I. OSMOSIS AND THE ACTION OF THE CELL
MATERIAL.—For experiments in osmosis provide fresh and boiled
slices of red beet, a fresh egg, a piece of ox bladder or some
parchment paper; glass tubing, thread, twine, elastic bands, salt
and sugar solutions. A common medicine dropper with the small end
cut off will answer instead of tubing for making an artificial
cell; or an eggshell may be used, by blowing out the contents
through a puncture in the small end, and carefully chipping away a
portion of the shell at the big end, leaving the lining membrane
intact. The different liquids can be put into the shell and the
exposed membrane placed in contact with the liquid in the glass,
by fitting over the latter a piece of cardboard with a hole in
the center large enough for the exposed surface to protrude
sufficiently to touch the water.
[Illustration: FIG. 71.—Artificial cell.]
=55. Object of the experiments.=—In order to understand clearly the
action of roots in absorbing nutrients from the soil, it will be
necessary to learn something about the movement of liquids through
the cells, upon which the physiological processes of the plant
depend. For this purpose make an artificial cell by tying a piece of
ox bladder or parchment paper tightly over one end of a small glass
tube, as shown in Fig. 71.
EXPERIMENT 39. HOW DOES ABSORPTION TAKE PLACE IN THE CELL?—(_a_)
Put some salt water in a wineglass, partly fill the tube of the
artificial cell with fresh water, and mark on the outside of both
vessels the height at which the contained liquid stands. Set the
tube in the glass of salt water and wait for results, having first
tested carefully to make sure that there are no leaks in the
membrane. After half an hour, notice whether there is any increase
of water in the glass, as indicated by the mark. If so, where did
it come from? Is there any loss of water in the tube? What has
become of it? How did it get out? Taste it to see if any of the
salt water has got in. Which is the heavier, salt water, or fresh?
(If you do not know, weigh an equal quantity of each.) In which
direction did the principal flow take place; from the heavier to
the lighter, or from the lighter to the heavier liquid?
(_b_) Put a sugar or salt solution in the tube, and clear, fresh
water in the glass, marking the height in each as before. Does the
liquid rise or fall in the tube? Does any of it escape into the
water of the glass, and if so, is it more or less than before?
Which now contains the denser fluid, the tube or the glass? What
principle governs the course of the liquid? Try the same experiment
with (_c_), the same liquid in both vessels, and notice whether
there is a greater flow in one direction than the other, as
indicated by a comparison with the marks on the outside. (_d_) Put
in the tube some of the white of a raw egg, insert in a glass of
pure water, and note the effect. (_e_) Reverse, with water in the
tube and white of egg in the glass. Does the water rise in the tube
as before? Test the contents for proteins; has any of the albumin
passed through the membrane into the tube?
EXPERIMENT 40. TO TEST THE BEHAVIOR OF LIVING AND DEAD CELLS.—Slice
a fresh piece of red beet into a vessel of water and of a boiled
one into another vessel of the same liquid at the same temperature.
What difference do you notice? Can you think of any reason why the
boiled one gives up its juices and the other one does not?
=56. Osmosis.=—The passage of liquids or of solids in solution
through membranes is known as _osmosis_. Our experiments have
shown that the principles governing the osmotic movement are: (1)
the passage of water from the thinner liquid toward the denser
takes place more rapidly than in the opposite direction; (2) the
rapidity of the transfer depends on the difference in density; (3)
crystallizable substances in solution, like sugar and salt, osmose
readily; (4) albuminous or gelatinous substances, such as the white
of an egg, osmose so slowly that the cell wall may be regarded as
practically impermeable to them.
=57. Osmosis a form of diffusion.=—Osmosis is related to diffusion
as a part to the whole. In other words, it is a name given to the
process when it takes place through a membrane, whether solid, as the
outer wall of the cell, or semi-fluid, as the inner wall of living
protoplasm. Diffusion may therefore take place without osmosis,
that is, in the absence of a membrane, as, for example, when we
sweeten our tea or coffee by allowing sugar to diffuse through it.
Many membranes offer little resistance to the osmotic movement of
crystallizable substances. Such membranes are said to be _permeable_.
Membranes which are not permeable to the dissolved solids, are called
_semi-permeable_, since they allow the diffusion of water but not of
the substances in solution. Living protoplasm is of this class. It is
only very slightly permeable to many substances toward which, when
dead, it acts as a permeable membrane.
=58. Absorption in living and dead cells.=—There is one great
difference between the action of the artificial cell used in the
foregoing experiments and that of the cells of which a living body
is built up. The living cell always has at least two membranes. One
of these, the cell wall, is readily permeable, while the other, the
protoplasm, is semi-permeable—that is, substances in solution usually
diffuse more or less slowly, while water diffuses rapidly. Hence
in the living cell the protoplasm exercises a power of absorption
independent of the cell wall, sometimes rejecting substances admitted
by the latter, sometimes retaining others to which it is permeable,
as shown in Exp. 40. In the boiled beet the protoplasm had been
killed and the red coloring matter passed through it unhindered,
while in the living one it was held back by the protoplasmic lining,
which is thus seen to control the absorptive properties of the cell.
=59. Plasmolysis.=—Cells can be killed or injured in other ways
than by heat; for example, by cold, by poisons, by starvation, and
by overfeeding through the use of too much fertilizer or too rich
a one. In this last case, the soil water becomes impregnated with
soluble matter from the manure, which may render it denser than the
sap in the roots. When this happens, it will cause the osmotic flow
to set outward and thus deplete the cell of its water; whence we have
the paradox that a cell, or even a whole plant, may be starved by
overfeeding. This action of osmosis in withdrawing the contents from
a cell is termed _plasmolysis_, and you can easily understand how
very important a knowledge of the principles governing it is to the
farmer in determining the application of fertilizers to his crops.
