_Demonstration_.--Roots of bean or pea.
_Demonstration or home experiment_.--Response of root to
gravity and to water. What part of root is most responsive?
_Laboratory work_.--Root hairs, radish or corn, position on
root, gross structure only. Drawing.
_Demonstration._--Root hair under compound microscope.
_Demonstration._--Apparatus illustrating osmosis.
_Demonstration or a home experiment._--Organic matter
present in soil.
_Demonstration._--Root tubercles of legume.
_Demonstration._--Nutrients present in some roots.
Uses of the Root.--If one of the seedlings of the bean spoken of in the
last chapter is allowed to grow in sawdust and is given light, air, and
water, sooner or later it will die. Soil is part of its natural
environment, and the roots which come in contact with the soil are very
important. It is the purpose of this chapter to find out just how the young
plant is fitted to get what it needs from this part of its environment;
namely, the soil.
The development of a bean seedling has shown us that the root grows first.
_One of the most important functions of the root to a young seed plant is
that of a holdfast, an anchor to fasten it in the place where it is to
develop._ It has many other uses, as the taking in of water with the
mineral and organic matter dissolved therein, the storage of food,
climbing, etc. All functions other than the first one stated arise after
the young plant has begun to develop.
[Illustration: A root system, showing primary and secondary roots.]
Root System.--If you dig up a young bean seedling and carefully wash the
dirt from the roots, you will see that a long root is developed as a
continuation of the hypocotyl. This root is called the _primary_ root.
Other smaller roots which grow from the primary root are called
_secondary_, or _tertiary_, depending on their relation to the first root
developed.
Downward Growth of Root. Influence of Gravity.--Most of the roots examined
take a more or less downward direction. We are all familiar with the fact
that the force we call gravity influences life upon this earth to a great
degree. Does gravity act on the growing root? This question may be answered
by a simple experiment.
[Illustration: Revolve this figure in the direction of the arrows to see if
the roots of the radish respond to gravity.]
Plant mustard or radish seeds in a pocket garden, place it on one edge and
allow the seeds to germinate until the root has grown to a length of about
half an inch. Then turn it at right angles to the first position and allow
it to remain for one day undisturbed. The roots now will be found to have
turned in response to the change in position, that part of the root near
the growing point being the most sensitive to the change. This experiment
seems to indicate that the roots are influenced to grow downward by the
force of gravity.
Experiments to determine the Influence of Moisture on a Growing Root.--The
objection might well be interposed that possibly the roots in the pocket
garden[8] grew downward after water. That moisture has an influence on the
growing root is easily proved.
Footnote 8: _The Pocket Garden._--A very convenient form of
pocket germinator may be made as follows. Obtain two cleaned
four by five negatives (window glass will do); place one
flat on the table and place on this half a dozen pieces of
colored blotting paper cut to a size a little less than the
glass. Now cut four thin strips of wood to fit on the glass
just outside of the paper. Next moisten the blotter, place
on it some well-soaked radish, mustard seeds or barley
grains, and cover with the other glass. The whole box thus
made should be bound together with bicycle tape. Seeds will
germinate in this box and with care may live for two weeks
or more.
Plant bird seed, mustard or radish seed in the underside of a sponge, which
should be kept wet, and may be suspended by a string under a bell jar in
the schoolroom window. Note whether the roots leave the sponge to grow
downward, or if the moisture in the sponge is sufficient to counterbalance
the force of gravity.
Water a Factor which determines the Course taken by Roots.--_Water, as well
as the force of gravity, has much to do with the direction taken by roots._
Water is always found below the surface of the ground, but sometimes at a
great depth. Most trees, and all grasses, have a greater area of surface
exposed by the roots than by the branches. The roots of alfalfa, a
cloverlike plant used for hay in the Western states, often penetrate the
soil after water for a distance of ten to twenty feet below the surface of
the ground.
Fine Structure of a Root.[9]--When we examine a delicate root in thin
longitudinal section under the compound microscope, we find the entire root
to be made up of cells, the walls of which are uniformly rather thin. Over
the lower end of the root is found a collection of cells, most of which are
dead, loosely arranged so as to form a cap over the growing tip. This is
evidently an adaptation which protects the young and actively growing cells
just under the root cap. In the body of the root a central cylinder can
easily be distinguished from the surrounding cells. In a longitudinal
section a series of tubelike structures may be found within the central
cylinder. These structures are cells which have grown together at the small
end, the long axis of the cells running the length of the main root. In
their development the cells mentioned have grown together in such a manner
as to lose their small ends, and now form continuous hollow tubes with
rather strong walls. Other cells have come to develop greatly thickened
walls; these cells give mechanical support to the tubelike cells.
