THE FLOWER—FERTILIZATION AND POLLINATION
[Illustration: FIG. 193.—_B_, POLLEN escaping from anther; _A_,
pollen germinating on a stigma. Enlarged. ]
=Fertilization.=—_Seeds result from the union of two elements or
parts._ One of these elements is a cell-nucleus of the pollen-grain.
The other element is the cell-nucleus of an egg-cell, borne in the
ovary. The pollen-grain falls on the stigma (Fig. 193). It absorbs
the juices exuded by the stigma, and grows by sending out a tube
(Fig. 194). This tube grows downward through the style, absorbing
food as it goes, and finally reaches the egg-cell in the interior of
an ovule in the ovary (Fig. 195), and =fertilization=, or union of a
nucleus of the pollen and the nucleus of the egg-cell in the ovule,
takes place. _The ovule and embryo within then develops into a seed._
The growth of the pollen-tube is often spoken of as germination of
the pollen, but it is not germination in the sense in which the word
is used when speaking of seeds.
[Illustration: FIG. 194.—A POLLEN-GRAIN AND THE GROWING TUBE.]
Better seeds—that is, those that produce stronger and more
fruitful plants—often result when the _pollen comes from another
flower_. Fertilization effected between different flowers is
cross-fertilization; that resulting from the application of
pollen to pistils in the same flower is =close-fertilization= or
=self-fertilization=. It will be seen that the cross-fertilization
relationship may be of many degrees—between two flowers in the same
cluster, between those in different clusters on the same branch,
between those on different plants. Usually fertilization takes place
only between plants of the same species or kind.
In many cases there is, in effect, _an apparent selection of pollen_
when pollen from two or more sources is applied to the stigma.
Sometimes the foreign pollen, if from the same kind of plant, grows,
and fertilization results, while pollen from the same flower is less
promptly effective. If, however, no foreign pollen is present, the
pollen from the same flower may finally serve the same purpose.
[Illustration: FIG. 195.—DIAGRAM TO REPRESENT FERTILIZATION.
_s_, stigma; _st_, style; _ov_, ovary; _o_, ovule; _p_,
pollen-grain; _pt_, pollen-tube; _e_, egg-cell; _m_, micropyle.]
In order that the pollen may grow, _the stigma must be ripe_. At
this stage the stigma is usually moist and sometimes sticky. A ripe
stigma is said to be =receptive=. The stigma may remain receptive
for several hours or even days, depending on the kind of plant, the
weather, and how soon pollen is received. Watch a certain flower
every day to see the anther locules open and the stigma ripen. When
fertilization takes place, the stigma dies. Observe, also, how soon
the petals wither after the stigma has received pollen.
=Pollination.=—The transfer of the pollen from anther to stigma is
known as =pollination=. The pollen may fall of its own weight on
the adjacent stigma, or it may be carried from flower to flower by
wind, insects, or other agents. There may be =self-pollination=
or =cross-pollination=, and of course it must always precede
fertilization.
[Illustration: FIG. 196.—ANTHER OF AZALEA, opening by terminal pores.]
Usually the pollen is discharged by the bursting of the anthers. The
commonest method of discharge is through a _slit_ on either side
of the anther (Fig. 193). Sometimes it discharges through a _pore_
at the apex, as in azalea (Fig. 196), rhododendron, huckleberry,
wintergreen. In some plants a part of the anther wall raises or falls
as a _lid_, as in barberry (Fig. 197), blue cohosh, May apple. The
opening of an anther (as also of a seed-pod) is known as =dehiscence=
(_de_, from; _hisco_, to gape). When an anther or seed-pod opens, it
is said to _dehisce_.
[Illustration: FIG. 197.—BARBERRY STAMEN, with anther opening by
lids.]
_Most flowers are so constructed as to increase the chances of
cross-pollination._ We have seen that the stigma may have the power
of choosing foreign pollen. The commonest means of necessitating
cross-pollination is the _different times of maturing of stamens
and pistils in the same flower_. In most cases the stamens mature
first: the flower is then =proterandrous=. When the pistils mature
first, the flower is =proterogynous=. (_Aner_, _andr_, is a Greek
root often used, in combinations, for stamen, and _gyne_ for pistil.)
The difference in time of ripening may be an hour or two, or it may
be a day. The ripening of the stamens and the pistils at different
times is known as =dichogamy=, and flowers of such character are
said to be dichogamous. There is little chance for dichogamous
flowers to pollinate themselves. Many flowers are _imperfectly
dichogamous_—some of the anthers mature simultaneously with the
pistils, so that there is chance for self-pollination in case foreign
pollen does not arrive. Even when the stigma receives pollen from
its own flower, cross-fertilization may result. The hollyhock is
proterandrous. Fig. 198 shows a flower recently expanded. The centre
is occupied by the column of stamens. In Fig. 199, showing an older
flower, the long styles are conspicuous.
[Illustration: FIG. 198.—FLOWER OF HOLLYHOCK; proterandrous.]