Dead cells, although powerless to carry on the life processes of a
plant, have nevertheless important uses in serving the purposes of
mechanical support and also to some extent in assisting in the work
of absorption, though their function here is a purely mechanical one.
[Illustration: FIG. 72.—Root of a tree enveloping a rock. The large
sycamore, whose base is partly concealed by the trumpet creeper on
the left of the picture, is growing in very hard, stony soil, and
one of its main roots has molded itself so completely to the ledge
of rock protruding on the right, that when a portion of it was torn
away, as shown where the light streak ends at _a_, the impress of its
fibers was so strongly marked on the rock as to give the latter the
appearance of a petrified root.]
=60. Selective absorption.=—Different plants through their roots
absorb different substances from the soil water, or the same
substance in varying degrees. Hence, one kind of crop will exhaust
the soil of certain minerals while leaving other kinds intact, or
very little diminished; and _vice versa_, another kind will take
up abundantly what its predecessor has rejected. In this sense,
plants are said to exercise a selective power in the absorption
of nutrients. The expression must not be understood, however, as
implying any kind of volitional discrimination. It is merely a short
and convenient way of saying that the cells of different plants
possess different degrees of permeability to certain substances, some
being more permeable to one thing, some to another. But beyond this
rejection of untransmissible substances there is no active power of
discrimination, any substance that can pass through the cell wall and
its protoplasmic lining being taken in, whether useful, unnecessary,
or even harmful. These may, however, be got rid of by excretion, as
the superfluous water taken in with dissolved minerals is exhaled
from the leaves; or if incapable of passing out by osmosis, rendered
harmless and retained in the form of the curious “crystalloids”
found in various parts of plants. But while the kind of selection
exercised by vegetable cells implies no power of choice, as a matter
of fact those substances most used by the plant in carrying on its
life processes are absorbed in much greater quantities than others,
being transferred to parts where growth or other changes in the plant
tissues are going on, and there used up in the work of nutrition, or
excreted in part as waste products. In either case their passage from
cell to cell will give rise to a continuous osmotic current in that
direction, and the absorption of new matter will go on in proportion
to the amounts used up.
[Illustration: FIG. 73.—Roots of elm and sycamore contending for
possession of the soil on a rocky bluff on the Potomac.]
=61. Definition.=—Tissue is a word used to denote any animal or
vegetable substance having a uniform structure organized to perform
a particular office or function. Thus, for instance, we have bony
tissue and muscular tissue in animals; that is, tissue made of bone
substance and muscle substance and doing the work of bone and muscle
respectively. Likewise in plants, we have strengthening tissue
made up of hard, thick-walled cells, serving mainly for purposes
of mechanical support, and vascular tissue, made up of conducting
vessels for conveying sap—and so on, for every separate function.
Practical Questions
1. Why do raspberries and strawberries have a flabby, wilted look
if sugar has been put on them too long before they are served? (7,
56.)
2. Where has the juice gone? What caused it to go out of the
berries? (56, 59.)
3. Is a knowledge of the principles governing osmosis of any
practical use to the housekeeper?
4. Why cannot roots absorb water as freely in winter as in summer?
(Suggestion: which is the heavier, cold or warm water?)
5. Why does fertilizing too heavily sometimes injure a crop? (59.)
6. Do you see any apparent contradiction between the action
of plasmolysis and the selective power of protoplasm? Can you
reconcile it?
7. If a piece of beet that has been frozen is placed in water
it will behave just as the slice of boiled beet did in Exp. 40;
explain. (58, 59.)
II. MINERAL NUTRIMENTS ABSORBED BY PLANTS
MATERIAL.—A dozen or two each of different kinds of seeds and
grains. A small portion from a growing shoot of a woody and a
herbaceous land plant, and of some kind of succulent water or marsh
plant, such as arrow grass (_Sagittaria_), water plantain, etc.
APPLIANCES.—A pair of scales; a lamp, stove, or other means of
burning away the perishable parts of the specimens to be studied.
EXPERIMENT 41.—DO THE TISSUES OF PLANTS CONTAIN MINERAL
MATTER?—Take about a dozen each of grains and seeds of different
kinds, weigh each kind separately, and then dry them at a high
temperature, but not high enough to scorch or burn them. After they
have become perfectly dry, weigh them again. What proportion of the
different seeds was water, as indicated by their loss of weight in
drying?
Burn all the solid part that remains, and then weigh the ash. What
proportion of each kind of seed was of incombustible material? What
proportion of the solid material was destroyed by combustion?
EXPERIMENT 42.—DO THEY CONTAIN DIFFERENT KINDS AND QUANTITIES OF
MINERALS?—Test in the same way the fresh, active parts of any kind
of ordinary land plant (sunflower, hollyhock, pea vines, etc.), and
of some kind of succulent water or marsh plant (Sagittaria, water
lily, fern). Do you notice any difference in the amount of water
given off and of solid matter left behind? In the character of the
ashes left? Have you observed in general any difference between the
ashes of different woods; as, for instance, hickory, pine, oak?
Compare with the residue left in Exp. 21; would you judge that the
residual substances are of the same composition?
[Illustration: FIG. 74.—Water cultures of buckwheat, showing effect
of the lack of the different food elements: 1, with all the elements;
2, without potassium; 3, with soda instead of potash; 4, without
calcium; 5, without nitrates or ammonia salts.]
=62. Essential constituents.=—The composition of the ash of any
particular plant will depend upon two things: the absorbent capacity
of the plant itself and the nature of the substances contained in the
soil in which it grows. But chemical analysis has shown that however
the ashes may vary, they always contain some proportion of the
following substances: potassium (potash), calcium (lime), magnesium,
phosphorus, and (in green plants) iron. These elements occur in all
plants, and if any one of them is absent, growth becomes abnormal if
not impossible.
