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.
[Illustration: FIG. 40.—WHEAT GROWING UNDER DIFFERENT SOIL
TREATMENTS. Soil deficient in nitrogen; commercial nitrogen applied
to pot 3 (on right). ]
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.
[Illustration: FIG. 41.—NODULES ON ROOTS OF RED CLOVER.]
=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, sulphur, 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.
[Illustration: FIG. 42.—NODULES ON VETCH.]
=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
plough 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_.
[Illustration: FIG. 43.—TWO KINDS OF SOIL THAT HAVE BEEN WET AND THEN
DRIED. The loamy soil above remains loose and capable of growing
plants; the clay soil below has baked and cracked. ]
=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._
[Illustration: FIG. 44.—ROOT-HAIRS OF THE RADISH. ]
=Roots need Air.=—Corn on land that has been flooded by heavy
rains loses its green colour 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.
[Illustration: FIG. 45.—CROSS-SECTION OF ROOT, enlarged, showing
root-hairs.]
_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.
[Illustration: FIG. 46.—ROOT-HAIR, much enlarged, in contact with
the soil particles (_s_). Air-spaces at _a_; water-films on the
particles, as at _w_. ]
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
mould-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.
[Illustration: FIG. 47.—GROWING POINT OF ROOT OF INDIAN CORN.
_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.]
=Microscopic Structure of Roots.=—Near the end of any young root or
shoot the cells are found to differ from one another 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 centre (_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 by the pupil 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.
[Illustration: FIG. 48.—THE MARKING OF THE STEM AND ROOT. ]
=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.
[Illustration: FIG. 49.—THE RESULT.]
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=.
[Illustration: FIG. 50.—THE GRASP OF A PLANT ON THE PARTICLES OF
EARTH. A grass plant pulled in a garden.]
[Illustration: FIG. 51.—PLANT GROWING IN INVERTED POT.]
[Illustration: FIG. 52.—HOLES IN SOIL MADE BY ROOTS, now decayed.
Somewhat magnified.]
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 grows 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 behaviour 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 ten cent piece. 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.