[Illustration: FIG. 199.—OLDER FLOWER OF HOLLYHOCK.]
_Some flowers are so constructed as to prohibit self-pollination._
Very irregular flowers are usually of this kind. With some of them,
the petals form a sac to inclose the anthers and the pollen cannot be
shed on the stigma but is retained until a bee forces the sac open;
the pollen is rubbed on the hairs of the bee and transported. Regular
flowers usually depend mostly on dichogamy and the selective power
of the pistil to insure crossing. _Flowers that are very irregular
and provided with nectar and strong perfume are usually pollinated by
insects._ Gaudy colours probably attract insects in many cases, but
perfume appears to be a greater attraction.
[Illustration: FIG. 200.—FLOWER OF LARKSPUR.]
[Illustration: FIG. 201.—ENVELOPES OF A LARKSPUR. There are five wide
sepals, the upper one being spurred. There are four small petals. ]
The insect _visits the flower for the nectar_ (for the making of
honey) _and may unknowingly carry the pollen_. Spurs and sacs in the
flower are =nectaries= (Fig. 200), but in spurless flowers the nectar
is usually secreted _in the bottom of the flower cup_. This compels
the insect to pass by the anther and rub against the pollen before
it reaches the nectar. Sometimes the anther is a long lever poised
on the middle point and the insect bumps against one end and lifts
it, thus bringing the other end of the lever with the pollen sacs
down on its back. Flowers that are pollinated by insects are said to
be =entomophilous= (“insect loving”). Fig. 200 shows a larkspur. The
envelopes are separated in Fig. 201. The long spur at once suggests
insect pollination. The spur is a sepal. Two hollow petals project
into this spur, apparently serving to guide the bee’s tongue. The two
smaller petals, in front, are peculiarly coloured and perhaps serve
the bee in locating the nectary. The stamens ensheath the pistils
(Fig. 202). As the insect stands on the flower and thrusts its head
into the centre, the envelopes are pushed downward and outward and
the pistil and stamens come in contact with its abdomen. Since the
flower is proterandrous, the pollen that the pistils receive from the
bee’s abdomen must come from another flower. Note a somewhat similar
arrangement in the toadflax or butter-and-eggs.
[Illustration: FIG. 202.—STAMENS OF LARKSPUR, surrounding the
pistils. ]
In some cases (Fig. 203) the stamens are longer than the pistil in
one flower and shorter in another. If the insect visits such flowers,
it gets pollen on its head from the long-stamen flower, and deposits
this pollen on the stigma in the long-pistil flower. Such flowers are
=dimorphous= (of two forms). If pollen from its own flower and from
another flower both fall on the stigma, the probabilities are that
the stigma will choose the foreign pollen.
[Illustration: FIG. 203.—DIMORPHIC FLOWERS OF PRIMROSE.]
_Many flowers are pollinated by the wind._ They are said to be
=anemophilous= (“wind loving”). Such flowers produce great
quantities of pollen, for much of it is wasted. They usually have
broad stigmas, which expose large surfaces to the wind. They are
usually lacking in gaudy colours and in perfume. Grasses and pine
trees are typical examples of anemophilous plants.
[Illustration: FIG. 204.—FLOWERS OF BLACK WALNUT: Two Pistillate
flowers at _A_, and staminate catkins at _B_.]
In many cases cross-pollination is assured because the _stamens
and the pistils are in different flowers_ (diclinous). Monœcious
and diœcious plants may be pollinated by wind or insects, or other
agents (Fig. 204). They are usually wind-pollinated, although willows
are often, if not mostly, insect-pollinated. The Indian corn is a
monœcious plant. The staminate flowers are in a terminal panicle
(tassel). The pistillate flowers are in a dense spike (ear), inclosed
in a sheath or husk. Each “silk” is a style. Each pistillate flower
produces a kernel of corn. Sometimes a few pistillate flowers are
borne in the tassel and a few staminate flowers on the tip of the
ear. Is self-fertilization possible with the corn? Why does a
“volunteer” stalk standing alone in a garden have only a few grains
on the ear? What is the direction of the prevailing wind in summer?
If only two or three rows of corn are planted in a garden where
prevailing winds occur, in which direction had they better run?
[Illustration: FIG. 205.—COMMON BLUE VIOLET. The familiar flowers
are shown, natural size. The corolla is spurred. Late in the season,
cleistogamous flowers are often borne on the surface of the ground. A
small one is shown at _a_. A nearly mature pod is shown at _b_. Both
_a_ and _b_ are one third natural size.]
Although most flowers are of such character as to insure or increase
the chances of cross-pollination, there are some _that absolutely
forbid crossing_. These flowers are usually borne beneath or on the
ground, and they lack showy colours and perfumes. They are known
as =cleistogamous= flowers (meaning self-fertilizing flowers). The
plant has normal showy flowers that may be insect-pollinated, and
in addition is provided with these simplified flowers. Only a few
plants bear cleistogamous flowers. Hog-peanut, common blue violet,
fringed wintergreen, and dalibarda are the best subjects in this
country. Fig. 205 shows a cleistogamous flower of the blue violet at
_a_. Above the true roots, slender stems bear these flowers, that
are provided with a calyx, and a curving corolla which does not
open. Inside are the stamens and the pistils. Late in the season
the cleistogamous flowers may be found just underneath the mould.
