SEEDS AND GERMINATION
The seed contains a _miniature plant_, or =embryo=. The embryo
usually has three parts that have received names: the stemlet, or
=caulicle=; the seed-leaf, or =cotyledon= (usually 1 or 2); the
bud, or =plumule=, lying between or above the cotyledons. These
parts are well seen in the common bean (Fig. 15), particularly
when the seed has been soaked for a few hours. One of the large
cotyledons—comprising half of the bean—is shown at _R_. The caulicle
is at _O_. The plumule is shown at _A_. The cotyledons are attached
to the caulicle at _F_: _this point may be taken as the first node or
joint_.
[Illustration: FIG. 15.— PARTS OF THE BEAN.
_R_, cotyledon; _O_, caulicle; _A_, plumule; _F_, first node.]
=The Number of Seed-leaves.=—All plants having _two seed-leaves_
belong to the group called =dicotyledons=. Such seeds in many
cases split readily in halves, _e.g._ a bean. Some plants have
only _one_ seed-leaf in a seed. They form a group of plants called
=monocotyledons=. Indian corn is an example of a plant with only one
seed-leaf: a grain of corn does not split into halves as a bean does.
Seeds of the pine family contain more than two cotyledons, but for
our purposes they may be associated with the dicotyledons, although
really forming a different group.
These two groups—the dicotyledons and the monocotyledons—represent
two great natural divisions of the vegetable kingdom. The
dicotyledons contain the woody bark-bearing trees and bushes
(except conifers), and most of the herbs of temperate climates
except the grasses, sedges, rushes, lily tribes, and orchids. The
flower-parts are usually in fives or multiples of five, the leaves
mostly netted-veined, the bark or rind distinct, and the stem often
bearing a pith at the centre. The monocotyledons usually have the
flower-parts in threes or multiples of three, the leaves long and
parallel-veined, the bark not separable, and the stem without a
central pith.
[Illustration: FIG. 16.—EXTERNAL PARTS OF BEAN. ]
Every seed is _provided with food_ to support the germinating
plant. Commonly this food is starch. The food may be stored _in the
cotyledons_, as in bean, pea, squash; or _outside the cotyledons_, as
in castor bean, pine, Indian corn. When the food is outside or around
the embryo, it is usually called =endosperm=.
=Seed-coats; Markings on Seed.=—The embryo and endosperm are inclosed
within a covering made of two or more layers and known as the
=seed-coats=. Over the point of the caulicle is a minute hole or a
thin place in the coats known as the =micropyle=. This is the point
at which the pollen-tube entered the forming ovule and through which
the caulicle breaks in germination. The micropyle is shown at _M_ in
Fig. 16. The scar where the seed broke from its funiculus (or stalk
that attached it to its pod) is named the =hilum=. It occupies a
third of the length of the bean in Fig. 16. The hilum and micropyle
are always present in seeds, but they are not always close together.
In many cases it is difficult to identify the micropyle in the
dormant seed, but its location is at once shown by the protruding
caulicle as germination begins. Opposite the micropyle in the
bean (at the other end of the hilum) is an elevation known as the
=raphe=. This is formed by a union of the funiculus, or seed-stalk,
with the seed-coats, and through it food was transferred for the
development of the seed, but it is now functionless.
Seeds differ wonderfully in size, shape, colour, and other
characteristics. They also vary in longevity. These characteristics
are _peculiar to the species or kind_. Some seeds maintain life
only a few weeks or even days, whereas others will “keep” for ten
or twenty years. In special cases, seeds have retained vitality
longer than this limit, but the stories that live seeds, several
thousand years old, have been taken from the wrappings of mummies are
unfounded.
=Germination.=—The embryo is not dead; it is only dormant. _When
supplied with moisture, warmth, and oxygen (air), it awakes and
grows: this growth is_ =germination=. The embryo lives for a time on
the stored food, but gradually the plantlet secures a foothold in the
soil and gathers food for itself. _When the plantlet is finally able
to shift for itself, germination is complete._
=Early Stages of Seedling.=—The germinating seed first _absorbs
water, and swells_. The starchy matters gradually become soluble.