The part of the dried substances that was burned away after expelling
the water consists, in all plants, mainly of carbon, hydrogen,
oxygen, nitrogen, and sulphur, in varying proportions. These five
rank first in importance among the essential elements of vegetable
life, and without them the plant cell itself, the physiological unit
of vegetable structure, could not exist. They compose the greater
part of the substance of every plant, carbon alone usually forming
about one half the dry weight. Other substances may be present in
varying proportions, but the two groups named above are found in all
plants without exception, and so we may conclude that (with the
possible addition of chlorine) they form the indispensable elements
of plant food. Carbon, hydrogen, oxygen, nitrogen, sulphur, and
phosphorus compose the structure of which the plant is built. The
other four ingredients do not enter into the substance as component
parts, but aid in the chemical processes by which the life functions
of the plant are carried on, and are none the less essential elements
of its food. Figure 74 shows the difference between a plant grown in
a solution where all the food elements are present, and others in
which some of them are lacking.
[Illustration: FIG. 75.—Roots of soy bean bearing tubercle-forming
bacteria.]
=63. How plants obtain their food material.=—Plants obtain their
supply of the various mineral salts from solutions in the soil
water which they absorb through their roots. With a few doubtful
exceptions, they cannot assimilate their food unless it is in a
liquid or gaseous form. Of the gases, carbon dioxide, oxygen, and
hydrogen can be freely absorbed from the air, or from water with
various substances in solution, but most plants are so constituted
that they cannot absorb free nitrogen from the air; they can take it
only in the form of compounds from nitrates dissolved in the soil,
and hence the importance of ammonia and other nitrogenous compounds
in artificial fertilizers. Some of the pea family, however, bear
on their roots little tubers formed by minute organisms called
bacteria, which have the power of extracting nitrogen directly from
the free air mingled with the soil; and hence the soil in which these
tuber-bearing legumes decay is enriched with nitrogen in a form ready
for use.
Practical Questions
1. Could any normal plant grow in a soil from which nitrogen was
lacking? Potash? Lime? Phosphorus? (62.)
2. Could it live in an atmosphere devoid of oxygen? Nitrogen?
Carbon dioxide? (62.)
3. Why are cow peas or other legumes planted on worn-out soil to
renew it? (63.)
4. Is the same kind of fertilizer equally good for all kinds of
soil? For all kinds of plants? (60, 62.)
5. Why does too much watering interfere with the nourishment of
plants? (Exps. 26, 27.)
6. Are ashes fit for fertilizers after being leached for lye? (62.)
7. Why will plants die, or make very slow growth, in pots, unless
the soil is renewed occasionally? (60, 62.)
III. STRUCTURE OF THE ROOT
MATERIAL.—Taproot of a young woody plant not over one or two years
old; apple and cherry shoots make good specimens. For showing root
hairs, seedlings of radish, turnip, or oat are good, also roots of
wandering Jew grown in water; for the rootcap, corn, sunflower,
squash.
[Illustration: FIG. 76.—Cross section of a young taproot; _a_, _a_,
root hairs; _b_, epidermis; _c_, cortical layer; _d_, fibrovascular
cylinder. Note the absence of medullary rays during the first year of
growth.]
=64. Gross anatomy of the root.=—Cut a cross section of any woody
taproot, about halfway between the tip and the ground level,
examine it with a lens, and sketch. Label the dark outer covering,
_epidermis_, the soft layer just within that, _cortex_, the hard,
woody axis that you find in the center, _vascular cylinder_, and
the fine silvery lines that radiate from the center to the cortex,
_medullary rays_ (in a very young root these will not appear). Cut
a section through a root that has stood in coloring fluid for about
three hours and note the parts colored by the fluid. What portion of
the root, would you judge from this, acts as a conductor of the water
absorbed from the ground?
Make a longitudinal section passing through the central portion of
the root and extending an inch or two into the lower part of the
stem. Do you find any sharp line of division between them? Notice the
hard, woody axis that runs through the center. This is the vascular
cylinder and contains the conducting vessels, the cut ends of which
were shown in cross section in Fig. 76.
[Illustration: FIG. 77.—Verti-section of branching root, showing the
branches, _n_, _n_, originating in the central axis, _f_, and passing
through the cortex, _r_, _r_.]
=65. Distinctions between root and stem.=—Pull off a branch from
the stem and one from the root; which comes off the more easily?
Examine the points of attachment of the two and see why this is so.
This mode of branching from the central axis instead of from the
external layers, as in the stem, is one marked distinction between
the structure of the two organs. In stems, moreover, branches occur
normally above the points of leaf insertion at the nodes (46), while
in the root they tend to arrange themselves in straight vertical
rows. The shoots and cions that often originate from them are not
normal root branches, but outgrowths from irregular or _adventitious_
buds, that may occur on any part of a plant. The root is not divided
into nodes like the stem, and never bears leaves.
[Illustration: FIG. 78.—Root of a tree on the side of a gulley,
acting as stem.]
=66. The active part of the root.=—It is only the newest and most
delicate parts of the root that produce hairs and are engaged in the
active work of absorption, the older parts acting mainly as carriers.
Hence, old roots lose much of their characteristic structure and
take on more and more of the office of the stem, until there is
practically no difference between them. On the sides of gullies,
where the earth has been washed from around the trees, we often
see the upper portion of the root covered with a thick bark and
fulfilling every office of a true stem.
