5. THE FOOD WHICH A GREEN PLANT OBTAINS FROM THE SOIL.
1. =A plant cannot grow permanently in damp sawdust or clean
sand.=—Notice that the seedlings which were grown in damp sawdust
presently wither and die, while those which were grown in soil
flourish, and, with proper care, come to maturity. Obtain some clean
sand, and, to be sure that there is nothing in it which water can
dissolve, wash the sand in several changes of clean water. Germinate
some seeds in the sand, keeping it damp. The resulting plants in this
case also wither and die. Evidently soil contains some plant-food which
the plant cannot obtain from sawdust or clean sand. What is this food?
[Illustration: FIG. 18.—Porcelain crucible heated by Bunsen burner.]
2. =The amount of water and mineral matter in plants.=—Take a healthy
plant, say a bean plant, and weigh it. Then dry it thoroughly in the
oven and weigh it again. It will be found very much lighter; the
difference in weight represents the water which has been driven off.
Burn the dried plant. When the flame goes out notice the black charcoal
which is obtained. Continue the heating and observe that at last
nothing is left but a little grey ash. This experiment can be performed
over an ordinary fire by using an old shovel or a tile, but if you can
use a porcelain crucible (without lid) and a Bunsen burner (Fig. 18)
you will get better results. If a chemical balance is available, weigh
the ash and compare it with the weight of ash obtained from an ordinary
bean seed, such as that which gave rise to the plant you have used.
The ash from the plant is much greater than that got from the seed.
This extra ash must have been taken from the soil during the plant’s
growth.
3. =A nutritive solution.=—Make, or ask a dispensing chemist to make,
the following solution:
Potassium nitrate (consists of _potassium_, _nitrogen_,
and _oxygen_), 1 gram.
Sodium chloride (consists of _sodium_ and _chlorine_), ½ ”
Calcium sulphate (consists of _calcium_, _sulphur_,
and _oxygen_), ½ ”
Magnesium sulphate (consists of _magnesium_,
_sulphur_, and _oxygen_), ½ ”
Calcium phosphate (consists of _calcium_, _phosphorus_,
and _oxygen_), ½ ”
Water, 1 litre.
(A few drops of a dilute solution of sulphate, or chloride, of _iron_
should be added.)
Water, with this solution, a plant growing in wet sand, and when it is
well grown, dry and burn it. As much ash is obtained as from a plant of
the same size grown in soil. Notice the difference between such a plant
and one which has had water only supplied to it.
4. =Water culture.=—Fix two similar young plants in corks as shown
in Fig. 19, and put the corks into two bottles, the first of which
contains pure water and the second the nutritive solution, and let the
roots of the seedlings dip into the liquids. Cover the outsides of
the bottles with rolls of paper to keep out the light. Notice that
the plant living in the nutritive solution thrives, while the other
presently withers. Dry and burn the former, and observe that it yields
more ash than does a seed such as that from which it sprang.
5. =Plants obtain their mineral food from the soil by their roots.=—As
the roots are the only parts of the plant which are in contact with the
nutritive solution, or which (under ordinary conditions) are in the
soil, the mineral matter must be taken in by the roots.
6. =The root-hairs.=—Take up a seedling which has been growing in
damp sand, and observe the small particles of sand adhering to the
root-hairs (p. 17). The hairs of a plant’s root and rootlets apply
themselves very closely to particles of soil (Fig. 20), and the mineral
food (dissolved in water) passes into the hairs and so gets into the
root and thence to the other parts of the plant.
7. =Roots as storehouses of food.=—Examine, before the plants flower,
the roots of a turnip, a carrot, and a radish, and notice how greatly
they are swollen. You know that these roots are valued as foods; of
what use do you think the stored food is _to the plants themselves_?
=The food of a young seedling.=—When such a seed as that of a bean is
germinated in damp sawdust or wet clean sand, and kept in a warm and
light place, it puts out a radicle, which grows downwards and becomes
the main root, and a stem which grows upwards and bears green leaves.
After a time the main root branches, giving off side roots, which
spread in all directions through the sawdust or sand. The main root and
the rootlets bear very fine fluffy hairs for a short length, which is
situated just behind their points (Fig. 11), and these root hairs come
into very close contact with particles of damp sawdust or sand (Fig.
20), the moisture passing into them and thus reaching the main root,
from which it is distributed to the various parts of the plant. The
stem likewise flourishes, growing in length and thickness, and putting
out new leaves.