The seed-coats are ruptured, the caulicle and plumule emerge. During
this process the seed _respires freely, throwing off carbon dioxide_
(CO_{2}).
[Illustration: FIG. 17.—PEA. Grotesque forms assumed when the roots
cannot gain entrance to the soil. ]
The caulicle usually elongates, and from its lower end roots are
emitted. The elongating caulicle is known as the =hypocotyl= (“below
the cotyledons”). That is, the hypocotyl is that part of the stem
of the plantlet lying between the roots and the cotyledon. _The
general direction of the young hypocotyl, or emerging caulicle,
is downwards._ As soon as roots form, it becomes fixed and its
subsequent growth tends to raise the cotyledons above the ground, as
in the bean. When cotyledons rise into the air, germination is said
to be =epigeal= (“above the earth”). Bean and pumpkin are examples.
When the hypocotyl does not elongate greatly and the cotyledons
remain under ground, the germination is =hypogeal= (“beneath the
earth”). Pea and scarlet runner bean are examples (Fig. 48). When
the germinating seed lies on a hard surface, as on closely compacted
soil, the hypocotyl and rootlets may not be able to secure a foothold
and they assume grotesque forms (Fig. 17). Try this with peas and
beans.
The first internode (“between nodes”) above the cotyledons is the
=epicotyl=. It elevates the plumule into the air, and _the plumule
leaves expand into the first true leaves of the plant_. These first
true leaves, however, may be very unlike the later leaves in shape.
[Illustration: FIG. 18.—COTYLEDONS OF GERMINATING BEAN SPREAD APART
TO SHOW ELONGATING CAUDICLE AND PLUMULE. ]
[Illustration: FIG. 19.—GERMINATION OF BEAN.]
=Germination of Bean.=—The common bean, as we have seen (Fig. 15),
has cotyledons that occupy all the space inside the seed-coats.
When the hypocotyl, or elongated caulicle, emerges, the plumule
leaves have begun to enlarge, and to unfold (Fig. 18). The hypocotyl
elongates rapidly. One end of it is held by the roots. The other
is held by the seed-coats in the soil. It therefore takes the form
of a loop, and the central part of the loop “comes up” first (_a_,
Fig. 19). Presently the cotyledons come out of the seed-coats, and
the plant straightens and the cotyledons expand. These cotyledons,
or “halves of the bean,” persist for some time (_b_, Fig. 19). They
often become green and probably perform some function of foliage.
Because of its large size, the Lima bean shows all these parts well.
[Illustration: FIG. 20.—SPROUTING OF CASTOR BEAN. ]
=Germination of Castor Bean.=—In the castor bean the hilum and
micropyle are at the smaller end (Fig. 20). The bean “comes up”
with a loop, which indicates that the hypocotyl greatly elongates.
On examining germinating seed, however, it will be found that the
cotyledons are contained inside a fleshy body, or sac (_a_, Fig. 21).
This sac is the endosperm. Against its inner surface the thin, veiny
cotyledons are very closely pressed, absorbing its substance (Fig.
22). The cotyledons increase in size as they reach the air (Fig. 23),
and become functional leaves.
[Illustration: FIG. 21.—GERMINATION OF CASTOR BEAN.
Endosperm at _a_.]
[Illustration: FIG. 22.—CASTOR BEAN.
Endosperm at _a_, _a_; cotyledons at _b_.]
