I. PROCESSES ACCOMPANYING GERMINATION
MATERIAL.—A pint or two of corn, peas, beans, or any quickly
germinating seed.
APPLIANCES.—Matches; wood splinters; gas jet or alcohol lamp; test
tubes; a small quantity of mercuric oxide; a thermometer; a couple
of two-quart preserve jars, and a smaller wide-mouthed bottle that
can be put into one of them; some limewater; a glass tube (the
straws used by druggists for soft drinks will answer).
=26. Preliminary exercises.=—Before taking up the study of
germinating seeds, it is important to learn from what sources the
organic substances used by the growing plant are derived, and some of
the processes that accompany growth and development.
EXPERIMENT 21. TO SHOW THE CHANGES THAT ACCOMPANY OXIDATION.—Strike
a match and let it burn out. Examine the burnt portion remaining
in your hand; what changes do you notice? These changes have been
caused by the union of some substance in the match with something
outside of it, in the act of burning; let us see if we can find out
what this outside substance is.
EXPERIMENT 22. TO SHOW THE ACTIVE AGENT IN OXIDATION.—Heat some
mercuric oxide in a test tube over the flame of a burner. The
heat will cause the oxygen to separate from the mercury, and in a
short time the tube will be filled with the gas. Extinguish the
flame from a lighted splinter and thrust the glowing end into the
tube; what happens? The oxygen unites with something in the wood
and causes it to burn just as the match did. Compare your burnt
splinter with the burnt end of the match; what resemblance do you
notice between them?
EXPERIMENT 23. TO SHOW THAT CARBON DIOXIDE IS A PRODUCT OF
OXIDATION.—Your experiment with the match showed that ignition
is accompanied by heat, and if active enough, by light, and also
that it left behind a solid substance in the form of charcoal. But
how about the part that united with the oxygen to produce these
results? Let us see what became of it. Hold a lighted candle
under the open end of a test tube, or under the mouth of a small
glass jar. Does any vapor collect on the inside? After two or
three minutes quickly invert the jar or the tube, and thrust in a
lighted match: what happens? Can the substance now in the jar be
ordinary air? Why not? (Exps. 21, 22.) Pour in a small quantity of
limewater, holding your hand over the mouth of the tube to prevent
the air from getting in; the gas inside, being heavier than air,
will not escape immediately unless agitated. What change do you
notice in the limewater?
It has been proved by experiment that the kind of gas formed by
the burning candle has the property of turning limewater milky;
hence, whenever you see this effect produced in limewater, you may
conclude that this gas, known as _carbon dioxide_, is present;
and conversely, the presence of carbon dioxide, especially if
accompanied by some of the other effects observed, as the giving
out of heat and moisture, may be taken as evidence that some
process similar to that going on in the burning candle is, or has
been, at work.
EXPERIMENT 24. DO THESE EFFECTS ACCOMPANY ANY OF THE LIFE PROCESSES
OF ANIMALS?—Blow your breath against the palm of your hand; what
sensation do you feel? Blow it against a mirror, or a piece of
common glass; what do you see? Blow through a tube into the bottom
of a glass containing limewater; how is the water affected? How do
these facts correspond with the results of Exp. 23?
EXPERIMENT 25. IS THERE ANY EVIDENCE THAT A SIMILAR PROCESS GOES ON
IN PLANTS?—(1) Half fill a small, wide-mouthed jar with limewater,
place it inside a larger one (Fig. 46), and fill the space between
them, up to the neck of the smaller vessel, with well-soaked
peas, beans, or barleycorns, on a bed of moist cotton or blotting
paper. Cover with a piece of glass and keep at a moderately warm
temperature. (2) As a control experiment, place beside this another
jar arranged in precisely the same way, except that seeds must
be used whose vitality has been destroyed by heat. To prevent
the entrance of germs among the dead seeds, which might cause
fermentation and thus interfere with the experiment, set the jar
containing them in a vessel of water and boil an hour or two before
the experiment begins. Otherwise, treat precisely as in (1).
[Illustration: FIG. 46.—Diagrammatic section, showing arrangement
of jars for Exp. 25.]
After germination has taken place in (1), what change do you notice
in the limewater? If the effect is not apparent, gently stir with a
straw or a glass rod to mix it with the gas in the larger jar. Has
the limewater in the control experiment undergone the same change?
(It may show a slight milkiness due to the carbon dioxide in the
air.) Insert a thermometer among the seeds in both of the larger
jars, and compare their temperature with that of the outside air;
which shows the greater rise? From this experiment and the last
one, what process, common to animals, would you conclude has been
going on in the germinating seeds?
NOTE.—Heat in germinating seeds is not always due to this cause
alone, but is sometimes increased by the presence of minute
organisms called bacteria. Germinating barley and rye in breweries
sometimes show an increase in temperature of 40 to 70 degrees, due
to these organisms, and spontaneous combustion in seed cotton has
been reported from the same cause.
=27. Oxidation.=—The process that brought about the results observed
in the foregoing experiments, and popularly known as _combustion_, is
more accurately defined by chemists as _oxidation._ It takes place
whenever substances enter into new combinations with oxygen. The most
familiar examples of it are when oxygen enters into combination with
substances containing carbon. It was the union of a portion of the
oxygen of the air in Exp. 21, and of that in the tube in Exp. 22,
with some of the carbon in the wood, that caused the burning. The
effect was more marked in the second case because the oxygen in the
tube was pure, while in the air it is mixed with other substances.
