SEEDS AND GERMINATION
[Illustration: FIG. 15.--PARTS OF THE BEAN.
_R_, cotyledon; _O_, caulicle; _A_, plumule; _F_, first node.]
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_.
=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 center. 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.
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=.
[Illustration: FIG. 16.--EXTERNAL PARTS OF BEAN.]
=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, color, 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 dioxid_ (CO₂).
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.
[Illustration: FIG. 17.--PEA. Grotesque forms assumed when the roots
cannot gain entrance to the soil.]
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 CAULICLE AND PLUMULE.]
=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. 19.--GERMINATION OF BEAN.]
[Illustration: FIG. 20.--SPROUTING OF CASTOR BEAN.]
[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 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. 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.]
=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. 28.--GERMINATION IS COMPLETE.
_p_, top of epicotyl; _o_, plumule-leaf; _m_, roots; _c_, lower roots.]
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 colors and markings of kernels of corn;
and of peas, beans, castor bean.
=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.
[Illustration: FIG. 29.--CONES OF HEMLOCK (ABOVE), WHITE PINE, PITCH
PINE.]
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. 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. Its
germination, as also that of the scarlet runner bean, is explained in
“Lessons with Plants.” 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
(color 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 a 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
dioxid. The air in the bottle may be tested for carbon dioxid 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 molds and bacteria which
might interfere with 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
for 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
foregoing 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 vigor
and rapidity. Most seeds will sprout in a week or less. Usually such
a tester must have fresh sand and paper after every test, for mold
fungi are likely to breed in it. If canton flannel is used, it may be
boiled. If possible, the seeds should not touch each other.
[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 jeweler’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 ¹⁄₂ 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.