[Illustration: FIG. 23.—GERMINATION COMPLETE IN CASTOR BEAN. ]
=Germination of Monocotyledons.=—Thus far we have studied
dicotyledonous seeds; we may now consider the monocotyledonous
group. Soak kernels of corn. Note that the micropyle and hilum are
at the smaller end (Fig. 24). Make a longitudinal section through
the narrow diameter; Fig. 25 shows it. The single cotyledon is
at _a_, the caulicle at _b_, the plumule at _p_. The cotyledon
remains in the seed. The food is stored both in the cotyledon and as
endosperm, chiefly the latter. The emerging shoot is the plumule,
with a sheathing leaf (_p_, Fig. 26). The root is emitted from the
tip of the caulicle, _c_. The caulicle is held in a sheath (formed
mostly from the seed-coats), and some of the roots escape through
the upper end of this sheath (_m_, Fig. 26). The epicotyl elongates,
particularly if the seed is planted deep or if it is kept for a time
confined. In Fig. 27 the epicotyl has elongated from _n_ to _p_. The
true plumule-leaf is at _o_, but other leaves grow from its sheath.
In Fig. 28 the roots are seen emerging from the two ends of the
caulicle-sheath, _c_, _m_; the epicotyl has grown to _p_; the first
plumule-leaf is at _o_.
[Illustration: FIG. 24.—SPROUTING INDIAN CORN.
Hilum at _h_; micropyle at _d_.]
[Illustration: FIG. 25.—KERNEL OF INDIAN CORN.
Caulicle at _b_; cotyledon at _a_; plumule at _p_.]
[Illustration: FIG. 26.—INDIAN CORN.
Caulicle at _c_; roots emerging at _m_; plumule at _p_.]
[Illustration: FIG. 27.—INDIAN CORN.
_o_, plumule; _n_ to _p_, epicotyl.]
In studying corn or other fruits or seeds, the pupil should note
how the seeds are arranged, as on the cob. Count the rows on a corn
cob. Odd or even in number? Always the same number? The silk is the
style: find where it was attached to the kernel. Did the ear have any
coverings? Explain. Describe colours and markings of kernels of corn;
and of peas, beans, castor bean.
[Illustration: FIG. 28.—GERMINATION IS COMPLETE.
_p_, top of epicotyl; _o_, plumule-leaf; _m_, roots; _c_, lower
roots.]
=Gymnosperms.=—The seeds in the pine cone, not being inclosed in a
seed vessel, readily fall out when the cone dries and the scales
separate. Hence it is difficult to find cones with seeds in them
after autumn has passed (Fig. 29). The cedar is also a gymnosperm.
Remove a scale from a pine cone and draw it and the seeds as they lie
in place on the upper side of the scale. Examine the seed, preferably
with a magnifying glass. Is there a hilum? The micropyle is at the
bottom or little end of the seed. Toss a seed upward into the air.
Why does it fall so slowly? Can you explain the peculiar whirling
motion by the shape of the wing? Repeat the experiment in the wind.
Remove the wing from a seed and toss it and an uninjured seed into
the air together. What do you infer from these experiments?
[Illustration: FIG. 29.—CONES OF HEMLOCK (ABOVE), WHITE PINE, PITCH
PINE. ]
[Illustration: FIG. 30.—MUSKMELON SEEDLINGS, with the unlike
seed-leaves and true leaves.]
SUGGESTIONS.—Few subjects connected with the study of plant-life
are so useful in schoolroom demonstrations as germination. The
pupil should prepare the soil, plant the seeds, water them, and
care for the plants. =10.= Plant seeds in pots or shallow boxes.
The box should not be very wide or long, and not over four inches
deep. Holes may be bored in the bottom so it will not hold water.
Plant a number of squash, bean, corn, pine, or other seeds about
an inch deep in damp sand or pine sawdust in this box. The depth
of planting should be two to four times the diameter of the seeds.
Keep the sand or sawdust moist but not wet. If the class is large,
use several boxes, that the supply of specimens may be ample. Cigar
boxes and chalk boxes are excellent for individual pupils. It is
well to begin the planting of seeds at least ten days in advance
of the lesson, and to make four or five different plantings at
intervals. A day or two before the study is taken up, put seeds to
soak in moss or cloth. The pupil then has a series from swollen
seeds to complete germination, and all the steps can be made out.
