part of the leaf
could not build starch~, so that exposure to the light and air seems
to be necessary, as we expected. This further suggests that it is only
in the daytime that the plant can build starch. You can see that this
is actually the case by testing leaves from the same tree at different
times of the day and comparing the starch in them. For example, test
a leaf from a certain plant in the early afternoon, when it has been
exposed all day to good sunlight, and compare it with one which is
gathered just before sunrise, if you can get up so soon (this is, of
course, easier in the spring or autumn, when the sun does not rise so
early as in midsummer). You will find that the leaves picked in the
early afternoon are packed with starch, while those picked before the
day begins show very little or none.
[Illustration: Fig. 15. Same leaf as in Fig. 14 treated with iodine. It
shows that the covered part had formed no starch.]
What then becomes of the starch during the night?
You will remember that we found much starch in potatoes, which you know
grow right underground, and therefore, according to the experiments
we have just done, should not contain starch. But it is found that
the starch is ~made in the leaves~ through the day, and is slowly
carried down the stems in solution, and then ~stored (not made) in the
underground parts~, such as bulbs, potatoes, thick roots, and many
others. It is like the shopkeeper, who collects some money each day and
sends it every evening to the bank to be stored for him.
The leaves of the plant are then fresh next day to begin the work of
building up more starch.
[Illustration: Fig. 16. Striped leaves; the white stripes show no
starch when stained with iodine.]
One of the great contrasts between the leaves in the air, and the
parts of the plant underground, is that the leaves are bright green in
colour, and the underground parts are yellowish or brown. It has been
found that the green colour in leaves is very important in the building
up of the starch. You can see this in the case of leaves which have
parts quite colourless, as in those which are variegated or striped.
Take the leaves of such a plant, which have been exposed to a good
light, and test them in the usual way for starch. You will find that
the pale stripes of the leaf show no colour with iodine, because they
are empty of starch, owing to the fact that the green colour was not
there to build it up. The value of the green colour is that it absorbs
the energy of the sunlight, and uses it to get the carbon from the
carbonic acid gas, and then to build the carbon into starch.
Now you will remember in doing the experiments on the food solutions
(~see~ p. 17), that one of the plants lost its green colour, turned
yellowish, and finally died. That was the plant which had no iron in
its food solution. We have found, therefore, that without iron a plant
cannot build up its green colour, and without its green colour it
cannot use the store of carbon in the air to build up its food. This
is only one example of the importance of mineral salts to the plant.
Salts containing nitrogen are equally vital, while a number of mineral
compounds are necessary for healthy growth. So that we see that ~the
minerals absorbed in solution by the roots~, as well as ~the carbonic
acid gas absorbed from the air by the leaves~, and ~the energy of light
absorbed by the green colour are all equally necessary to the life of
the plant, as all help in the building up of its food~.
We have now seen that plants require food just as much as animals do;
but that they use different and simpler elements from which they build
it up for themselves, unlike the animals, which require their starchy
foods to be ready built up for them. The foods which plants make they
use in growing, and the other activities of their lives, just as
animals do.