Collections of such tubes and supporting woody cells together make up what
are known as _fibrovascular bundles_.
Footnote 9: Sections of tradescantia roots are excellent for
demonstration of these structures.
[Illustration: Cross section of a young taproot; _a_, _a_, root hairs; _b_,
outer layer of bark; _c_, inner layer of bark; _d_, wood or central
cylinder.]
Root Hairs.--Careful examination of the root of one of the seedlings of
mustard, radish, or barley grown in the pocket germinator shows a covering
of tiny fuzzy structures. These structures are very minute, at most 3 to 4
millimeters in length. They vary in length according to their position on
the root, the most and the longest root hairs being found near the point
marked _R. H._ in the figure. These structures are outgrowths of the outer
layer of the root (the _epidermis_), and are of very great importance to
the living plant.
[Illustration: Young embryo of corn, showing root hairs (_R. H._) and
growing stem (_P._).]
Structure of a Root Hair.--A single root hair examined under a compound
microscope will be found to be a long, round structure, almost colorless in
appearance. The wall, which is very flexible and thin, is made up of
cellulose, a substance somewhat like wood in chemical composition, through
which fluids may easily pass. Clinging close to the cell wall is the
protoplasm of the cell. The interior of the root hair is more or less
filled with a fluid called _cell sap_. Forming a part of the living
protoplasm of the root hair, sometimes in the hairlike prolongation and
sometimes in that part of the cell which forms the epidermis, is found a
_nucleus_. The protoplasm and nucleus are alive; the cell wall formed by
the living matter in the cell is dead. _The root hair is a living plant
cell_ with a wall so delicate that water and mineral substances from the
soil can pass through it into the interior of the root.
[Illustration: Diagram of a root hair; _CS_, cell sap; _CW_, cell wall;
_P_, protoplasm; _N_, nucleus; _S_, particles of soil.]
How the Root absorbs Water.--The process by which the root hair takes up
soil water can better be understood if we make an artificial root hair
large enough to be easily seen. An egg with part of the outer shell removed
so as to expose the soft skinlike membrane underneath is an example.
Better, an artificial root hair may be _made_ in the following way. Pour
some soft celloidin into a test tube; carefully revolve the test tube so
that an even film of celloidin dries on the inside. This membrane is
removed, filled with white of egg, and tied over the end of a rubber cork
in which a glass tube has previously been inserted. When placed in water,
it gives a very accurate picture of the root hair at work. After a short
time water begins to rise in the tube, having passed through the film of
celloidin. If grape sugar, salt, or some other substance which will
dissolve in water were placed in the water outside the artificial root
hair, it could soon be proved by test to pass through the wall and into the
liquid inside.
Osmosis.--To explain this process we must remember that gases and liquids
of different densities, when separated by a membrane, tend to flow toward
each other and mingle, the greater flow always being in the direction of
the denser medium. _The process by which two gases or fluids, separated by
a membrane, tend to pass through the membrane and mingle with each other,
is called osmosis._ The method by which the root hairs take up soil water
is exactly the same process. It is by osmosis. The white of the egg is the
best possible substitute for living matter; the celloidin membrane
separating the egg from the water is much like the delicate membrane-like
wall which separates the protoplasm of the root hair from the water in the
soil surrounding it. The fluid in the root hair is denser than the soil
water; hence the greater flow is toward the interior of the root hair.[10]
Footnote 10: For an excellent elementary discussion of
osmosis see Moore, _Physiology of Man and Other Animals_.
Henry Holt and Company.
[Illustration: The soil particles are each surrounded with a delicate film
of water. How might the root hairs take up this water?]
Passage of Soil Water within the Root.--We have already seen that in an
exchange of fluids by osmosis the greater flow is always toward the denser
fluid. Thus it is that the root hairs take in more fluid than they give up.
The cell sap, which partly fills the interior of the root hair, is a fluid
of greater density than the water outside in the soil. When the root hairs
become filled with water, the density of the cell sap is lessened, and the
cells of the epidermis are thus in a position to pass along their supply of
water to the cells next to them and nearer to the center of the root. These
cells, in turn, become less dense than their inside neighbors, and so the
transfer of water goes on until the water at last reaches the central
cylinder. Here it is passed over to the tubes of the woody bundles and
started up the stem. The pressure created by this process of osmosis is
sufficient to send water up the stem to a distance, in some plants, of 25
to 30 feet. Cases are on record of water having been raised in the birch a
distance of 85 feet.