=28. Carbon.=—The black substance left in your hand after oxidation
of the wood in Exps. 21 and 22 is _carbon_. It composes the greater
part of most plant bodies, and, in fact, is the most important
element in the realm of organic nature. There is not a living thing
known, from the smallest microscopic germ to the most gigantic tree
in existence, that does not contain carbon as one of its essential
constituents.
=29. Carbon dioxide.=—The gas produced by the burning candle in Exp.
23, by the germinating seeds in Exp. 25, and expelled from your own
lungs in Exp. 24, is carbon dioxide. Chemists designate it by the
symbol CO₂, which means that it consists of one part carbon to two
parts oxygen. It is an invariable product wherever the oxidation of
substances containing carbon goes on. Heat and moisture are evolved
at the same time, and if oxidation is very active, as in Exps. 21 and
22, light also. When the process takes place very slowly, no light is
evolved, and so little heat as to be imperceptible without special
observation. Hence, oxidation may go on around us and even in our own
bodies without our being conscious of the fact.
Carbon dioxide is of prime importance to the well-being of plants. It
furnishes the material from which the greater part of their organic
food is derived, as will be seen when we take up the study of the
leaf and its work. To animals, on the contrary, its presence is so
injurious that if the proportion of it in the air we breathe ever
rises much above 1 part to 1000, the ill effects become painfully
sensible. It is not, however, as was formerly supposed, a poison,
the harm it does being to decrease the proportion of oxygen in the
atmosphere so that animals cannot get enough of it to breathe, and
die of suffocation.
=30. Respiration in plants and in animals.=—It was shown in Exp.
24 that respiration in animals is accompanied by the products of
oxidation; hence we conclude that respiration is a form of oxidation.
And since these same products are given off by plants (Exp. 25), the
inference is clear that the same process goes on in them. But in
plants the life functions are so much more sluggish than in animals
that it is only in their most active state, during germination and
flowering, that evidence of it is to be looked for.
=31. Respiration and energy.=—In plants, as in animals, respiration
is the expression or measure of energy. Sleeping animals breathe
more slowly than waking ones, snakes and tortoises more slowly than
hares and hawks. The more we exert ourselves and the more vital
force we expend, the harder we breathe; hence, respiration is more
active in children than in older persons and in working people than
in those at rest. It is the same with plants; respiration is most
perceptible in germinating seeds and young leaves, in buds and
flowers, where active work is going on. Hence, in this condition
they consume proportionately larger quantities of oxygen and
liberate correspondingly larger quantities of carbon dioxide, with
a proportionate increase of heat. In some of the arums,—calla lily,
Jack-in-the-pulpit, colocasia, etc.,—and in large heads of compositæ,
like the sunflower, where a great number of small flowers are brought
together within the same protecting envelope, the rise of temperature
is sometimes so marked that it may be perceived by placing a flower
cluster against the cheek.
Practical Questions
1. What is charcoal? (28.)
2. Is any of this substance contained in the seed? in the flour and
meal made from seed? (28; Exp. 25.)
3. What combination takes place when the cook lets the stove get
too hot and burns the biscuits? (27, 28.)
4. Of what does the burned part consist? (28.) What was it before
it was burned? (27, 28).
5. Which burns the more readily, an oily seed or a starchy one?
Which leaves the more solid matter behind? (Suggestion: test by
putting a bean, or a large grain of corn, and an equal quantity
of the kernel of a Brazil nut on the end of a piece of wire and
thrusting into a flame.)
6. Is there any rational ground for the statement that the wooden
buildings formerly used on Southern plantations as cotton ginneries
were sometimes destroyed through spontaneous combustion due to the
heat generated by piles of decaying cotton seed? (Exp. 25, Note.)
II. CONDITIONS OF GERMINATION
MATERIAL.—Several ounces each of various kinds of seed. For the
softer kinds, pea, bean, corn, oats, wheat are recommended; for
those with harder coverings, squash, castor bean, apple, pear, or,
where obtainable, cotton; for still harder kinds, persimmon and
date seeds, or the stones of plum and cherry.
APPLIANCES.—1 dozen common earthenware plates for germinators; 1
dozen two-ounce wide-mouthed bottles; 2 common glass tumblers;
clean sand, sawdust, or cotton batting, for bedding; a double
boiler; a gas burner, or a lamp stove.
=32. Recording observations.=—For this purpose a page should be ruled
off in the notebook of each student, after the model here given,
and the facts brought out by the different experiments set down as
observed.
NUMBER OF SEEDS GERMINATED
==============+=+==+==+==+====+====+====+====+====+=====+====
No. of hours | |24|48|72|4 d.|5 d.|6 d.|7 d.|8 d.|10 d.|2 w.
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|1| | | | | | | | | |
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|2| | | | | | | | | |
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|3| | | | | | | | | |
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|4| | | | | | | | | |
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|5| | | | | | | | | |
+-+--+--+--+----+----+----+----+----+-----+----
No. of vessel|6| | | | | | | | | |
==============+=+==+==+==+====+====+====+====+====+=====+====
EXPERIMENT 26. CAN SEEDS HAVE TOO MUCH MOISTURE?—Drop a number
of dry beans or grains of corn, oats, or other convenient seed,
into a vessel with a bedding of cotton or paper that is barely
moistened, and an equal number of soaked seeds of the same kind
into another vessel with a saturated bedding of the same material.