All this time the young bean plant is living on the food material
stored up in its cotyledons (p. 6); and if the sand or sawdust is
kept moist, with even pure water, this seed food is at first quite
sufficient. When at last the seed food is all used up, however, and all
that remains of the cotyledons is a shrivelled skin, the plant begins
to droop and wither from lack of food.
=Plants obtain food from the soil.=—Contrast this with the condition of
a seedling which has been grown in soil. It still flourishes, even when
the seed food is used up, for it is drawing up food from the soil—food
which could not be obtained from the damp sawdust or clean sand.
That the plant really has taken up some solid matter from the soil can
be proved by a few simple experiments. A plant which has been growing
in soil for some time after its seed food is used up is dried and
burnt, and the ashes are weighed. The weight of ash or mineral matter
thus obtained is found to be considerably greater than that of the ash
obtained from an ungerminated seed, or from a seedling grown in damp
sawdust or sand which has only been supplied with pure water.
=The mineral food of plants.=—The composition of the ash obtained
from various plants has been carefully determined by chemists, and in
this manner they have been able to find out what substances must be
present in soil in order that the plant may obtain all the mineral
food it requires. A mixture of potassium nitrate (nitre), sodium
chloride (common salt), calcium sulphate (plaster of Paris), magnesium
sulphate (Epsom salts), calcium phosphate, and chloride (or sulphate)
of iron—dissolved in water in the proportions specified on p. 27—has
been found to supply the necessary elements of the mineral food in a
form which the plant can readily use. That such a mixture is capable
of supporting the plant, while water alone is incapable of doing so,
may be seen by growing a plant—in the manner shown in Fig. 19—in this
solution. If, in addition, the plant is supplied with light and fresh
air, it will grow in a perfectly healthy and normal manner. If any of
the constituents (except the common salt) are omitted, the plant will
suffer. On the other hand, a plant which is growing in pure water will
presently die, from the lack of the necessary mineral food.
[Illustration: FIG. 19.—Plant growing in a nutritive solution of salts.
The bottle should be covered with a roll of paper to keep out the
light.]
=The work of the roots.=—These experiments show that water—of which
a large proportion of a plant consists—and the mineral constituents
of its food (dissolved in the soil-water) are taken up from the soil
by the roots. In ordinary soil the rootlets spread out on all sides,
dividing and subdividing, seeking for this very weak solution of
mineral salts. Even when soil appears practically dry, a very thin film
of moisture covers each little particle of earth, and the root hairs
become closely applied to these little particles (Fig. 20), so that the
water passes through their walls and gradually makes its way to the
main root, the stem, and the leaves.
[Illustration: FIG. 20.—Tip of a root hair with adhering particles of
soil. (× 240.)]
Roots sometimes perform other duties in addition to those of fixing
the plant in the soil and providing it with water and mineral food.
It is usual, for example, for =biennial plants=—which produce flowers
and seeds in their second year, and then die—to take in much more food
during their first season than they require at the time, and to store
up the surplus in readiness for the great effort of the second year.
These reserve materials are often stored in the roots, which then
become swollen and fleshy, like those of the turnip, radish, and carrot.
6. THE FOOD WHICH A GREEN PLANT OBTAINS FROM THE AIR.
1. =Plants contain much carbon.=—Char a stick, and notice the black
charcoal which is formed. Charcoal is an impure form of carbon.
2. =Carbon dioxide gas is formed when wood burns.=—Fasten a shaving
or a splinter of wood on a piece of wire, light it, and lower it into
a clean glass jar. When the wood has burned for a few seconds take it
out, and pour a little clear lime-water into the jar. The lime-water
turns milky. Similarly, pour a little lime-water into a jar in which
nothing has been burning, and notice that it remains clear. There is
evidently a difference in the nature of the air of the two jars. The
difference is caused by the burning of the wood, during which some of
the carbon unites with the oxygen of the air in the jar, forming an
invisible gas, called _carbon dioxide_. Carbon dioxide can always be
detected by the milkiness it causes in clear lime-water.
3. =Carbon dioxide present in ordinary air.=—Pour some clear lime-water
into a blue saucer and let it stand exposed to the air for half an
hour, then examine it. A white scum has formed on the surface of the
lime-water. Stir with a glass rod; the solution becomes milky. The
scum and the milkiness are produced by the union of the lime with
carbon dioxide from the air. Carbon (in the form of carbon dioxide) is
therefore present in the air.
