CHAPTER XIX

THE FLOWER--FERTILIZATION AND POLLINATION

=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. 193.--_B_, POLLEN escaping from anther; _A_, pollen germinating on a stigma. Enlarged.]

[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.

[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 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.

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.

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. 196.--ANTHER OF AZALEA, opening by terminal pores.]

[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 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 center 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 colors 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 colored 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 center, 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 colors 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 insured because the _stamens and 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 would 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 colors and perfumes. They are known as =cleistogamous= flowers (meaning “hidden 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 the Northern states. 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 pistils. Late in the season the cleistogamous flowers may be found just underneath the mold. 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 the North 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 colored 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.--FIG. The fig is a hollow torus with flowers borne on the inside, and pollinated by insects that enter at the apex.]