=67. Minute structure of the root.=—(_a_) Mount in water and place
under the microscope a portion of the root of an oat or radish
seedling containing a number of hairs. In studying the thin,
transparent roots of very young seedlings a section will not be
necessary. Observe whether the hairs originate from the epidermis or
from the interior. Are they true roots, or mere outgrowths from the
cells of the epidermis? Do they consist of a single cell or a number
of cells each? Notice what very thin cell walls the hairs have; is
there any advantage in this? The interior, transparent portion of
the hair contains the sap, and the protoplasm forms a thin lining on
the inner surface of the wall; why not the sap next the wall and the
protoplasm in the interior? (58, 60.)
[Illustration: FIG. 79.—Longitudinal section through the tip of
a young root, somewhat diagrammatic: _h_, _h_, root hairs; _ep_,
epidermis; _a_, cortex; _b_, central cylinder; _e_, sheath of the
cylinder (endodermis); _g_, growing point; _c_, root cap; _d_, dead
and dying cells loosened from the extremity of the cap.]
(_b_) Next examine a portion of the body of the root and try to
make out the parts as shown in Fig. 79, and compare them with your
observations in 64. The light line running through the middle is
the _central cylinder_, up which the water passes, as was shown by
the colored liquid in 64. Outside this is a darker portion (_a_,
Fig. 79), corresponding to the cortex (_rr_, Fig. 77). Besides other
uses, the cortex serves to prevent the loss of water as it passes up
to the stem, and also, in fleshy roots like the carrot and turnip,
for the storage of nourishment. Its innermost row of cells is
thickened into the sheath, or _endodermis_ (_e_), which serves as an
additional protection to the conducting tissues. The extreme outer
layer, from the cells of which the root hairs are developed, is, as
already stated, the epidermis, and in the older and more exposed
parts of perennial roots is displaced by the bark, which becomes
indistinguishable from that of the stem. (66.)
(_c_) Look at the tip of the root for a loose structure (_c_) fitting
over it like a thimble. This is the rootcap. Do you see any loose
cells that seem to have broken away from it? These are old cells that
have been pushed to the front by the formation of new growth back of
them, and, being of no further use, are rubbed off by friction as the
root bores its way through the soil. Draw a longitudinal section of
the root as it appears under the microscope, labeling all the parts.
If they cannot be made out distinctly in the specimen examined, use
sections of young corn or bean roots, which are larger and show the
parts more distinctly.
[Illustration: FIG. 80.—Cross section of a young root, magnified:
_h_, hairs; _a_, cortex; _b_, central cylinder; _e_, sheath or
endodermis; _ep_, epidermis; _sp_, cut ends of the ducts.]
(_d_) Place under the microscope a thin cross section through the
hairy portion of a primary root of a bean or pea seedling, and try
to make out the parts noted above and shown in cross section in
Fig. 80. Make a sketch of what you see, labeling all the parts you
can recognize. Show in your drawing the differences in the size
and shape of the cells composing the different tissues. Notice in
the central cylinder (Fig. 80) several groups of what look in the
section like little round pits, or holes, _sp_. These are the cut
ends of large-sized tubes or _ducts_ that convey the water absorbed
by the roots to the stem. Each set of these tubes, together with a
number of smaller ones belonging to the same group, constitutes a
_fibrovascular bundle_—a very important element in the structure of
all roots and stems, as these bundles make up the conducting system
of the plant body.
IV. THE WORK OF ROOTS
MATERIAL.—Germinating seedlings of radish, bean, corn, etc.;
a potted plant of calla, fuchsia, tropæolum, touch-me-not
(_Impatiens_), or corn; a plant that has been growing for some time
in a porous earthen jar.
APPLIANCES.—Glass tumblers; coloring fluid; wax; some coarse
netting; dark wrapping paper, or a long cardboard box; a sheet of
oiled paper; some half-inch glass tubing; a few inches of rubber
tubing; an ounce of mercury; some blue litmus paper; a flower pot
full of earth; a few handfuls of sand, clay, and vegetable mold; a
pair of scales; a half dozen straight lamp chimneys, or long-necked
bottles from which the bottoms have been removed as directed in
Exp. 53.
EXPERIMENT 43. USE OF THE EPIDERMIS.—Cut away the lower end
of a taproot; seal the cut surface with wax so as to make it
perfectly water-tight, and insert it in red ink for at least half
the remaining length, taking care that there is no break in the
epidermis. Cut an inch or two from the tip of the lower piece, or
if material is abundant, from another root of the same kind, and
without sealing the cut surface, insert it in red ink, beside the
other. At the end of three or four hours, examine longitudinal
sections of both pieces. Has the liquid been absorbed equally by
both? If not, in which has it been absorbed the more freely? What
conclusion would you draw from this, as to the passage of liquids
through the epidermis?
From this experiment we see that the epidermis, besides protecting
the more delicate parts within from mechanical injury by hard
substances contained in the soil, serves by its comparative
imperviousness to prevent evaporation, or the escape of the sap by
osmosis as it flows from the root hairs up to the stem and leaves.
EXPERIMENT 44. TO SHOW THAT ROOTS ABSORB MOISTURE.—Fill two pots
with damp earth, put a healthy plant in one, and set them side by
side in the shade. After a few days examine by digging into the
soil with a fork and see in which pot it is drier. Where has the
moisture gone? How did it get out?
EXPERIMENT 45. TO SHOW THAT ROOTS SHUN THE LIGHT.—Cover the top
of a glass of water with thin netting, and lay on it sprouting
mustard or other convenient seed. Allow the roots to pass through
the netting into the water, noting the position of root and stem.
Envelop the sides of the glass in heavy wrapping paper, admitting
a little ray of light through a slit in one side, and after a few
days again observe the relative position of the two organs. How is
each affected by the light?