In a third vessel place the same number of soaked seed, covering
them partially with water, and in a fourth cover the same number
entirely. Label them 1, 2, 3, and 4; keep all together in a warm,
even temperature, and observe at intervals of twenty-four hours for
a week. What condition as to moisture do you find most favorable
to germination? Would seeds germinate in the entire absence of
moisture? How do you know?
EXPERIMENT 27. WAS IT THE PRESENCE OF TOO MUCH WATER, OR THE LACK
OF AIR CAUSED BY IT, THAT INTERFERED WITH GERMINATION IN THE LAST
EXPERIMENT?—To answer this question experimentally is not easy,
since it is difficult to obtain a complete vacuum without special
appliances. The simplest way is to fill with mercury a glass tube
30 inches long, closed at one end, and invert it over a small
vessel—a teacup, or an egg cup will answer—containing mercury
enough to cover the bottom to a depth of two or three centimeters
(see Appendix, Weights and Measures, for English equivalents.)
The tube must be supported in such a way that its lower end will
dip into the mercury without touching the bottom of the vessel.
With a pair of forceps insert under the mouth of the tube two or
three seeds that have been well soaked in water deprived of air
by previous boiling. Being lighter than mercury, they will float
to the top, where there is a complete absence of air while other
conditions favorable to germination are present. Before releasing,
they should be well shaken under the mercury to free them from air
bubbles, and if the coats are loose fitting so that they can be
removed without injury to the parts inclosed in them, they should
be slipped off in order to get rid of any imprisoned air they
may contain. Additional moisture may be supplied, if necessary,
by injecting, by means of a medicine dropper inserted under the
mouth of the tube, a drop or two of water that has been previously
boiled. Keep in a warm, even temperature, under conditions
favorable to germination, and compare the behavior of the seeds
with those placed in the different vessels in Exp. 26.
If appliances for this experiment are lacking, a rough
approximation can be made by using the seeds of aquatic plants,
such as the lotus, water lily, and the so-called Chinese sacred
bean, sold in the variety stores, which we know are capable of
germinating in the limited amount of air contained in ordinary
soil water. Place an equal number of such seeds, of about the same
size and weight, on a bedding of common garden soil in two glass
tumblers. Fill one vessel a little over half full of ordinary soil
water and the other to the same height with water from which the
air has been expelled by boiling. Pour over the liquid a film of
sweet oil or castor oil, to prevent the access of air, leaving the
surface of the water in the other vessel exposed. In which do the
seeds come up most freely?
[Illustration: FIG. 47.—To find out the proper depth at which to
plant seeds.]
Some seeds, especially those rich in proteins, as peas and beans,
will germinate in a vacuum, because oxygen is supplied for a time
by the chemical decomposition of substances in their tissues which
contain it, but when these are exhausted, respiration ceases and
death ensues.
EXPERIMENT 28. DOES THE DEPTH AT WHICH SEEDS ARE PLANTED AFFECT
THEIR GERMINATION?—Plant a number of peas or grains of corn at
different depths in a wide-mouthed glass jar filled with moist
sand, as shown in Fig. 47, the lowest ones at the bottom, the top
ones barely covered. Try different kinds of seed and grain,—radish,
squash, cotton, or wheat,—and watch them make their way to the
surface. Do you notice any difference in this respect between large
seed and small ones? Between those with thick cotyledons and thin
ones? At what depth do you find, from your recorded observations,
that seed germinate best?
EXPERIMENT 29. WHAT TEMPERATURE IS MOST FAVORABLE TO
GERMINATION?—Put half a dozen soaked beans on moist cotton or
sawdust in three wide-mouthed bottles of the same size or in
germinators arranged as in Figs. 48, 49, the seed also being
selected with a view to similarity of size and weight. Keep one at
a freezing temperature; the second in a temperature of 15° to 20°
C. (see Appendix for Fahrenheit equivalents); and the third, at
30° C. If a place can be found near a stove or a register, where
an even temperature of about 125° F. is maintained, place a fourth
receptacle there. Observe at intervals of twenty-four hours for a
week or ten days, keeping the temperature as even as possible, and
maintaining an equal quantity of moisture in each vessel. Make a
daily record of your observations. What temperature do you find
most favorable to germination?
[Illustration: FIGS. 48, 49.—Home-made germinators: 48, closed; 49,
showing interior arrangement.]
EXPERIMENT 30. AT WHAT TEMPERATURE DO SEEDS LOSE THEIR
VITALITY?—Place about two dozen each of grains of corn, beans,
squash seed, and castor beans, with an equal number of plum or
cherry stones, in water, and heat to a temperature of 150° F. After
an exposure of ten minutes, take out six of each kind and place
in germinators made of two plates with moist sand or damp cloth
between them, as shown in Figs. 48, 49. Raise the temperature to
175° F., and after ten minutes take out six more of each kind of
seed and place in another germinator. Raise the water in the vessel
to 200°, take out another batch of seeds; raise to the boiling
point for ten minutes more, and plant the remaining six of each
lot. Number the four germinators, and observe at intervals of
twenty-four hours for two weeks. The harder kinds should be kept
under observation for three or four weeks, as they germinate slowly.
Try the same experiments with the same kinds of seeds at a dry
heat, using a double boiler to prevent scorching, and record
observations as before.
EXPERIMENT 31. TIME REQUIRED FOR GERMINATION.—Arrange in
germinators seeds of various kinds, such as corn, wheat, peas,
turnip, apple, orange, grape, castor bean, etc. “Clip” some of the
harder ones and keep all the kinds experimented with under similar
conditions as to moisture, temperature, etc., and record the time
required for each to sprout. What is the effect of clipping, and
why?