Physiological Importance of Osmosis.--It is not an exaggeration to say that
osmosis is a process not only of great importance to a plant, but to an
animal as well. Foods are digested in the food tube of an animal; that is,
they are changed into a soluble form so that they may pass through the
walls of the food tube and become part of the blood. The inner lining of
part of the food tube is thrown into millions of little fingerlike
projections which look somewhat, in size at least, like root hairs. These
fingerlike processes are (unlike a root hair) made up of many cells. But
they serve the same purpose as the root hairs, for they absorb liquid food
into the blood. This process of absorption is largely by osmosis. Without
the process of osmosis we should be unable to use much of the food we eat.
Composition of Soil.--If we examine a mass of ordinary loam carefully, we
find that it is composed of numerous particles of varying size and weight.
Between these particles, if the soil is not caked and hard packed, we can
find tiny spaces. In well-tilled soil these spaces are constantly being
formed and enlarged. They allow air and water to penetrate the soil. If we
examine soil under the microscope, we find considerable water clinging to
the soil particles and forming a delicate film around each particle. In
this manner most of the water is held in the soil.
[Illustration: Inorganic soil is being formed by weathering.]
How Water is held in Soil.--To understand what comes in with the soil
water, it will be necessary to find out a little more about soil.
Scientists who have made the subject of the composition of the earth a
study, tell us that once upon a time at least a part of the earth was
molten. Later, it cooled into solid rock. Soil making began when the ice
and frost, working alternately with the heat, chipped off pieces of rock.
These pieces in time became ground into fragments by action of ice,
glaciers, running water, or the atmosphere. This process is called
weathering. Weathering is aided by oxidation. A glance at almost any
crumbling stones will convince you of this, because of the yellow oxide of
iron (rust) disclosed. So by slow degrees this earth became covered with a
coating of what we call inorganic soil. Later, generation after generation
of tiny plants and animals which lived in the soil died, and their remains
formed the first organic materials of the soil.
[Illustration: This picture shows how the forests help to cover the
inorganic soil with an organic coating. Explain how.]
You are all familiar with the difference between the so-called rich soil
and poor soil. The dark soil contains more dead plant and animal matter,
which forms the portion called _humus_.
[Illustration: Apparatus for testing the capacity of soils to take in and
retain moisture.]
Humus contains Organic Matter.--It is an easy matter to prove that black
soil contains organic matter, for if an equal weight of carefully dried
humus and soil from a sandy road is heated red-hot for some time and then
reweighed, the humus will be found to have lost considerably in weight, and
the sandy soil to have lost very little. The material left after heating
is inorganic material, the organic matter having been burned out.
Soil containing organic materials holds water much more readily than
inorganic soil, as a glance at the accompanying figure shows. If we fill
each of the vessels with a given weight (say 100 grams each) of gravel,
sand, barren soil, rich loam, leaf mold, and 25 grams of dry, pulverized
leaves, then pour equal amounts of water (100 c.c.) on each and measure all
that runs through, the water that has been retained will represent the
water supply that plants could draw on from such soil.
[Illustration: Soil particles cling to root hairs. Why?]
The Root Hairs take more than Water out of the Soil.--If a root containing
a fringe of root hairs is washed carefully, it will be found to have little
particles of soil still clinging to it. Examined under the microscope,
these particles of soil seem to be cemented to the sticky surface of the
root hair. The soil contains, besides a number of chemical compounds of
various mineral substances,--lime, potash, iron, silica, and many
others,--a considerable amount of organic material. Acids of various kinds
are present in the soil. These acids so act upon certain of the mineral
substances that they become dissolved in the water which is absorbed by the
root hairs. Root hairs also give off small amounts of acid. An interesting
experiment may be shown (see Figure on page 80) to prove this. A solution
of _phenolphthalein_ loses its color when an acid is added to it. If a
growing pea be placed in a tube containing some of this solution the latter
will quickly change from a rose pink to a colorless solution.
A Plant needs Mineral Matter to Make Living Matter.--Living matter
(protoplasm), besides containing the chemical elements carbon, hydrogen,
oxygen, and nitrogen, contains a very minute proportion of various elements
which make up the basis of certain minerals. These are calcium (lime),
sulphur, iron, potassium, magnesium, phosphorus, sodium, and chlorine.
That plants will not grow well without certain of these mineral substances
can be proved by the growth of seedlings in a so-called nutrient
solution.[11] Such a solution contains all the mineral matter that a plant
uses for food. If certain ingredients are left out of this solution, the
plants placed in it will not live.
Footnote 11: See Hunter's _Laboratory Problems in Civic
Biology_ for list of ingredients.
[Illustration: Effect of root hairs on phenolphthalein solution. The change
of color indicates the presence of acid.]