EXPERIMENT 46. TO FIND OUT WHETHER ROOTS NEED AIR.—Remove a plant
from a porous earthenware pot in which it has been growing for some
time; the roots will be found spread out in contact with the walls
of the pot instead of embedded in the soil at the center. Why is
this?
EXPERIMENT 47. TO SHOW THAT ROOTS SEEK WATER.—Stretch some coarse
netting covered with moist batting over the top of an empty
tumbler. Lay on it some seedlings, as in Exp. 45, allowing the
roots to pass through the meshes of the netting. Keep the batting
moist, but take care not to let any of the water run into the
vessel. Observe the position of the roots at intervals, for twelve
to twenty-four hours, then fill the glass with water to within
10 millimeters (a half inch, nearly) or less of the netting, let
the batting dry, and after eight or ten hours again observe the
position of the roots. What would you infer from this experiment as
to the affinity of roots for water?
EXPERIMENT 48. WHAT BECOMES OF THE WATER ABSORBED BY ROOTS.—Cover
a calla lily, young cornstalk, sunflower, tropæolum, or other
succulent herb with a cap of oiled paper to prevent evaporation
from the leaves, set the pot containing it in a pan of tepid water,
and keep the temperature unchanged. After a few hours look for
water drops on the leaves. Where did this water come from? How did
it get up into the leaves?
EXPERIMENT 49. TO SHOW THE FORCE OF ROOT PRESSURE.—Cut off the stem
of the plant 6 or 8 centimeters (3 or 4 inches) from the base. Slip
over the part remaining in the soil a bit of rubber tubing of about
the same diameter as the stem, and tie tightly just below the cut.
Pour in a little water to keep the stem moist, and slip in above,
a short piece of tightly fitting glass tubing. Watch the tube for
several days and note the rise of water in it. The same phenomenon
may be observed in the “bleeding” of rapidly growing, absorbent
young shoots, such as grape, sunflower, gourd, tobacco, etc., if
cut off near the ground in spring when the earth is warm and moist.
By means of an arrangement like that shown in Fig. 81, the force
of the pressure exerted can be measured by the displacement of the
mercury. This flow cannot be due to the giving off of moisture
by the leaves, since they have been removed. Their action, when
present, by causing a deficiency of moisture in certain places may
influence the direction and rapidity of the current, but does not
furnish the motive power, which evidently comes, in part at least,
from the roots, and is the expression of their absorbent activity.
[Illustration: FIG. 81.—Arrangement for estimating the force of root
pressure: _s_, stub of the cut stem; _g_, glass tubing joined by
means of the rubber tubing, _t_, to the stem; _m_, mercury forced up
the glass tube by water, _w_, pumped from the soil by the roots.]
EXPERIMENT 50. TO SHOW THAT ROOTS CAUSE THE OCCURRENCE OF
ACIDS.—Lay a piece of blue litmus paper on a board or on a piece
of glass slightly tilted at one end to secure drainage. Cover the
surface with an inch of moist sand and plant in it a number of
healthy seedlings. Acids have the property of changing blue litmus
to red; hence, if you find any red stains on the paper where the
roots have penetrated, what are you to conclude?
Carbon dioxide has a slight acid reaction and is caused to form in
varying quantities by all roots. Probably other substances, and
these not a few, are actually excreted.
EXPERIMENT 51. CAN THE ABSORBENT POWER OF ROOTS BE INTERFERED
WITH?—Place the roots of a number of seedlings with well-developed
hairs in a weak solution of saltpeter—10 grams (about ⅓ of an
ounce) to a pint of water, and others in a stronger solution—say 30
grams, or 1 ounce, to a pint. Try the same experiment with weak and
strong solutions of any conveniently obtainable liquid fertilizer.
After 45 minutes or an hour examine the roots under a lens and note
the change that has taken place. What has gone out of them? What
caused the loss of the contained sap?
EXPERIMENT 52. TO TEST THE WEIGHT OF SOILS.—Thoroughly dry and
powder a pint each of sand and clay, measure accurately, and
balance against each other in a pair of scales. Which weighs more,
bulk for bulk, a “light” soil, or a “heavy” one? (77.)
EXPERIMENT 53. TO TEST THE CAPACITY OF SOILS FOR ABSORBING AND
RETAINING MOISTURE.—Arrange, as shown in Fig. 82, a number of
long-necked bottles from which the bottom has been removed. This
can be done by making a small indentation with a file at the point
desired and leading the break round the circumference with the end
of a glowing wire or a red-hot poker. The crack will follow the
heated object with sufficient regularity to answer the purpose.
Tie a piece of thin cloth over the mouth of each bottle and invert
with the necks extending an inch or two into empty tumblers placed
beneath. Fill all to the same height with soils of different
kinds—sand, clay, gravel, loam, vegetable mold, etc.—and pour over
each the same quantity of water from above. Watch the rate at which
the liquid filters through into the tumblers. Which loses its
moisture soonest? Which retains it longest?
[Illustration: FIG. 82.—Apparatus for testing the capacity of soils
to take in and retain moisture.]
Next leave the soils in the bottles dry, fill the tumblers up to
the necks of the bottles, and watch the rate at which the water
rises in the different ones. The power of soils to absorb moisture
is called _capillarity_. Which of your samples shows the highest
capillarity? Which the lowest? Do you observe any relation between
the capillarity of a soil and its power of retention?
=68. Roots as holdfasts.=—One use of ordinary roots is to serve as
props and stays for anchoring plants to the soil. Tall herbs and
shrubs, and vegetation generally that is exposed to much stress of
weather, are apt to have large, strong roots. Even plants of the same
species will develop systems of very different strength according as
they grow in sheltered or exposed places.
[Illustration: FIG. 83.—Dandelion: _a_, common form, grown in plains
region at low altitude; _b_, alpine form.]