EXPERIMENT 32. ARE VERY YOUNG OR IMMATURE SEEDS CAPABLE OF
GERMINATING?—Plant some seeds from half-grown tomatoes, and grains
of wheat, oats, or barley before they are ready for harvesting. Try
as many kinds as you like, and see how many will come up. Notice
whether there is any difference in the health and vigor of plants
raised from seeds in different stages of maturity.
EXPERIMENT 33. THE RELATIVE VALUE OF PERFECT AND INFERIOR
SEED.—From a number of seeds of the same species select half a
dozen of the largest, heaviest, and most perfect, and an equal
number of small, inferior ones. If a pair of scales is at hand, the
different sets should be weighed and a record kept for comparison
with the seedlings at the end of the experiment. Plant the two sets
in pots containing exactly the same kind of soil, and keep under
identical conditions as to light, temperature, and moisture. Keep
the seedlings under observation for two or three weeks, making
daily notes and occasional drawings of the height and size of the
stems, and the number of leaves produced by each.
[Illustration: FIGS. 50, 51.—Stem development of seedlings: 50,
raised from healthy grains of barley; weight, 39.5 grams (about 500
grs.); 51, raised under exactly similar conditions from the same
number of inferior grains; weight, 23 grams (about 350 grs.).]
[Illustration: FIGS. 52, 53.—Improvement of corn by selection: 52,
original type; 53, improved type developed from it.]
=33. Resistance to heat and cold.=—In making experiments with regard
to temperature, notice how the extremes tolerated are influenced,
first, by the length of time the seeds are exposed; second, by the
amount of water contained in them; and third, by the nature of the
seed coats. Every farmer knows that the effect of freezing is much
more injurious to plants or parts of plants when full of sap (water)
than when dry. This, in the opinion of the most recent investigators,
is because the water in the spaces outside the cells freezes first
and as moisture is gradually withdrawn from the inside to take its
place, the soluble salts which may be present in the cell sap become
more concentrated, and by their chemical action on the contained
proteins cause them to be precipitated, or “salted out,” as we see
sugar or salt precipitated from solutions of those substances when
water is withdrawn by evaporation. In this way, it is believed, the
fundamental protoplasm of the cell may be so disorganized that death
ensues if the freezing is continued long enough, since the protein
precipitates become “denatured” and cannot be reabsorbed if kept
in a solid state too long. The length of time necessary to produce
death from this cause is, of course, different in different plants,
according to the kind of salts dissolved in the sap and the nature of
the proteins acted on by them. The proteins in the sap of Begonia,
or Pelargonium, plants which are very sensitive to cold, yield a
denatured precipitate at, or a little below the freezing point of
water, while those of winter rye withstand a temperature of -15° C.,
and of pine needles, -40° C.
Mechanical injury through rupture of parts by freezing is not apt
to cause serious damage except in cases of sudden and violent cold
at a time when the tissues are gorged with sap, as not infrequently
happens during the abrupt changes of temperature which sometimes
occur in spring after the trees have put forth their leaves. In an
extreme case of this kind, the writer has seen the trunk of an oak
a foot or more in diameter split in deep seams from the effects of
freezing.
=34. The length of time during which seeds may retain their
vitality.=—No direct experiment can be made to test this point,
since it would require months, or even years, covering in some
instances more than the lifetime of a generation. It has been stated
on good authority that seeds of the water chinquapin (Nelumbo) have
germinated after more than a hundred years, and moss spores preserved
in herbariums, after fifty. But the records in such cases are not
always trustworthy, and there is absolutely no foundation for the
statements sometimes made about the germination of wheat grains found
preserved with mummies over two thousand years old. If kept perfectly
dry, however, seed may sometimes be preserved for months, or even
years. Peas have been known to sprout after ten years, red clover
after twelve, and tobacco after twenty. Ordinarily, however, the
vitality of seeds diminishes with age, and in making experiments it
is best to select fresh ones. Those used for comparison should also,
as far as possible, be of the same size and weight.
=35. Effect of precocious germination.=—It has been found by
experiment that plants raised from immature seed, when they will
germinate at all (Exp. 32), yield earlier and larger crops than the
same kinds from mature seed. Early tomatoes and some other vegetables
are produced in this way. The majority of seeds, however, require
a period of rest before beginning their life work. Those that are
forced to take up the burden of “child labor” show the effect of such
abnormal condition by yielding fruits that are smaller and less firm
than those raised from mature seed, so that they do not keep well and
have to be marketed quickly. Under what circumstances does it pay to
cultivate such fruits?
Practical Questions
1. What are the principal external conditions that affect
germination? (Exps. 26-29.)
2. What effect has cold? want of air? too much water?
3. Is light necessary to germination?
4. What is the use of clipping seeds? (Exps. 12, 13, 14, and
Material, p. 12.)
5. In what cases should it be resorted to? (Exp. 31.)
6. Why will seed not germinate in hard, sunbaked land without
abundant tillage? Why not on undrained or badly drained land?
(Exps. 26, 27.)
7. Will seeds that have lost their vitality swell when soaked?
(Exp. 16.)
8. Are there any grounds for the statement that the seeds of plums
boiled into jam have sometimes been known to germinate?[1] (33;
Exp. 30.)
9. Could such a thing happen in the case of apple or sunflower
seed, and why or why not? (33.)
10. Does it make any difference in the health and vigor of a plant
whether it is grown from a large and well-developed seed or from a
weak and puny one? (Exp. 33.)