Dry seeds should be had for comparison. If there is no special
room for laboratory, nor duplicate apparatus for every pupil, each
experiment may be assigned to a committee of two pupils to watch
in the schoolroom. =11.= Good seeds for study are those detailed
in the lesson, and buckwheat, pumpkin, cotton, morning glory,
radish, four o’clock, oats, wheat. It is best to use familiar
seeds of farm and garden. Make drawings and notes of all the
events in the germination. Note the effects of unusual conditions,
as planting too deep and too shallow and different sides up. For
hypogeal germination, use the garden pea, scarlet runner, or Dutch
case-knife bean, acorn, horse-chestnut. Squash seeds are excellent
for germination studies, because the cotyledons become green and
leafy and germination is rapid. Onion is excellent, except that
it germinates too slowly. In order to study the root development
of germinating plantlets, it is well to provide a deeper box with
a glass side against which the seeds are planted. =12.= Observe
the germination of any common seed about the house premises. When
elms, oaks, pines, or maples are abundant, the germination of their
seeds may be studied in lawns and along fences. =13.= When studying
germination the pupil should note the differences in shape and
size between cotyledons and plumule leaves, and between plumule
leaves and the normal leaves (Fig. 30). Make drawings. =14=. Make
the tests described in the introductory experiments with bean,
corn, the castor bean, and other seed for starch and proteids.
Test flour, oatmeal, rice, sunflower, four o’clock, various nuts,
and any other seeds obtainable. Record your results by arranging
the seeds in three classes, 1. Much starch (colour blackish or
purple), 2. Little starch (pale blue or greenish), 3. No starch
(brown or yellow). =15.= _Rate of growth of seedlings as affected
by differences in temperature._ Pack soft wet paper to the depth
of an inch in the bottom of four glass bottles or tumblers. Put
ten soaked peas or beans into each. Cover each securely and set
them in places having different temperatures that vary little. (A
furnace room, a room with a stove, a room without stove but reached
by sunshine, an unheated room not reached by the sun). Take the
temperatures occasionally with the thermometer to find difference
in temperature. The tumblers in warm places should be covered very
tightly to prevent the germination from being retarded by drying
out. Record the number of seeds which sprout in each tumbler within
1 day, 2 days, 3 days, 4 days, etc. =16.= _Is air necessary for
the germination and growth of seedlings?_ Place damp blotting
paper in the bottom of a bottle and fill it three-fourths full of
soaked seeds, and close it tightly with a rubber stopper or oiled
cork. Prepare a “check experiment” by having another bottle with
all conditions the same except that it is covered loosely that
air may have access to it, and set the bottles side by side (why
keep the bottles together?). Record results as in the preceding
experiment. =17.= _What is the nature of the gas given off by
germinating seeds?_ Fill a tin box or large-necked bottle with dry
beans or peas, then add water; note how much they swell. Secure two
fruit jars. Fill one of them a third full of beans and keep them
moist. Allow the other to remain empty. In a day or two insert a
lighted splinter or taper into each. In the empty jar the taper
burns: it contains oxygen. In the seed jar the taper goes out: the
air has been replaced by carbon dioxide. The air in the bottle
may be tested for carbon dioxide by removing some of it with a
rubber bulb attached to a glass tube (or a fountain-pen filler)
and bubbling it through lime water. =18.= _Temperature._ Usually
there is a perceptible rise in temperature in a mass of germinating
seeds. This rise may be tested with a thermometer. =19.= _Interior
of seeds._ Soak seeds for twenty-four hours and remove the coat.
Distinguish the embryo from the endosperm. Test with iodine. =20.=
_Of what utility is the food in seeds?_ Soak some grains of corn
overnight and remove the endosperm, being careful not to injure
the fleshy cotyledon. Plant the incomplete and also some complete
grains in moist sawdust and measure their growth at intervals.