=69. Root pull.=—Roots are not mere passive holdfasts, but exert
an active downward pull upon the stem. Notice the rooting end of a
strawberry or raspberry shoot and observe how the stem appears to
be drawn into the ground at the rooting point. In the leaf rosettes
of herbs growing flat on the ground or in the crevices of walls and
pavements, the strong depression observable at the center is due to
root pull. (Fig. 84.)
[Illustration: FIG. 84.—Raspberry stolon showing root pull.]
=70. Storage of food.=—Another office of roots is to store up food
for the use of the plant. This is done chiefly in the tissues of
fleshy roots and tubers, and gives to them their great economic
value. Next to grains and cereals, roots probably furnish a larger
portion of food to the human race than any other crop. In addition to
this they are also the source of valuable drugs, condiments, and dyes.
=71. Absorption and conveyance of sap.=—But the most important
function of roots is that of absorption. By their action the soil
water and the minerals contained in it are drawn up into the plant
body and made available for conversion by the leaves into organic
foods, as will be explained in another chapter. From the nature
of their function, most roots have naturally a strong affinity
for water, and its presence or absence has a marked influence on
their direction of growth, being often sufficient to overcome that
of geotropism (Exp. 47). There are many trees and shrubs, notably
willow, sweet bay, red birch, and the like, that grow best on the
banks of streams and ponds, where their roots can have direct access
to water. Excess of moisture, however, is injurious to most land
plants by preventing the roots from getting sufficient air for
respiration.
=72. The conditions of absorption.=—The sap in the root cells is
normally denser than the water in the soil, so there is a continuous
flow from the latter to the former. But if, for any reason, the
density of the liquids should be reversed, the flow would set in the
opposite direction, and if continued long enough, the strength of the
plant would be literally “sapped” by the exhaustion of its tissues,
so that it would die. What is this process of cell exhaustion called?
=73. The use of acid secretions to the root.=—It was shown in Exp.
50 that carbon dioxide and probably other substances occur in the
immediate vicinity of roots. Carbon dioxide is an active agent in
dissolving the various mineral matters contained in the soil, and as
these last can be absorbed only in a liquid or a gaseous state (63),
the advantage to the root as an absorbent organ, of being able to
secrete such active solvents, is obvious.
[Illustration: FIG. 85.—A natural root etching, found on a piece of
slate.]
=74. Relation of roots to the soil.=—In order to perform their work
of absorption, roots must have access to a suitable soil. To produce
the best results a soil must contain (1) all the essential mineral
constituents (62); (2) moisture for dissolving these materials; and
(3) air enough to supply the oxygen which is necessary to the life
processes of all green plants.
=75. Composition of soils.=—Sand, clay, and humus, or vegetable
mold, with the various substances dissolved in them, constitute the
basis of cultivated soils. A mixture of sand, clay, and humus is
called loam. When the proportion of humus is very large and well
decomposed, the mixture is called _muck_. Pure sand contains but
little nourishing matter and is too porous to retain water well. Pure
clay is too compact to be easily permeable to either air or water.
Most soils are composed of a mixture of the two with vegetable mold
in varying proportions, giving a sandy loam, or a clay loam, as the
case may be.
=76. Tillage.=—The advantages of tillage are: (_a_) that by breaking
up the hard lumps it renders the soil more permeable to air and water
and more easily penetrable by the roots in their search for food;
(_b_) the covering of loose, friable earth left by the plow and the
harrow acts as a mulch, and by shading the soil below, prevents
too rapid a loss of water by evaporation. Where the essential food
ingredients are present, good tillage counts for more in making a
crop than the original quality of the soil.
=77. Light and heavy soils.=—These terms are used by farmers not
in relation to the weight of soils, but in reference to the ease
or difficulty with which they are worked. Light soils contain a
preponderance of sand; heavy ones, of clay.
Practical Questions
1. Will plants grow better in an earthen pot or a wooden box than
in a vessel of glass or metal? Why? (Exp. 46.)
2. Which absorb more from the soil, plants with light roots and
abundant foliage, or those with heavy roots and scant foliage?
(Suggestion: roots absorb from the soil; leaves, mainly from the
air.)
3. Why are willows so generally selected for planting along the
borders of streams in order to protect the banks from washing? (71.)
4. Why are the conducting tissues of roots at the center instead of
near the surface as in stems? (67, _b_.)
5. Why does corn never grow well in swampy ground? (74; Exp. 46.)
6. Why are fleshy roots so much larger in cultivated plants than in
wild ones of the same species? (74, 76.)
7. When the use of a particular kind of fertilizer causes the
leaves of the plants to which it has been applied to turn brown, so
that the farmer says they have been “burned” by it, to what cause
is the trouble due? (59, 72.)
8. Why do farmers speak of turnips and other root crops as “heavy
feeders”? (70, 71.)
9. Which is more exhausting to the soil, a crop of beets, or one of
oats? Onions, or green peas? (See 2, suggestion.)
10. Why will inserting the end of a wilted twig in warm water
sometimes cause it to revive? (Exps. 48, 49.)
V. DIFFERENT FORMS OF ROOTS
MATERIAL.—Examples of taproots: bean, pea, cotton, maple seedlings,
or any kind of very young woody root. Fibrous: any kind of grass
or grain. Fleshy: parsnip, turnip, carrot, dahlia, sweet potato.
Water: duckweed, pondweed, or a cutting of wandering Jew grown
in water. Parasitic: mistletoe, dodder, beech drops. Aërial and
adventitious: the aërial roots of old scuppernong vines, climbing
roots of ivy and trumpet vine, prop roots from the lower nodes of
cornstalks and sugar cane.