11. Would a farmer be wise who should market all his best grain and
keep only the inferior for seed?
12. What would be the result of repeated plantings from the worst
seed?
13. Of constantly replanting the best and most vigorous?
14. Suppose seed would germinate without moisture; would this be an
advantage, or a disadvantage to agriculturists?
15. Why is a cool, dry place best for keeping seeds? (Exps. 26, 29.)
16. Why are the earliest tomatoes found in the market usually
smaller than those offered later? (35.)
17. Why is continued rain so injurious to wheat, oats, and other
grains before they are mature enough to be harvested? (35; Exp. 32.)
18. Would the same effect be likely to occur in the case of very
oily seeds, such as flax and castor beans? Why? (Suggestion: try
the effect of putting water on a piece of oiled paper.)
19. Explain why many seeds cannot germinate successfully without
air. (30, 31; Exp. 25.)
20. Mention some of the practical advantages that a farmer, a
gardener, or a careful housewife might gain from experiments like
those made in this section.
21. Explain why seeds can endure so much greater extremes of
temperature than growing plants. (23, 33.)
III. DEVELOPMENT OF THE SEEDLING
MATERIAL.—Seedlings of various kinds in different stages of growth.
It is recommended that the same species be used that were studied
in Section III, Chapter I, or such equivalents as may have been
substituted for them. Enough should be provided to give each pupil
three or four specimens in different stages of development. Seeds,
even of the same kind, develop at such different rates that it
will probably not be necessary to make more than two plantings of
each sort, from 2 to 5 days apart. Soaked seeds of corn and wheat
will germinate in from 3 to 7 days, according to the temperature;
oats in 1 to 4; beans in 4 to 6; squash and castor beans in from
8 to 10. Very obdurate ones may be hastened by clipping. Keep the
germinators in an even temperature, at about 70° to 80° F.
Pine is a very difficult seed to germinate, requiring usually
from 18 to 21 days. By soaking the mast for twenty-four hours and
planting in damp sand or sawdust kept at an even temperature of 23°
C. or about 75° F., specimens may be obtained.
[Illustration: FIGS. 54, 55.—Seedling of corn (_after_ GRAY): 54,
early stage of germination; 55, later stage.]
=36. Seedlings of monocotyls.=—Examine a seedling of corn that has
just begun to sprout; from which side does the seedling spring, the
plain or the grooved one? Refer to your sketch of the dry grain and
see if this agrees with the position of the embryo as observed in the
seed. Make sketches of four or five seedlings in different stages of
advancement, until you reach one with a well-developed blade. From
what part of the embryo has each part of the seedling developed?
Which part first appeared above ground? Is it straight, or bent in
any way? In what direction does the plumule grow? The hypocotyl? Does
the cotyledon appear above ground at all? Slip off the husk and see
if there is any difference in the size and appearance of the contents
as you proceed from the younger to the older plants. How would you
account for the difference?
=37. The root.=—Examine the lower end of the hypocotyl and find where
the roots originate; would you say that they are an outgrowth from
the stem, or the stem from the root? Observe that the root of the
corn does not continue to grow in a single main axis like that of
the castor bean, but that numerous adventitious and secondary roots
spring from various points near the base of the hypocotyl and spread
out in every direction, thus giving rise to the fibrous roots of
grains and grasses.
=38. Root hairs.=—Notice the grains of sand or sawdust that cling to
the rootlets of plants grown in a bedding of that kind. Examine with
a lens and see if you can account for their presence. Lay the root
in water on a bit of glass, hold up to the light and look for root
hairs; on what part are they most abundant?
[Illustration: FIG. 56.—Seedling of wheat, with root hairs.]
The hairs are the chief agents in absorbing moisture from the
soil. They do not last very long, but are constantly dying and
being renewed in the younger and tenderer parts of the root. These
are usually broken away in tearing the roots from the soil, so
that it is not easy to detect the hairs except in seedlings, even
with a microscope. In oat, maple, and radish seedlings they are
very abundant and clearly visible to the naked eye. The amount of
absorbing surface on a root is greatly increased by their presence.
[Illustration: FIG. 57.—Diagrammatic section of a root tip: _a_,
cortex; _b_, central cylinder in which the conducting vessels are
situated; _c_, root cap; _g_, growing point.]
=39. The root cap.=—Look at the tip of the root through your lens
and notice the soft, transparent crescent or horseshoe-shaped mass
in which it terminates. This is the root cap and serves to protect
the tender parts behind it as the roots burrow their way through the
soil. Being soft and yielding, it is not so likely to be injured by
the hard substances with which it comes in contact as would be the
more compact tissue of the roots. It is composed of loose cells out
of which the solid root substance is being formed; the growing point
of the root, _g_, is at the extremity of the tip just behind the
cap, _c_ (Fig. 57). The cap is very apparent in a seedling of corn,
and can easily be seen with the naked eye, especially if a thin
longitudinal section is made. It is also well seen in the water roots
of the common duckweed (_Lemna_), and on those developed by a cutting
of the wandering Jew, when placed in water. Are there any hairs on
the root cap? Can you account for their absence?
NOTE.—For a minute study of the structure of roots, see =67=.
=40. Organs of vegetation.=—The three parts, root, stem, and leaf,
are called organs of vegetation in contradistinction to the flower
and fruit, which constitute the organs of reproduction. The former
serve to maintain the plant’s individual existence, the latter to
produce seed for the propagation of the species, so we find that the
seed is both the beginning and the end of vegetable life.
