Chapter VIII: The Root.--Function and Structure

=The function of roots is twofold=,--to provide _support or anchorage_ for the plant, and to _collect and convey food_ materials. The first function is considered in Chapter VII; we may now give attention in more detail to the second.

The feeding surface of the roots is _near their ends_. As the roots become old and hard, they serve only as _channels through which food passes_ and as _hold-fasts or supports_ for the plant. The root-hold of a plant is very strong. Slowly pull upwards on some plant, and note how firmly it is anchored in the soil.

=Roots have power to choose their food=; that is, they do not absorb all substances with which they come in contact. They do not take up great quantities of useless or harmful materials, even though these materials may be abundant in the soil; but they may take up a greater quantity of some of the plant-foods than the plant can use to advantage. _Plants respond very quickly to liberal feeding_,--that is, to the application of plant-food to the soil (Fig. 40). The poorer the soil, the more marked are the results, as a rule, of the application of fertilizers. Certain substances, as common salt, will kill the roots.

=Roots absorb Substances only in Solution.=--Substances cannot be taken in solid particles. These materials are in solution in the soil water, and the roots themselves also have the power to dissolve the soil materials to some extent by means of substances that they excrete. The materials that come into the plant through the roots are _water and mostly the mineral substances_, as compounds of potassium, iron, phosphorus, calcium, magnesium, sulfur, and chlorine. These mineral substances compose the ash when the plant is burned. The carbon is derived from the air through the green parts. Oxygen is derived from the air and the soil water.

=Nitrogen enters through the Roots.=--All plants must have nitrogen; yet, although about four fifths of the air is nitrogen, plants are not able, so far as we know, to take it in through their leaves. It enters through the roots in combination with other elements, chiefly in the form of nitrates (certain combinations with oxygen and a mineral base). The great family of leguminous plants, however (as peas, beans, cowpea, clover, alfalfa, vetch), _use the nitrogen contained in the air in the soil_. They are able to utilize it through the _agency of nodules_ on their roots (Figs. 41, 42). These nodules contain bacteria, which appropriate the free or uncombined nitrogen and pass it on to the plant. The nitrogen becomes incorporated in the plant tissue, so that these crops are high in their nitrogen content. Inasmuch as nitrogen in any form is expensive to purchase in fertilizers, the use of leguminous crops to plow under is a very important agricultural practice in preparing the land for other crops. In order that leguminous crops may acquire atmospheric nitrogen more freely and thereby thrive better, _the land is sometimes sown or inoculated with the nodule-forming bacteria_.

=Roots require moisture= in order to serve the plant. The soil water that is valuable to the plant is not the free water, but the _thin film of moisture which adheres to each little particle of soil_. The finer the soil, the greater the number of particles, and therefore the greater is the quantity of film moisture that it can hold. This moisture surrounding the grains may not be perceptible, yet the plant can use it. _Root absorption may continue in a soil which seems to be dust dry._ Soils that are very hard and “baked” (Fig. 43) contain very little moisture or air,--not so much as similar soils that are granular or mellow.

=Proper Temperature for Root Action.=--_The root must be warm in order to perform its functions._ Should the soil of fields or greenhouses be much colder than the air, the plant suffers. When in a warm atmosphere, or in a dry atmosphere, plants need to absorb much water from the soil, and the roots must be warm if the root-hairs are to supply the water as rapidly as it is needed. _If the roots are chilled, the plant may wilt or die._

=Roots need Air.=--Corn on land that has been flooded by heavy rains loses its green color and turns yellow. _Besides diluting plant-food, the water drives the air from the soil, and this suffocation of the roots is very soon apparent in the general ill health of the plant._ Stirring or tilling the soil aërates it. Water plants and bog plants have adapted themselves to their particular conditions. They get their air either by special surface roots, or from the water through stems and leaves.

=Rootlets.=--_Roots divide into the thinnest and finest fibrils: there are roots and there are rootlets._ The smallest rootlets are so slender and delicate that they break off even when the plant is very carefully lifted from the soil.

_The rootlets, or fine divisions, are clothed with the_ =root-hairs= (Figs. 44, 45, 46). _These root-hairs attach to the soil particles, and a great amount of soil is thus brought into actual contact with the plant._ These are very _delicate prolonged surface cells of the roots_. They are borne for a short distance just back of the tip of the root.

_Rootlet and root-hair differ._ The rootlet is a _compact cellular structure. The root-hair is a delicate tubular cell_ (Fig. 45), _within which is contained living matter (protoplasm); and the protoplasmic lining membrane of the wall governs the entrance of water and substances in solution_. Being long and tube-like, these root-hairs are especially adapted for taking in the largest quantity of solutions; and they are the principal means by which plant-food is absorbed from the soil, although the surfaces of the rootlets themselves do their part. Water plants do not produce an abundant system of root-hairs, and such plants depend largely on their rootlets.

The root-hairs are very small, often invisible. They, with the young roots, are usually broken off when the plant is pulled up. They are best seen when seeds are germinated between layers of dark blotting paper or flannel. On the young roots, they will be seen as a mold-like or gossamer-like covering. _Root-hairs soon die_: they do not grow into roots. New ones form as the root grows.