4. =No carbon in the food solution.=—Examine again the list of
elements (p. 27), which compose the mineral salts which have been found
to replace satisfactorily the food which a plant obtains from the soil.
_There is no carbon in it._ A plant evidently does not depend on the
soil for the carbonaceous part of its food. From what other source can
a plant obtain its carbon? Carbon has just been proved to be present in
the air. Does the plant obtain its carbon from the air?
5. =Green leaves contain starch after exposure to sunlight.=—Take a
green leaf from a plant which has been exposed to the sunlight, boil
the leaf in water for a minute or two to kill it. Then put it in
methylated spirit until the leaf-green is dissolved out. When the leaf
is bleached rinse it in water, then put it into a dilute solution of
iodine (p. 2) and notice that it becomes blue or purplish brown. The
formation of this colour proves the presence of _starch_ in the leaf.
6. =Starch contains carbon.=—Char a piece of laundry starch and observe
the charcoal formed.
7. =Green leaves do not contain starch after being left in the dark for
24 hours.=—Keep a leafy plant in the dark for 24 hours and then test
a leaf as in the previous experiment. No starch can be detected. Put
the plant in the sunlight for an hour or two and test another leaf. It
contains starch. Plainly, starch is only formed in leaves if they are
exposed to light, and any starch previously present disappears when the
plant is kept in the dark.
8. =A green plant kept in air from which the carbon dioxide has been
removed will not form starch in its leaves.=—Obtain a large glass
bottle, such as those used by confectioners, and fit it with a cork
or india-rubber stopper through which passes a glass tube bent[5] as
in Fig. 21. Care should be taken to make all the joints tight, and it
may be necessary to soak the cork in melted paraffin to ensure this.
Pack the bend of the tube loosely with pieces of soda lime, and in
the bottle place a small jar containing lumps or a strong solution of
caustic soda. When the apparatus is ready, place in the bottle a small
plant or a leafy twig in water (fuchsia answers very well), which has
been kept in the dark for 24 hours. The caustic soda in the jar very
soon absorbs all the carbon dioxide which is present, and the soda lime
in the bend of the tube prevents any carbon dioxide from getting into
the bottle from the outside air. Place the jar in bright sunlight for
a few hours and then test a leaf for starch. None can be detected. It
is plain that one of the carbonaceous food stuffs—starch—is not formed
in the leaves of plants unless the plant is grown (in the light) in air
containing carbon dioxide.
9. =Seedlings are at first independent of light.=—Germinate pea or bean
seeds in wet sand or sawdust in the dark. Notice that for some time
the seedlings grow almost as well as when in the light. Soon, however,
the stem becomes long, weak and straggling, and the leaves are pale in
colour, even if the plant is supplied with mineral food.
10. =The formation of sugar in a germinating pea-seed.=—Take up a
pea-seedling when the stem is one or two inches long, and chew the
partly shrunken cotyledons. Notice the slightly sweet taste. Contrast
this with the taste of an ungerminated seed.
=A plant contains much carbon.=—When a piece of wood or other part
of a plant is strongly heated it first blackens or chars, showing
the presence of a large proportion of charcoal or impure carbon. On
continued heating, this carbon “burns away.” In the process of burning
it unites with some of the oxygen of the air, and forms a colourless,
invisible gas known as =carbon dioxide=. Though this substance cannot
be seen, its presence can be easily detected by means of clear
lime-water, which, when exposed to the gas, absorbs it and becomes
milky owing to the formation of a white precipitate of chalk. If, for
example, a splinter of wood is burnt in a glass jar, and a little clear
lime-water is immediately afterwards poured into the jar and shaken up,
a milkiness at once proclaims the presence of carbon dioxide.
=The air contains carbon.=—If lime-water is poured into a blue saucer
and left exposed to the air for half an hour, a white scum of chalk
is seen to have formed on its surface, showing that carbon dioxide is
present in the atmosphere. It is important that the student should
realise this presence of carbon—as invisible carbon dioxide gas—in
the air. Although the proportion is very small, amounting to only 3
parts of carbon dioxide in 10,000 parts of fresh country air, it is of
incalculable importance to plants, and indirectly to ourselves and all
other animals.