[Illustration: FIG. 58.—Seedlings of bean in different stages
of growth: _cc_, cotyledons, showing the plumule and hypocotyl
before germination; _a_, _b_, _d_, and _e_, successive stages of
advancement. At _d_ the arch of the hypocotyl is beginning to
straighten; at _e_ it has entirely erected itself.]
=41. Definitions.=—Organ is a general name for any part of a living
thing, whether animal or vegetable, set apart to do a certain work,
as the heart for pumping blood, or the stem and leaves of a plant for
conveying and digesting sap. By “function” is meant the particular
work or office that an organ has to perform.
=42. Seedlings of dicotyls. The bean.=—Sketch, without removing it, a
bean seedling that has just begun to show itself above ground; what
part is it that protrudes first? Sketch in succession four or five
others in different stages of advancement. Notice how the hypocotyl
is arched where it breaks through the soil. Does this occur in the
monocotyls examined? Do the cotyledons of the bean appear above
ground? How do they get out? Can you perceive any advantage in their
being dragged out of the ground backwards in this way rather than
pushed up tip foremost? What changes have the cotyledons undergone
in the successive seedlings? Remove from the earth a seedling just
beginning to sprout and sketch it. From what point does the hypocotyl
protrude through the coats? Does this agree with its position as
sketched in your study of the seed? In which part of the embryo does
the first growth take place?
Remove in succession the several seedlings you have sketched and
note their changes. How does the root differ from that of the corn
and oats? The first root formed by the extension of the hypocotyl is
the _primary_ root and should be so labeled in your drawings; the
branches that spring from it are _secondary_ roots. Look for root
hairs; if there are any, where do they occur?
[Illustration: FIG. 59.—Stages in the germination of a typical
seedling of the squash family: _a_, a seed before germination; _b_,
_c_, _e_, the same in different stages of growth; _d_, the empty
testa, with kernel removed; _hi_, hilum; _m_, micropyle; _p_, _p_,
the peg in the heel; _h_, _h_, _h_, the hypocotyl; _ar_, arch of the
hypocotyl; _co_, cotyledons; _pl_, plumule; _pr_, primary root; _sc_,
secondary roots.]
=43. Germination of the squash.=—How does the manner of breaking
through the soil compare with that of the bean? With the corn?
From which end of the seed, the large or the small one, does the
hypocotyl spring? Do the cotyledons come above ground? How do they
get out of the seed coat? Notice the thick protuberance developed
by the hypocotyl and pressing against the lower half of the coat at
the point where the hypocotyl breaks through. This is called the
“peg”; can you tell its use? Could the cotyledons get out of their
hard covering without it? Slip the peg below the coat in one of your
growing specimens, leave it in the soil, and see what will happen.
How do the cotyledons of the squash differ from those of the bean
as they come out of the seed cover? Do they act as foliage leaves?
Do you see any difference in the development of the plumule in the
two seeds (Figs. 19, 25) to account for the different behavior of
the cotyledons? Sketch three seedlings in different stages, labeling
correctly the parts observed. Make a similar study of the castor
bean, or other seedling selected by your teacher, and illustrate by
drawings.
=44. Arched and straight hypocotyls.=—This difference in the manner
of getting above ground is an important one. That by means of the
arched hypocotyl is, in general, characteristic of the process of
germination in which the cotyledons come above ground, while the
straight kind, which was illustrated in the corn and wheat, is the
prevailing method when the cotyledons remain below ground. Can you
give a reason for the difference?
[Illustration: FIG. 60.—Pine seedling (_After_ GRAY).]
=45. Polycotyledons; germination of the pine.=—Examine a pine
seedling just beginning to sprout. What part emerges first from
the seed coat? Where does it break through? Where did you find the
micropyle in the pine seed? (15.) Can you give a reason why the
hypocotyl in seeds should break through the coats at this point? How
do the cotyledons get out of the testa? Is the hypocotyl arched or
straight in germination? How does it compare with the bean and squash
in this respect? With the corn? Is any endosperm left in the testa
after the cotyledons have come out? What has become of it? Do the
cotyledons function as leaves? How many of them have the specimen
you are studying? Notice the little knob or button at the upper
end of the hypocotyl, just above the point where the cotyledons are
attached; this is the _epicotyl_, or part above the cotyledons, here
identical with the plumule; does it develop as rapidly as in the
other seedlings you have examined?
=46. Relation of parts in the seedling.=—Before leaving this subject,
it is important to fix clearly in mind the different parts of the
germinating seedling and their relation to both the embryo from
which they originated and the plant into which they are to develop.
The part labeled “hypocotyl” in your sketches is all that portion
of the embryo below the point of attachment of the cotyledons. In
germination its upper part will become the stem, and in the embryo
constitutes the _caulicle_, or stemlet, while its lower part, from
which the root will develop, is the _radicle_, or rootlet; hence
the term “hypocotyl” includes both the future root and stem. The
plumule is that part of the embryo between the cotyledons and _above_
their point of attachment to the caulicle. It is the upward growing
point of the young plant, and hence the place of attachment of the
cotyledon is the first _node_, or point of leaf origin, on the stem.
The epicotyl, in contradistinction to the hypocotyl, is all that
part of the plant _above_ the insertion of the cotyledons. Before
germination it is identical with the plumule. As the seedling grows,
the epicotyl advances its growing point by adding new nodes and
_internodes_, as the spaces between the successive points of leaf
insertion are called.