=Osmosis.=--The water with its nourishment goes through the thin walls of the root-hairs and rootlets by the process of osmosis. If there are two liquids of different density on the inside and outside of an organic (either vegetable or animal) membrane, the liquids tend to mix through the membrane. The _law of osmosis_ is that _the most rapid flow is toward the denser solution_. The protoplasmic lining of the cell wall is such a membrane. The soil water being a weaker solution than the sap in the roots, the flow is into the root. A strong fertilizer sometimes causes a plant to wither, or “burns it.” Explain.

=Structure of Roots.=--The root that grows from the lower end of the caulicle is the _first_ or =primary root=. =Secondary roots= branch from the primary root. Branches of secondary roots are sometimes called =tertiary roots=. Do the secondary roots grow from the cortex, or from the central cylinder of the primary root? Trim or peel the cortex from a root and its branches and determine whether the branches still hold to the central cylinder of the main root.

=Internal Structure of Roots.=--A section of a root shows that it consists of a _central cylinder_ (see Fig. 45) surrounded by a layer. This layer is called the =cortex=. The outer layer of cells in the cortex is called the =epidermis=, and some of the cells of the epidermis are prolonged and form the delicate root-hairs. The cortex resembles the bark of the stem in its nature. The central cylinder contains many tube-like canals, or “vessels” that convey water and food (Fig. 45). Cut a sweet potato across (also a radish and a turnip) and distinguish the central cylinder, cortex and epidermis. Notice the hard cap on the tip of roots. Roots differ from stems in having no real pith.

=Microscopic Structure of Roots.=--Near the end of any young root or shoot the cells are found to differ from each other more or less, according to the distance from the point. _This differentiation takes place in the region just back of the growing point._ To study growing points, use the hypocotyl of Indian corn which has grown about one half inch. Make a longitudinal section. Note these points (Fig. 47): (_a_) the tapering root-cap beyond the growing point; (_b_) the blunt end of the root proper and the rectangular shape of the cells found there; (_c_) the group of cells in the middle of the first layers beneath the root-cap,--this group is the growing point; (_d_) study the slight differences in the tissues a short distance back of the growing point. There are four regions: the =central cylinder=, made up of several rows of cells in the center (_pl_); the =endodermis=, (_e_) composed of a single layer on each side which separates the central cylinder from the bark; the =cortex=, or inner bark, (_e_) of several layers outside the endodermis; and the =epidermis=, or outer layer of bark on the outer edges (_d_). Make a drawing of the section. If a series of the cross-sections of the hypocotyl should be made and studied, beginning near the growing point and going upward, it would be found that these four tissues become more distinctly marked, for at the tip the tissues have not yet assumed their characteristic form. The central cylinder contains the ducts and vessels which convey the sap.

_d_, _d_, cells which will form the epidermis; _p_, _p_, cells that will form bark; _e_, _e_, endodermis; _pl_, cells which will form the axis cylinder; _i_, initial group of cells, or growing point proper; _c_, root-cap.]

=The Root-cap.=--Note the form of the root-cap shown in the microscopic section drawn in Fig. 47. Growing cells, and especially those which are forming tissue by subdividing, are very delicate and are easily injured. The cells forming the root-cap are older and tougher and are suited for pushing aside the soil that the root may penetrate it.

=Region of most Rapid Growth.=--The roots of a seedling bean may be marked at equal distances by waterproof ink or by bits of black thread tied moderately tight. The seedling is then replanted and left undisturbed for two days. When it is dug up, the region of most rapid growth in the root can be determined. Give a reason why _a root cannot elongate throughout its length_,--whether there is anything to prevent a young root from doing so.

In Fig. 48 is shown a germinating scarlet runner bean with a short root upon which are marks made with waterproof ink; and the same root (Fig. 49) is shown after it has grown longer. Which part of it did not lengthen at all? Which part lengthened slightly? Where is the region of most rapid growth?

=Geotropism.=--Roots turn toward the earth, even if the seed is planted with the micropyle up. This phenomenon is called =positive geotropism=. Stems grow away from the earth. This is =negative geotropism=.