They never rise above ground. The following summer one may find a
seedling plant, in some kinds of plants, with the remains of the
old cleistogamous flower still adhering to the root. Cleistogamous
flowers usually appear after the showy flowers have passed. They
seem to insure a crop of seed by a method that expends little of the
plant’s energy. The pupil will be interested to work out the fruiting
of the peanut (Fig. 206). Unbaked fresh peanuts grow readily and can
easily be raised in Canada, in a warm sandy garden.
[Illustration: FIG. 206.—PODS OF PEANUTS RIPENING UNDERGROUND.]
[Illustration: FIG. 207.—STRUGGLE FOR EXISTENCE AMONG THE APPLE
FLOWERS.]
SUGGESTIONS.—=152.= _Not all the flowers produce seeds._ Note that
an apple tree may bloom very full, but that only relatively few
apples may result (Fig. 207). _More pollen is produced than is
needed to fertilize the flowers_; this increases the chances that
sufficient stigmas will receive acceptable pollen to enable the
plant to perpetuate its kind. At any time in summer, or even in
fall, examine the apple trees carefully to determine whether any
dead flowers or flower stalks still remain about the apple; or,
examine any full-blooming plant to see whether any of the flowers
fail. =153.= Keep watch on any plant to see whether insects visit
it. What kind? When? What for? =154.= Determine whether the calyx
serves any purpose in protecting the flower. Very carefully remove
the calyx from a bud that is normally exposed to heat and sun and
rain, and see whether the flower then fares as well as others.
=155.= Cover a single flower on its plant with a tiny paper or
muslin bag so tightly that no insect can get in. If the flower sets
fruit, what do you conclude? =156.= Remove carefully the corolla
from a flower nearly ready to open, preferably one that has no
other flowers very close to it. Watch for insects. =157.= Find the
nectar in any flower that you study. =158.= Remove the stigma. What
happens? =159.= Which of the following plants have perfect flowers:
pea, bean, pumpkin, cotton, clover, buckwheat, potato, Indian
corn, peach, chestnut, hickory, watermelon, sunflower, cabbage,
rose, begonia, geranium, cucumber, calla, willow, cottonwood,
cantaloupe? What have the others? =160.= On wind-pollinated plants,
are either anthers or stigmas more numerous? =161.= Are very small
coloured flowers usually borne singly or in clusters? =162.= Why
do rains at blooming time often lessen the fruit crop? =163.= Of
what value are bees in orchards? =164.= _The crossing of plants to
improve varieties or to obtain new varieties._—It may be desired to
perform the operation of pollination by hand. In order to insure
the most definite results, every effort should be made rightly to
apply the pollen which it is desired shall be used, and rigidly
to exclude all other pollen. (_a_) The first requisite is to
remove the anthers from the flower which it is proposed to cross,
and they _must be removed before the pollen has been shed_. The
flower-bud is therefore opened and the anthers taken out. Cut off
the floral envelopes with small, sharp-pointed scissors, then cut
out or pull out the anthers, leaving only the pistil untouched; or
merely open the corolla at the end and pull out the anthers with
a hook or tweezers; and this method is often the best one. It is
best to delay the operation as long as possible and yet not allow
the bud to open (and thereby expose the flower to foreign pollen)
nor the anthers to discharge the pollen. (_b_) _The flower must
next be covered with a paper bag to prevent the access of pollen_
(Figs. 208, 209). If the stigma is not receptive at the time (as
it usually is not), the desired pollen is not applied at once. The
bag may be removed from time to time to allow of examination of
the pistil, and when the stigma is mature, which is told by its
glutinous or roughened appearance, the time for pollination has
come. If the bag is slightly moistened, it can be puckered more
tightly about the stem of the plant. The time required for the
stigma to mature varies from several hours to a few days. (_c_)
When the stigma is ready, an unopened anther from the desired
flower is crushed on the finger nail or a knife blade, and the
pollen is rubbed on the stigma by means of a tiny brush, the
point of a knife blade, or a sliver of wood. The flower is _again
covered with the bag_, which is allowed to remain for several days
until all danger of other pollination is past. Care must be taken
completely to cover the stigmatic surface with pollen, if possible.
The seeds produced by a crossed flower produce _hybrids_, or plants
having parents belonging to different varieties or species. =165.=
One of the means of securing new forms of plants is by making
hybrids. Why?
[Illustration: FIG. 208.—A PAPER BAG, with string inserted.]
[Illustration: FIG. 209.—THE BAG TIED OVER A FLOWER.]
[Illustration: FIG. 210.—The fig is a hollow torus with flowers borne
on the inside, and pollinated by insects that enter at the apex.]