=47. Botanical terms.=—As the prefixes _hypo_ and _epi_ are of
frequent occurrence in botanical works, it will aid in understanding
their various compounds if you will remember that _hypo_ always
refers to something below or beneath, and _epi_, to something over
or above. With this idea in mind you will see that botanical terms
are a labor-saving device, since it is much easier, in making notes,
to use a single descriptive word than to write out the long English
equivalent, such as “the part under (or over) the cotyledons.”
Practical Questions
1. Do the cotyledons, as a general thing, resemble the mature
leaves of the same plants?
2. Name some plants in which you have observed differences, and
account for them; could convenience of packing in the seed coats,
for instance, or of getting out of them, have any bearing on the
matter?
3. Does the position in which seeds are planted in the ground have
anything to do with the position of the seedlings as they appear
above the surface?
4. Is this fact of any importance to the farmer?
5. Will grain that has begun to germinate make good meal or flour?
Why? (27, 36; Exp. 25.)
IV. GROWTH
MATERIAL.—Two young potted plants; some lily or hyacinth
bulbs; seedlings of different kinds,—some with well-developed
taproots,—apple, cotton, and maple are good examples.
APPLIANCES.—A small flat dish, some mercury, and a piece of cork.
[Illustration: FIGS. 61, 62.—Seedling of corn, marked to show
region of growth: 61, early stage of germination; 62, later stage.]
EXPERIMENT 34. HOW DOES THE ROOT INCREASE IN LENGTH?—Mark off the
root of a very young corn seedling into sections by moistening
a piece of sewing thread with indelible ink and applying it to
the surface of the root at intervals of about two millimeters (⅒
of an inch), or by tying a thread lightly around it at the same
intervals. Lay the seedling on a moist bedding between two panes
of glass kept apart by a sliver of wood to prevent their injuring
the root by pressure. Watch for a day or two, and you will see that
growth takes place from a point just back of the tip (Figs. 61, 62).
Mark off a seedling of the bean in the same way and watch to see
whether it increases in the same manner as the corn.
EXPERIMENT 35. HOW DOES THE STEM INCREASE IN LENGTH?—Mark off a
portion of the stem of a bean seedling as explained in the last
experiment, and find out how it grows. Allow a seedling to develop
until it has put forth several leaves and measure daily the spaces
between them. Label these spaces in your drawings, “internodes,”
and the points where the leaves are attached, “nodes.” Does an
internode stop growing when the one next above it has formed? When
is growth most rapid? Reverse the position of a number of seedlings
that have just begun to sprout and watch what will happen. After a
few days reverse again and note the effect.
[Illustration: FIGS. 63, 64.—Root of bean seedling, measured to show
region of growth: 63, early stage of germination; 64, later stage.
FIGS. 65, 66.—Stem of bean seedling, measured to show region of
growth: 65, early stage of growth; 66, later stage.]
EXPERIMENT 36. CAN PLANTS GROW AND LOSE WEIGHT AT THE SAME
TIME?—Remove the scales from a white lily bulb, weigh them, and
lay in a warm, but not too damp place, away from the light. After
a time bulblets will form at the bases of the scales. Weigh
them again, and if there has been any loss, account for it. The
experiment may be tried by allowing a potato tuber or a hyacinth
bulb to germinate without absorbing moisture enough to affect its
weight.
[Illustration: FIGS. 67, 68.—Experiment showing the direction of
growth in stems: 67, young potato planted in an inverted position;
68, the same after an interval of eight days.]
EXPERIMENT 37. IS THE DIRECTION OF GROWTH A MATTER OF ANY
IMPORTANCE?—Plant in a pot suspended as shown in Fig. 67, a healthy
seedling of some kind, two or three inches high, so that the
plumule shall point downward through the drain hole and the root
upward into the soil. Watch the action of the stem for six or
eight days, and sketch it at successive intervals. After the stem
has directed itself well upward, invert the pot again, and watch
the growth. After a week remove the plant and notice the direction
of the root. Sketch it entire, showing the changes in direction of
growth.
At the same time that this experiment is arranged, lay another pot
with a rapidly growing plant on one side, and every forty-eight
hours reverse the position of the pot, laying it on the opposite
side. At the end of ten or twelve days remove the plant and
examine. How has the growth of root and stem been affected?
What do we learn from these experiments and from Exp. 35 as to the
normal direction of growth in these two organs respectively? Can
you think of any natural force that might influence this direction?
[Illustration: FIG. 69.—Experiment showing the root of a seedling
forcing its way downward through mercury.]
EXPERIMENT 38. TO SHOW THAT PLANTS WILL EXERT FORCE RATHER THAN
CHANGE THEIR DIRECTION OF GROWTH.—Pin a sprouted bean to a cork and
fasten the cork to the side of a flat dish, as shown in Fig. 69.
Cover the bottom of the dish with mercury at least half an inch
deep, and over the mercury pour a layer of water. Cover the whole
with a pane of glass to keep the moisture in, and leave for several
days. The root will force its way downward into the mercury,
although the latter is fourteen times heavier than an equal bulk
of the bean root substance, and the root must thus overcome a
resistance equal to at least fourteen times its own weight.
=48. What growth is.=—With the seedling begins the growth of the
plant. Most people understand by this word mere increase in size; but
growth is something more than this. It involves a change of form,
usually, but not necessarily, accompanied by increase in bulk. Mere
mechanical change is not growth, as when we bend or stretch an organ
by force, though if it can be kept in the altered position till such
position becomes permanent, or as we say in common speech, “till it
grows that way,” the change may become growth. To constitute true
growth, the change of form must be permanent, and brought about, or
maintained, by forces within the plant itself.