SUGGESTIONS (Chaps. VII and VIII).--=25.= _Tests for food._ Examine
a number of roots, including several fleshy roots, for the presence
of food material, making the tests used on seeds. =26.= _Study of
root-hairs._ Carefully germinate radish, turnip, cabbage, or other
seed, so that no delicate parts of the root will be injured. For
this purpose, place a few seeds in packing-moss or in the folds of
thick cloth or of blotting paper, being careful to keep them moist
and warm. In a few days the seed has germinated, and the root has
grown an inch or two long. Notice that, except at a distance of
about a quarter of an inch behind the tip, the root is covered with
minute hairs (Fig. 44). They are actually hairs; that is, root-hairs.
Touch them and they collapse, they are so delicate. Dip one of the
plants in water, and when removed the hairs are not to be seen.
The water mats them together along the root and they are no longer
evident. Root-hairs are usually destroyed when a plant is pulled out
of the soil, be it done ever so carefully. They cling to the minute
particles of soil (Fig. 46). The hairs show best against a dark
background. =27.= On some of the blotting papers, sprinkle sand;
observe how the root-hairs cling to the grains. Observe how they
are flattened when they come in contact with grains of sand. =28.=
_Root hold of plant._ The pupil should also study the root hold. Let
him carefully pull up a plant. If a plant grow alongside a fence or
other rigid object, he may test the root hold by securing a string to
the plant, letting the string hang over the fence, and then adding
weights to the string. Will a stake of similar size to the plant
and extending no deeper in the ground have such firm hold on the
soil? What holds the ball of earth in Fig. 50? =29.= _Roots exert
pressure._ Place a strong bulb of hyacinth or daffodil on firm-packed
earth in a pot; cover the bulb nearly to the top with loose earth;
place in a cool cellar; after some days or weeks, note that the bulb
has been raised out of the earth by the forming roots. All roots
exert pressure on the soil as they grow. Explain. =30.= _Response
of roots and stems to the force of gravity, or geotropism._ Plant a
fast-growing seedling in a pot so that the plumule extends through
the drain hole and suspend the pot with mouth up (_i.e._ in the
usual position). Or use a pot in which a plant is already growing,
cover with cloth or wire gauze to prevent the soil from falling,
and suspend the pot in an inverted position (Fig. 51). Notice the
behavior of the stem, and after a few days remove the soil and
observe the position of the root. =31.= If a pot is laid on one side,
and changed every two days and laid on its opposite side, the effect
on the root and stem will be interesting. =32.= If a fleshy root
is planted wrong end up, what is the result? Try it with pieces of
horse-radish root. =33.= By planting radishes on a slowly revolving
wheel the effect of gravity may be neutralized. =34.= _Region of
root most sensitive to gravity._ Lay on its side a pot containing a
growing plant. After it has grown a few days, wash away the earth
surrounding the roots. Which turned downward most decidedly, the tip
of root or the upper part? =35.= _Soil texture._ Carefully turn up
soil in a rich garden or field so that you have unbroken lumps as
large as a hen’s egg. Then break these lumps apart carefully with
the fingers and determine whether there are any traces or remains
of roots (Fig. 52). Are there any pores, holes, or channels made by
roots? Are the roots in them still living? =36.= Compare another lump
from a clay bank or pile where no plants have been growing. Is there
any difference in texture? =37.= Grind up this clay lump very fine,
put it in a saucer, cover with water, and set in the sun. After a
time it will have the appearance shown in the lower saucer in Fig.
43. Compare this with mellow garden soil. In which will plants grow
best, even if the plant-food were the same in both? Why? =38.= _To
test the effect of moisture_ on the plant, let a plant in a pot or
box dry out till it wilts; then add water and note the rapidity with
which it recovers. Vary the experiment in quantity of water applied.
Does the plant call for water sooner when it stands in a sunny window
than when in a cool shady place? Prove it. =39.= Immerse a potted
plant above the rim of the pot in a pail of water and let it remain
there. What is the consequence? Why? =40.= _To test the effect of
temperature on roots._ Put one pot in a dish of ice water, and
another in a dish of warm water, and keep them in a warm room. In a
short time notice how stiff and vigorous is the one whose roots are
warm, whereas the other may show signs of wilting. =41.= _The process
of osmosis._ Chip away the shell from the large end of an egg so as
to expose the uninjured membrane beneath for an area about as large
as a dime. With sealing-wax, chewing-gum, or paste stick a quill
about three inches long to the smaller end of the egg. After the
tube is in place, run a hat pin into it so as to pierce both shell
and membrane; or use a short glass tube, first scraping the shell
thin with a knife and then boring through it with the tube. Now set
the egg upon the mouth of a pickle jar nearly full of water, so that
the large end with the exposed membrane is beneath the water. After
several hours, observe the tube on top of the egg to see whether the
water has forced its way into the egg and increased its volume so
that part of its contents are forced up into the tube. If no tube is
at hand, see whether the contents are forced through the hole which
has been made in the small end of the egg. Explain how the law of
osmosis is verified by your result. If the eggshell contained only
the membrane, would water rise into it? If there were no water in
the bottle, would the egg-white pass down into the bottle? =42.=
_The region of most rapid growth._ The pupil should make marks with
waterproof ink (as Higgins’ ink or indelible marking ink) on any soft
growing roots. Place seeds of bean, radish, or cabbage between layers
of blotting paper or thick cloth. Keep them damp and warm. When stem
and root have grown an inch and a half long each, with waterproof ink
mark spaces exactly one quarter inch apart (Figs. 48, 49). Keep the
plantlets moist for a day or two, and it will be found that on the
stem some or all of the marks are more than one quarter inch apart;
on the root the marks have not separated. The root has grown beyond
the last mark.

NOTE TO TEACHER.--The microscopic structure of the root can be
determined only by the use of the compound microscope; but a good
general conception of the structure may be had by a careful attention
to the text and pictures and to explanations by the teacher, if such
microscopes are not to be had. See note at close of Chapter X.