=78. Basis of distinction.=—Roots vary in form and external structure
according to their origin, function, and surroundings. In reference
to the first, they are classed as primary or secondary; in regard to
the second, as dry or fleshy; while as to surroundings, they may be
adapted to either the soil, water, air, or the parasitic habit. Soil
roots are the normal form. According to their mode of growth they are
either fibrous or axial.
[Illustration: PLATE 3.—Aërial roots of a Mexican “strangling” fig,
enveloping the trunk of a palm (_From_ “Rep’t. Mo. Bot. Garden”).]
=79. Taproots.=—These are the common form of the axial type. Compare
the root of any young hardwood cion a year or two old with one
of a mature stalk of corn or other grain, and with the roots of
seedlings of the same species. Notice the difference in their mode
of growth. In the first kind a single stout prolongation called a
taproot proceeds from the lower end of the hypocotyl and continues
the axis of growth straight downward, unless turned aside by some
external influence. A taproot may be either simple, as in the turnip,
radish, and dandelion, or branched, as in most shrubs and trees. In
the latter case the main axis is called the primary root, and the
branches are secondary ones.
[Illustration: FIG. 86.—Branched taproot of maple.]
=80. Fibrous and fascicled roots.=—Where the main axis fails
to develop, as in the corn and grasses generally, a number of
independent branches take its place, forming what are known as
fibrous roots. Both fibrous and taproots may be either hard or
fleshy. The turnip and carrot are examples of fleshy taproots, the
dahlia and rhubarb of fascicled roots. The function of both is the
storage of nourishment. The sweet potato is an example of a tuberous
root.
=81. Practical importance of this distinction.=—The difference
between axial and fibrous roots has important bearings in
agriculture. The first kind, which are characteristic of most
dicotyls, strike deep and draw their nourishment from the lower
strata of the soil, while the fibrous and fascicled, or radial kinds,
as we may call them for want of a better name, spread out near the
surface and are more dependent on external conditions.
[Illustration: FIG. 87.—Fibrous root.]
=82. Roots that grow above ground.=—The kinds of roots that have
just been considered are all subterranean, and bring the plant
into relation with the earth, whether for the purpose of absorbing
nourishment, or of mechanical support, or, as in the majority of
cases, for both. Many plants, however, do not get their mineral
nutrients directly from the soil, and these give rise to various
forms suited to other conditions of alimentation.
=83. Adventitious roots.=—This name applies to any kinds of roots
that occur on stems, or in other unusual positions. They may be
considered as intermediate between the two classes named in 81; for
while their starting point is above ground, they generally end by
fixing themselves in the soil, where they often function as normal
roots. Familiar examples are the roots that put out from the lower
nodes of corn and sugar cane stalks, and serve both to supply
additional moisture and to anchor the plant more firmly to the soil.
Most plants will develop adventitious roots if covered with earth, or
even if merely kept in contact with the ground. The gardener takes
advantage of this capacity when he propagates by cuttings and layers.
=84. Water roots.=—These are generally white and threadlike and more
tender and succulent than ordinary soil roots, because they have less
work to do. Floating and immersed plants, such as bladderwort and
hornwort (_Ceratophyllum_) have no need of absorbent roots, since the
greater part of their surface is in contact with water and can absorb
directly what is needed.
Land plants will often develop water roots and thrive for a time if
the liquid holds in solution a sufficient quantity of air and mineral
nutrients. Place a cutting of wandering Jew in a glass of clear
water, and in from four to six days it will develop beautiful water
roots in which both hairs and cap are clearly visible to the naked
eye.
=85. Haustoria=, from a Latin word meaning to drain, or exhaust,
is a name given to the roots of parasitic plants, or such as live
by attaching themselves to some other living organism, from which
they draw their nourishment ready made. Their roots are adapted
to penetrating the substance of the _host_, as their victim is
called, and absorbing the sap from it. Dodder and mistletoe are the
best-known examples of plant parasites, though the latter is only
partially parasitic, as it merely takes up the sap from the host and
manufactures its own food by means of its green leaves.
[Illustration: FIG. 88.—Beech root: _A_, grown in unsterilized wood
humus: _p_, strands of fungal hyphæ, associated at _a_, with humus;
_B_, grown in wood humus freed from fungus by sterilization—it is not
provided with fungal hyphæ, and has root hairs, _h_. (_A_ and _B_
both several times magnified.)]
=86. Saprophytes.=—Akin to parasites are saprophytes, which live
on dead and decaying vegetable matter. They are only partially
parasitic and do not bear the haustoria of true parasites. Many of
them, of which the Indian pipe (_Monotropa_) and coral root are
familiar examples, obtain their nourishment in part, at least, by
association with certain saprophytic fungi, which enmesh their roots
in a growth of threadlike fibers that take the place of root hairs
and absorb organic food from the rich humus in which these plants
grow. Such growths are called _mycorrhiza_, meaning “fungal roots.”
Similar associations are formed by some of the higher plants also.
The rootlets of the common beech and of certain of the pine family,
for instance, are often enveloped in a network of fungus fibers,
and in this case root hairs are developed very poorly, or not at
all. Besides greatly increasing the absorbent surface by their
ramification through the soil, the mycorrhizal threads may possibly
benefit the plant in other ways also, as, for instance, by bringing
about chemical changes that might aid in the work of nutrition.
[Illustration: FIG. 89.—An air plant (_Tillandsia_), growing on the
underside of a bough.]
[Illustration: FIG. 90.—A single strand of _Tillandsia usneoides_, a
rootless epiphyte belonging to the pineapple family; better known as
the “Spanish moss” that drapes the boughs of trees so conspicuously
in the warm parts of America. Two-thirds natural size. (Photographed
by C. F. O’Keefe.)]