=A green plant obtains its carbon from the air.=—Since the parts of
a plant contain much carbon, and the food which a plant obtains from
the soil need not contain any carbon; while the air, on the other
hand, does contain carbon, it seems likely that a plant obtains
its carbonaceous food from the air. This surmise is confirmed by
experiments. One of the most easily recognisable of plant products
containing carbon is =starch=, for it yields a very characteristic
blue, or purplish-brown, colour when treated with iodine solution. By
means of this test starch can easily be proved to be present in the
green leaves of a plant which has been exposed to the air and sunlight.
The leaf is first killed by being boiled in water for a minute or two,
and then its green colouring matter is dissolved out by immersion in
alcohol (methylated spirit). The bleached leaf is rinsed in water and
then put in iodine solution, and the blue or purplish-brown colour
which is formed shows the presence of starch. There is a marked
difference when a leaf, which has been kept in the dark for twenty-four
hours, is similarly tested. In this case no starch can be detected.
One compound of carbon—_i.e._ starch—may thus be recognised easily; and
if we found that a leaf made no starch when supplied only with air from
which the carbon dioxide had been removed, this fact would be strong
evidence in favour of the conclusion that a green plant obtains its
carbon from the carbon dioxide of the air. To test this, a large bottle
is fitted, by means of a tightly fitting cork or stopper, with a tube
containing lumps of soda lime, a substance which eagerly absorbs carbon
dioxide from air. A small jar of caustic soda is placed inside the
bottle (Fig. 21). A green plant or a leafy twig, which has been kept
for twenty-four hours in the dark to free it from starch, is then put
in the bottle, and the whole exposed to sunlight for a few hours. At
the end of this time it is found, on testing the leaves, that no starch
has been formed. By this, and other experiments, botanists have proved
that _green plants obtain all their carbon from the carbon dioxide of
the air, and that sunlight is indispensable for the process_. We shall
examine this question more fully when we study leaves (Chapter III.).
[Illustration: FIG. 21.—Experiment to prove that leaves do not make
starch unless the air with which they are supplied contains carbon
dioxide. _S_, tube containing lumps of soda lime; _S′_, jar containing
a solution of caustic soda.]
=The carbonaceous food of a young seedling.=—Just as a bean or pea
seedling is for a time independent of an outside supply of mineral
food—its roots needing only to be supplied with water—so there is
enough carbonaceous food also stored up in the seed to satisfy for some
time the needs of the growing plant—the stored starch being gradually
changed into sugar and absorbed. For this reason a young seedling will
live healthily in the dark. When, however, the seed food is exhausted,
or nearly so, the plant draws upon the store of carbon, which is
present as carbon dioxide in the air, for a renewal of the starch and
allied substances which are necessary to it. As it cannot make use of
this atmospheric carbon in the dark, it must henceforth be supplied
with sunlight or it will not thrive. Plants kept in the dark after
their seed-food is exhausted are pale in colour and unhealthy. Their
stems grow long and straggling (Fig. 22), but are usually too weak to
stand upright.
EXERCISES ON CHAPTER II.
1. Make experiments to discover the effects, upon
seedlings growing in a nutritive solution (p. 27), of each
of the following modifications in the composition of the
solution: (_a_) omit the sodium chloride; (_b_) omit the
potassium nitrate; (_c_) omit the compound of iron; (_d_)
omit the magnesium sulphate; (_e_) substitute sodium nitrate
for potassium nitrate.
2. Explain how it is that a green plant cannot carry on
its nutrition in darkness. (1892)
3. What part of its food does a green plant obtain from
the air? In what form, and under what conditions, is it
taken in? (1889)
4. Describe the method of water cultures, and give the
general results of a set of experiments. (1898)
5. Give experimental proof that green plants require to be
fed with combined nitrogen. (1897)
6. What are the necessary conditions for the formation of
starch in a plant? Mention experiments which support your
statements. (1896)
7. Explain the influence of light on a growing plant.
Illustrate your answer by reference to the changes in a
ripening and germinating bean.
(King’s Schol. 1903)
8. How can it be proved experimentally that a green plant
draws some, but not the whole, of its nourishment from the
air? (1904)
[Illustration: FIG. 22.—Two mustard seedlings of equal age. _E_, grown
in the dark; _N_, grown in ordinary daylight.]
FOOTNOTE:
[5] Cheap glass tubing can easily be made soft enough to bend by
heating it in an ordinary batswing gas burner.