Nitrogen in a Usable Form necessary for Growth of Plants.--A chemical
element needed by the plant to make protoplasm is nitrogen. The air can be
proven by experiment to be made up of about four fifths nitrogen, but this
element cannot be taken from either soil, water, or air in a pure state,
but is usually obtained from the organic matter in the soil, where it
exists with other substances in the form of _nitrates_. Ammonia and other
organic compounds which contain nitrogen are changed by two groups of
little plants called _bacteria_, first into nitrites and then nitrates.[12]
Footnote 12: It has recently been discovered that under some
conditions these bacteria are preyed upon by tiny one-celled
animals (_protozoa_) living in the soil and are so reduced
in numbers that they cannot do their work effectively. If,
then, the soil is heated artificially or treated with
antiseptics so as to kill the protozoa, the bacteria which
escape multiply so rapidly as to make the land much richer
than before.
[Illustration: Diagram to show how the nitrogen-fixing bacteria prepare
nitrogen for use by plants; _t_, tubercles.]
Relation of Bacteria to Free Nitrogen.--It has been known since the time of
the Romans that the growth of clover, peas, beans, and other legumes in
soil causes it to become more favorable for growth of other plants. The
reason for this has been discovered in late years. On the roots of the
plants mentioned are found little swellings or nodules; in the nodules
exist millions of bacteria, which take nitrogen from the atmosphere and fix
it so that it can be used by the plant; that is, they assist in forming
nitrates for the plants to use. Only these bacteria, of all the living
plants, have the power to take the free nitrogen from the air and make it
over into a form that can be used by the roots. As all the compounds of
nitrogen are used over and over again, first by plants, then as food for
animals, eventually returning to the soil again, or in part being turned
into free nitrogen, it is evident that any _new_ supply of usable nitrogen
must come by means of these nitrogen-fixing bacteria.
Rotation of Crops.--The facts mentioned above are made use of by careful
farmers who wish to make as much as possible from a given area of ground in
a given time. Such plants as are hosts for the nitrogen-fixing bacteria are
planted early in the season. Later these plants are plowed in and a second
crop is planted. The latter grows quickly and luxuriantly because of the
nitrates left in the soil by the bacteria which lived with the first crop.
For this reason, clover is often grown on land in which it is proposed to
plant corn, the nitrogen left in the soil thus giving nourishment to the
young corn plants. In scientifically managed farms, different crops are
planted in a given field on different years so that one crop may replace
some of the elements taken from the soil by the previous crop. This is
known as rotation of crops.[13] The annual yield of the average farm may
thus be greatly increased.
Footnote 13: That crop rotation is not primarily a process
to conserve the fertility of the soil, but is a sanitary
measure to prevent infection of the soil, is the latest
belief of the scientist.
[Illustration: Nitrogen in the soil is necessary for plants. Explain from
this diagram how nitrogen is put into the soil by some plants and taken out
by others.]
Five of the elements necessary to the life of the plant which may be taken
out of the soil by constant use are calcium, nitrogen, phosphorus,
potassium, and sulphur. Several methods are used by the farmer to prevent
the exhaustion of these and other raw food materials from the soil. One
method known as _fallowing_ is to allow the soil to remain idle until
bacteria and oxidation have renewed the chemical materials used by the
plants. This is an expensive method, if land is dear. The most common
method of enriching soil is by means of fertilizing material rich in plant
food. Manure is most frequently used, but many artificial fertilizers, most
of which contain nitrogen in the form of some nitrate, are used, because
they can be more easily transported and sold. Such are ground bone, guano
(bird manure), nitrate of soda, and many others. These also contain other
important raw food materials for plants, especially potash and phosphoric
acid. Both of these substances are made soluble so as to be taken into the
roots by the action of the carbon dioxide in the soil.
The Indirect Relation of this to the City Dweller.--All of us living in the
city are aware of the importance of fresh vegetables, brought in from the
neighboring market gardens. But we sometimes forget that our great staple
crops, wheat and other cereals, potatoes, fruits of all kinds, our cotton
crop, and all plants we make use of grow directly in proportion to the
amount of raw food materials they take in through the roots. When we also
remember that many industries within the cities, as mills, bakeries, and
the like, as well as the earnings of our railways and steamship lines, are
largely dependent on the abundance of the crops, we may recognize the
importance of what we have read in this chapter.
Food Storage in Roots of Commercial Importance.--Some plants, as the
parsnip, carrot, and radish, produce no seed until the second year, storing
food in the roots the first year and using it to get an early start the
following spring, so as to be better able to produce seeds when the time
comes. This food storage in roots is of much practical value to mankind.
Many of our commonest garden vegetables, as those mentioned above, and the
beet, turnip, oyster plant, sweet potato and many others, are of value
because of the food stored. The sugar beet has, in Europe especially,
become the basis of a great industry.