=49. Conditions of growth.=—The internal conditions depend upon
the organization of the plant. The essential external conditions
are the same as those required for germination: food material,
water, oxygen, and a sufficient degree of warmth. It may be greatly
influenced by other circumstances, such as light, gravitation,
pressure, and (probably) electricity; but the four first named are
the essential conditions without which no growth is possible.
=50. Cycle of growth.=—When an organ becomes rigid and its
form fixed, there is no further growth, but only nutrition and
repair,—processes which must not be confounded with it. Every plant
and part of a plant has its period of beginning, maximum, decline,
and cessation of growth. The cycle may extend over a few hours, as in
some of the fungi, or, in the case of large trees, over thousands of
years.
=51. Geotropism.=—The general tendency of the growing axes of
plants to take an upward and downward course as shown in Exp.
37—in other words, to point to and from the center of the earth—is
called _geotropism_. It is _positive_ when the growing organs point
downward, as most primary roots do; _negative_ when they point
upward, as in most primary stems; and _transverse_, or _lateral_,
when they extend horizontally, as is the case with most secondary
roots and branches.
=52. Gravity and growth.=—It cannot be proved directly that
geotropism is due to gravity, because it is not possible to remove
plants from its influence so as to see how they would behave in
its absence. The effect of gravity may be neutralized, however, by
arranging a number of sprouting seeds on the vertical disk of a
clinostat, an instrument fitted with a clockwork movement by means of
which they may be kept revolving steadily for several days. By this
constant change of position gravity is made to act on them in all
directions alike, which is the same in some respects as if it did not
act at all. If the disk is made to revolve rapidly, the growing root
tips turn toward the axis of motion, without showing a tendency to
grow downward. We may then conclude that geotropism is a reaction to
gravity.
=53. Geotropism an active force.=—It must be noted, however, that the
force here alluded to is not the mere mechanical effect of gravity,
due to weight of parts, as when the bough of a fruit tree is bent
under the load of its crop, but a certain stimulus to which the plant
reacts by a spontaneous adjustment of its growing parts. In other
words, geotropism is an active, not a passive function, and the plant
will overcome considerable resistance in response to it. (Exp. 38).
=54. Other factors.=—The direction of growth is influenced by many
other factors, such as light, heat, moisture, contact with other
bodies, and perhaps by electricity. The result of all these forces is
an endless variety in the forms and growth of organs that seems to
defy all law.
[Illustration: FIG. 70.—A piece of a haulm of millet that has been
laid horizontally, righting itself through the influence of negative
geotropism.]
Heat, unless excessive, generally stimulates growth; contact
sometimes stimulates it, causing the stem to curve away from the
disturbing object, and sometimes retards it, causing the stem to
curve toward the object of contact by growing more rapidly on the
opposite side, as in the stems of twining vines. Light stimulates
nutrition, but generally retards growth. The movements of plants
toward the light are effected in this way; growth being checked on
that side, the plant bends toward the light.
Practical Questions
1. Why do stems of corn, wheat, rye, etc., straighten themselves
after being prostrated by the wind? (51, 54.)
2. Do plants grow more rapidly in the daytime, or at night? (54.)
3. Reconcile this with the fact that green plants will die if
deprived of light.
4. Which grows more rapidly, a young shoot or an old one? (31, 50.)
5. Which, as a general thing, are the more rapid growers, annuals
or perennials? Herbaceous or woody-stemmed plants?
6. Name some of the most rapid growers you know.
7. Of what advantage is this habit to them?
8. Why do roots form only on the under side of subterraneous stems?
(51.)
9. Why do new twigs develop most freely on the upper side of
horizontal branches? (51.)
Field Work
(1) Notice the various seedlings met with in your walks and see how
many you can recognize by their resemblance to the mature plants.
Account for any differences you may observe between seedlings
and older plants of the same species. Observe the cotyledons as
they come up and their manner of getting out of the ground, and
notice the ways in which this is influenced by moisture, light,
and the nature of the soil. Where the cotyledons do not appear,
dig into the ground and find out the reason. Notice which method
of emergence occurs in each case, the arched, or straight, and
account for it. Observe particularly the behavior of seedlings in
hard, sunbaked soil. If you see any of them lifting cakes of earth,
compare the size and weight of the cake with that of the seed; if
there is any disparity, what does this imply? What is the force
called which the plant exercises in lifting the weight? (51.)
(2) Notice if there are any seeds germinating successfully on top
of the ground, and find out by what means their roots get into the
soil. Observe what effect sun and shade, moisture and drought, and
the nature of the soil have on the process. Find out whether roots
exercise force in penetrating the soil; what kinds they penetrate
most readily, and what kinds, if any, they fail to penetrate at
all. Notice whether seedlings with taproots, like the turnip and
castor bean, or those with fibrous roots, like corn and wheat, are
more successful in working their way downward.
(3) Look for tree seedlings. Explain why seedlings of fruit trees
are so much more widely distributed in cultivated districts, and so
much easier to find than those of forest trees. Where do the latter
occur, as a general thing? Account for the fact that seedling
trees are so much more rare than germinating herbs, and why trees
like the oak and chestnut and black walnut propagate so much more
slowly, in a state of nature, than the pine, cedar, ash, and maple.
(4) Observe the direction of growth in plants on the sides of
gullies and ravines, and tell how it is influenced by geotropism.
Notice whether there are other influences at work; for instance,
light, or in the case of roots, the attraction of moisture.