FOOD MATERIALS OF THE OLDER PLANT
(1) IN THE SOIL
As we have just seen, young seedlings are supplied with stores of food,
starch, and other things, which are packed in their cotyledons and are
used up by them as they grow. But we also saw that as the plant gets
older these stores get emptier, and finally the nurse leaves shrivel
up entirely when their contents are exhausted. All the same, however,
the plant continues to grow. Surely it cannot do this on nothing, any
more than an animal could? When the young calves cease to be fed with
milk, their food changes, and they begin to eat grass; this gives them
individually more work, for grass is not a “prepared food” like milk.
Very much the same thing happens with seedlings. ~Their prepared food
supply gets used up, and they must find food for themselves. Where do
they find it?~
When you remember the fine hairs on the young parts of roots which
absorb water from the soil or sawdust, it is quite natural to think at
once of the soil as a possible place for them to find their food; and,
indeed, this is partly the case. The water in the soil is not perfectly
pure, for there are many different “salts” dissolved in it. By “salts”
one does not mean only table salt, but also any kind of mineral in
solution, such as salts of iron or portions of chalk or limestone,
or even some of the minerals which make up granite. These may all be
dissolved in rain-water just as sugar is dissolved in your tea, and so
spread equally through it. As the water enters the roots of plants
through the hairs, these dissolved salts come in with it, and so get
distributed over the whole plant. The root hairs cannot “eat” particles
of soil, but they twine in among the fine grains and absorb the little
films of water which cling to them.
[Illustration: Fig. 9. Root hairs growing among soil particles.
(~Much magnified.~)]
You can find out some of =the importance of these mineral salts= in the
life of the plant, if you do the following experiment.
Take several seedlings which have already grown enough to have nearly
exhausted the supply of food in their cotyledons. These you must grow
in jars of pure distilled water, to which you have added certain salts
which have been found to be the important ones in the soil water and
plant food. By giving the plant nothing but these salts and distilled
water you know just what it gets. Distilled water is made by catching
and condensing steam, and it has no salts dissolved in it; while
ordinary tap water has run off some mountain side or risen in some
spring from the rocks, and it has many salts in it already, so that it
is useless for this experiment.
Take three big glass jars, each with one litre of distilled water, and
label them A, B, and C. Into A put nothing further, into B put the
following salts, which have been weighed out carefully either by you or
by a chemist:--
Potassium nitrate 1 gramme
Calcium sulphate 1/2 „
Sodium chloride 1/2 „
Magnesium sulphate 1/2 „
Calcium phosphate 1/2 „
then add to C all these salts, and also one or two drops of a dilute
solution of iron chloride.
Into the jars fit corks which are split, with a hole in the centre,
and pack a plant into each with the part of the stem between the corks
wrapped round with cotton wool (~see~ fig. 10), and so fix the plant
that its roots are in the solution and its stem and leaves in the
air[2] (~see~ fig. 11). Wrap black cotton or paper round the jars so as
to keep the roots dark as they would be in the soil.
[2] Weigh the plant, which you are putting in jar C, carefully, and
keep a record of its weight for future use (~see~ p. 18).
[Illustration: Fig 10. Plant packed in split cork. (~h~) Hole in cork;
(~c~) cotton-wool packing the stem.]
Do not use too small vessels; in fact, if you had bigger jars and took
double quantities of everything it would be better.
You may make the experiment more complete by preparing a whole series
of solutions with one of the salts left out each time. In this way
you would be able to see the effect of the different elements on the
growth of the plants, and you would find nitrates are very important.
Put a plant, similar to the one you are experimenting with, into a pot
of soil or the garden, and keep it well watered. This is called the
“control plant.”
Very soon you will find that the plant in jar A (the one with only
distilled water) is not growing so fast as the others, and after a
time will die off completely. The one in jar C with all the salts, on
the other hand, should grow quite as well as the control plant in the
garden, which you should take as the standard.
The plant in jar B, when it has everything but iron, should act in a
curious manner. At first it should grow all right and outlive the one
in distilled water, but after a time its leaves should get paler, till
the new ones formed are quite yellow instead of green, and soon after
this the plant will die. If, however, you add two drops of the iron
solution before it dies, it may recover, become green again, and go on
living. It turned a whitish yellow colour because there was no iron in
its supply of salts and water. Just as when people get pale and white
the doctor orders them iron, so it is necessary for plants to have iron
when they begin to lose their green colour. Later on you will find how
very important the green colour is, for without it they cannot grow
(~see~ Chap. VI.).[3]
[3] This experiment is sometimes difficult to manage successfully,
though it appears so simple. Great care should be taken not to overdose
the plant with iron.
[Illustration: Fig. 11. Three jars in which seedlings of the same age
are growing; A, in distilled water; B, in the food solution without
iron; and C, in the complete food solution.]
From these experiments you see that ~it is not the soil which is
necessary to the plants, but that certain salts in solution in the
water~ held by the soil particles are very important. When all the
salts are present in the water, as was the case in jar C, the plant can
grow just as well as one in the soil; but when it has not these salts
it must die. The salts in solution, therefore, must be a very important
part of the food. Are they the only food the plant gets?