=87. Epiphytes, or air plants.=—In the proper meaning of the word
these are not parasitic, but use their host merely as a mechanical
support to bring them into better light relations. The name, however,
is loosely applied to all plants that find a lodgment on the trunks
and branches of trees, whether parasites or true epiphytes that draw
no nourishment from the host. Not infrequently the latter is killed
by them through suffocation, overweighting, or the constriction of
the stems by close clinging twiners.
=88. Aërial roots= are such as have no connection at all with the
soil or with any host plant, except as they may lodge upon the trunks
and branches of trees for a support. In other than purely epiphytic
plants, which get all their nourishment from the air, they are
generally subsidiary to soil roots, like the long dangling cords that
hang from some species of old grapevines; or they subserve other
purposes altogether than absorbing nourishment, as the climbing roots
of the trumpet vine and poison ivy. A very remarkable development
of aërial roots takes place in the “strangling fig” of Mexico and
Florida, which begins life as a small epiphyte, from seeds dropped by
birds on the boughs or trunks of trees. When it gets well started,
the young plant sends down enormous aërial roots, which find their
way to the ground, and in time so completely envelop the host that it
is literally strangled to death (Plate 3, p. 73). When this support
is removed, the sheathing roots take its place and become to all
intents and purposes the stem of the fig tree, which now leads an
independent life.
[Illustration: FIG. 91.—Root system of a tobacco plant.]
=89. The root system.=—The entire mass of roots belonging to a
plant, with all its ramifications and subdivisions, composes a root
system. The extent of root expansion is in general about equal to
that of the crown, thus bringing the new and active parts under the
drip of the boughs where the moisture is most abundant. Some plants
have root systems out of all seeming proportion to their size. A
catalpa seedling six months old showed, by actual measurement,
250 feet of root growth, and it is estimated that the roots of a
thrifty cornstalk, if laid end to end, would extend a mile. In the
development of the root system, a great deal depends upon external
conditions. In a poor, dry soil, the roots have to travel farther in
search of a livelihood, and so a larger system has to be developed
than in a more favorable location.
Practical Questions
1. Which is better to succeed a crop of turnips on the same land,
hay or carrots? (81.)
2. Write out what you think would be a good rotation for four or
five successive crops based on the forms of the roots.
3. Study the following rotations and give your opinion about them,
on the same principle. Suggest any improvements that may occur
to you, and give a reason for the change. Beets, barley, clover,
wheat; cotton, oats, peas, corn; oats, melons, turnips; cotton,
oats, corn and peas mixed, melons; cotton, hay, corn, peas.
4. Give three good reasons in favor of a rotation over a
single-crop system. (24, 60, 62, 81.)
5. Which will require deeper tillage, a bed of carrots or one of
strawberries? (81.)
6. Explain why some plants keep green and fresh when the surface of
the soil is dry, while others wilt or die. (81, 89.)
7. Which will better withstand drought, a crop of alfalfa or one of
Indian corn? Why? (81.)
8. Which will interfere less with the trees if planted in an
orchard, beets or onions? (81.)
9. Ought a crop of hemp and tobacco to succeed each other on the
same land? (81, 89.)
10. Why does a gardener manure a grass plot by scattering the
fertilizer on the surface, while he digs around the roses and
lilacs and deposits it under ground? (81.)
11. Do the adventitious roots of such climbers as ivy and trumpet
vine draw any nourishment from the objects to which they cling?
(83-88.)
12. How can you tell?
13. Do partial dependents of this kind injure trees by climbing
upon them; and if so, how? (87, 88.)
14. What is the use of the aërial roots of the scuppernong grape?
(88.)
15. Is the resurrection fern (_Polypodium incanum_), that grows on
tree trunks in our Southern States, a parasite or an air plant?
(87.)
16. On what plants in your neighborhood does mistletoe grow most
abundantly? Dodder?
17. Is mistletoe injurious to the host? (85.)
18. Name some plants that are propagated mainly, or solely, by
roots and cuttings.
19. Where do aërial roots get their nourishment? (88.)
20. Would they be of any use to a plant in a very cold or dry
climate?
21. Where should manure be placed to benefit a tree or shrub with
wide-spreading roots? (66, 89.)
22. Is it a wise practice to mulch a tree by raking up dead leaves
and piling them around the base of the trunk, as is often done?
Why, or why not? (66, 89.)
Field Work
(1) Examine the underground parts of hardy winter herbs in your
neighborhood, also of any weeds or grasses that are particularly
troublesome, and see if there is anything about the structure of
these parts to account for their persistence. Note the difference
between roots of the same species in low, moist places and in
dry ones; between those of the same kind of plants in different
soils; in sheltered and in exposed situations. Study the direction
and position of the roots of trees and shrubs with reference to
any stream or body of water in the neighborhood. (The elm, fig,
mulberry, and willow are good subjects for such observations.)
Notice also whether there is any relation between the underground
parts and the leaf systems of plants in reference to drainage and
transpiration.
(2) Observe the effect of root pull upon low herbs. Look along
washes and gullies for roots doing the office of stems, and note
any changes of structure consequent thereon. Study the relative
length and strength of the root systems of different plants, with
reference to their value as soil binders, or their hurtfulness in
damaging the walls of cellars, wells, sewers, etc. Dig your trowel
a few inches into the soil of any grove or copse you happen to
visit, note the inextricable tangle of roots, and consider the
fierce competition for living room in the vegetable world that it
implies.
(3) Tests might be made of the different soils in the neighborhood
of the schoolhouse by planting seeds of various kinds and noting
the rate of germination; first, without fertilizers, then by adding
the different elements in succession to see what is lacking. The
field for study suggested by this subject is almost inexhaustible.