(Boiling the sawdust will destroy moulds and bacteria which might
interfere with the experiment.) Peas or beans may be sprouted on
damp blotting paper; the cotyledons of one may be removed, and this
with a normal seed equally advanced in germination may be placed
on a perforated cork floating in water in a jar so that the roots
extend into the water. Their growth may be observed for several
weeks. =21.= _Effect of darkness on seeds and seedlings._ A box may
be placed mouth downward over a smaller box in which seedlings are
growing. The empty box should rest on half-inch blocks to allow
air to reach the seedlings. Note any effects on the seedlings of
this cutting off of the light. Another box of seedlings not so
covered may be used as a check. Lay a plank on green grass and
after a week note the change that takes place beneath it. =22.=
_Seedling of pine._ Plant pine seeds. Notice how they emerge. Do
the cotyledons stay in the ground? How many cotyledons have they?
When do the cotyledons get free from the seed-coat? What is the
last part of the cotyledon to become free? Where is the growing
point or plumule? How many leaves appear at once? Does the new
pine cone grow on old wood or on wood formed the same spring with
the cone? Can you always find partly grown cones on pine trees
in winter? Are pine cones when mature on two-year-old wood? How
long do cones stay on a tree after the seeds have fallen out? What
is the advantage of the seeds falling before the cones? =23.=
_Home experiments._ If desired, nearly all of the fore-going
experiments may be tried at home. The pupil can thus make the
drawings for the notebook at home. A daily record of measurements
of the change in size of the various parts of the seedling should
also be made. =24.= _Seed-testing._—It is important that one know
before planting whether seeds are good, or able to grow. A simple
seed-tester may be made of two plates, one inverted over the other
(Fig. 31). The lower plate is nearly filled with clean sand, which
is covered with cheese cloth or blotting paper on which the seeds
are placed. Canton flannel is sometimes used in place of sand and
blotting paper. The seeds are then covered with another blotter or
piece of cloth, and water is applied until the sand and papers are
saturated. Cover with the second plate. Set the plates where they
will have about the temperature that the given seeds would require
out of doors, or perhaps a slightly higher temperature. Place 100
or more grains of clover, corn, wheat, oats, rye, rice, buckwheat,
or other seeds in the tester, and keep record of the number that
sprout. The result will give a percentage measure of the ability
of the seeds to grow. Note whether all the seeds sprout with equal
vigour and rapidity. Most seeds will sprout in a week or less.
Usually such a tester must have fresh sand and paper after each
test, for mould fungi are likely to breed in it. If canton flannel
is used, it may be boiled. If possible, the seeds should not touch
one another.
[Illustration: FIG. 31.—A HOME-MADE SEED-TESTER.]
NOTE TO TEACHER.—With the study of germination, the pupil will need
to begin dissecting.
=For dissecting=, one needs a lens for the examination of the
smaller parts of plants and animals. It is best to have the lens
mounted on a frame, so that the pupil has both hands free for
pulling the part in pieces. An ordinary pocket lens may be mounted
on a wire in a block as in Fig. A. A cork is slipped on the top of
the wire to avoid injury to the face. The pupil should be provided
with two dissecting needles (Fig. B), made by securing an ordinary
needle in a pencil-like stick. Another convenient arrangement is
shown in Fig. C. A small tin dish is used for the base. Into this
a stiff wire standard is soldered. The dish is filled with solder
to make it heavy and firm. Into a cork slipped on the standard, a
cross wire is inserted, holding on the end a jeweller’s glass. The
lens can be moved up and down and sidewise. This outfit can be made
for about seventy-five cents. Fig. D shows a convenient hand-rest
or dissecting-stand to be used under this lens. It may be 16 in.
long, 4 in. high, and 4 or 5 in. broad.
Various kinds of dissecting microscopes are on the market, and
these are to be recommended when they can be afforded.
[Illustration: _A._—IMPROVISED STAND FOR LENS.]
[Illustration: _B._—DISSECTING NEEDLE ½ natural size.]
[Illustration: _C._—DISSECTING GLASS.]
[Illustration: _D._—DISSECTING STAND.]
Instructions for the use of the compound microscope, with which
some schools may be equipped, cannot be given in a brief space;
the technique requires careful training. Such microscopes are not
needed unless the pupil studies cells and tissues.