George H. Vansell, W. H. Griggs.
Plants have sexes somewhat as animals do. Many plants carry both the male and female elements on the same individual. Other plants have the sex organs in separate plants that is, a plant may be strictly male or female. In any case, pollen from the male part of the plant must come in contact with the female element if seed is to result.
Reproduction is the sole function of a flower. In a typical flower the essential female parts, regardless of their variable form and number, are ovary, style, and stigma. The ovary, the basal part, becomes the fruit. The style is a column of tissue arising from the top of the ovary. The expanded or otherwise modified tip of the style is the stigma. In many plants the surface of the stigma has a sticky secretion to which the pollen adheres. In a typical male part of the flower, the anther simply produces pollen, which is the functional male sex element.
Pollination is the transfer of pollen from the anther to the stigma or the distribution of pollen. Pollination must be accomplished before fertilization (the union of the male germ cell, contained in the pollen grains, with the female germ cell or egg in the ovary) and eventual reproduction can take place.
If pollen is transferred from an anther to the stigma of the same flower or to the stigma of another flower on the same plant, self-pollination is said to have taken place. The transfer of pollen from an anther to the stigma of a flower of another individual plant is spoken of as cross-pollination. Those are botanical definitions and do not consider the varietal factor, which is of such great importance to fruit production. Self-pollination, as used in -fruit production, also includes the transfer of pollen from the anthers of a flower of one variety to the stigma of a flower of the same variety. Cross-pollination, in the horticultural sense, refers to the transfer of pollen from the flower of one variety to a flower of a different variety.
Charles Darwin, the English naturalist, concluded from his observations and exhaustive experiments with many plant families that plants resulting from cross-pollination generally had greater vigor, weight, and height and produced flowers earlier than those resulting from self-pollination. It has since been shown that the advantages of cross-pollination and the disadvantages of self-pollination are not always so decisive as Darwin supposed. Nevertheless, there is a long list of species and varieties of plants that are self-sterile and require cross-pollination; the advantages of hybrid vigor in some modern cropping practices also is well established.
Darwin listed several ways in which plants are constructed to avoid self -pollination and insure cross-pollination:
1. By the separation of the sexes, in which staminate and pistillate flowers are borne on separate plants, as in the hemp, willow, holly, and date.
2. By a difference in the time of maturity of the pollen and stigma in the same flower, as in the red clover, beet, plantain, and avocado.
3. By special mechanical contrivances that prevent self-pollination or that favor insect pollination, as in many orchids, legumes, and mints.
4. By producing different forms of flowers on the same plant with different lengths of stamens and pistils, as in the Chinese primrose.
5. By complete or partial sterility of the flowers to their own pollen or the prepotency of pollen from another individual or variety over the plant's own pollen, as in lobelia, mignonette, mullein, and many varieties of apple, pear, cherry, plum, and almond.
Honey bee on comb.
No difficulty is encountered in the pollination and fertilization of some self-fertile plants because both sexual elements develop so close together that pollen is directly deposited on the stigma. Wheat illustrates such a situation it is self-pollinating and at the same time self-fertile. Self-fertile plants exist, however, which are wholly or partly incapable of fertilizing themselves without the aid of a transferring agent. Cantaloupe is a prime example of this condition.
Other plants cannot fertilize themselves following self-pollination by a transferring agent even though both sexual parts mature at the same time. Varieties of sweet cherry and almond illustrate this situation; pollen from some other variety is required to effect fertilization. Not only are all the varieties of these fruits self-unfruitful; there are instances as well of interin-compatible varieties.
Varieties of certain fruits, such as the apple, pear, and plum, produce some fruit as a response to self-pollination, but not enough for a profitable crop. Such varieties are said to be partially self-fruitful. In such instances, agents for the transfer of pollen between different varieties are essential for commercial production.
Even in many cases of self-fruitfulness, like the French prune, the activity of insects on the blossoms greatly increases fruit production through better pollen distribution. French prune trees, enclosed in tents, gave sets of 19.0 percent when bees were present and 0.34 percent when bees were absent. Similar results were obtained when trees of the self-fruitful sour cherry Montmorency were caged in Michigan.
It is perhaps significant that because California has such a large population of honey bees the yield of fruit, lint, and seed from many plants, such as plums, cotton, alfalfa, Ladino clover, onions, carrots, cantaloupes, lima beans, and white mustard, often is outstanding.
A NUMBER OF AGENTS may be necessary for the distribution of pollen from plant to plant, because some pollens are dry and light and others are moist and heavy. The commonest agents of pollen distribution are gravity, wind, and insects. For example, corn pollen drops from the tassel to the silks. Date palm pollen is a fine dust, which floats away like fog. Pine pollen, with its bladderlike wings, is readily carried by wind. Deciduous fruit pollens are rather gummy and generally must be transferred by insects. The huge pollen grain of cotton is thickly dotted with sticky fluid, which makes it far too heavy to be carried by the wind but adapts it well to sticking to the hairs that cover the bodies of pollinating insects. Pollination may sometimes also be accomplished by rain, birds, and artificial means devised by man.
Even among many plants designated botanically as pollinated by wind and gravity, insects are sometimes a factor in the collection and distribution of pollen. For example, the pollens from bee colonies often contain liberal quantities from corn, oak, pine, walnut, ryegrass, Sudangrass, Canary Island palm, date palm, juniper, cypress, elm, or redwood. Few of those plants, if any, produce visible nectar for the attraction of insects to the male blossom parts; therefore they are probably visited only by the pollen-collecting insects.
Fruits develop in many plants without pollination and fertilization or the subsequent development of seeds. A number of our cultivated plants regularly bear such parthenocarpic seedless fruits. Among them are seedless raisin grapes, English forcing cucumbers, navel oranges, bananas, pineapples, and some varieties of pears, figs, and Japanese persimmons. Sometimes mere pollination without subsequent fertilization may be sufficient to start fruit development. The application of synthetic plant hormones to the flowers and leaves has also been found to stimulate parthenocarpy in tomatoes, Smyrna figs, holly, pears, and others.
NATIVE WILD BEES (bumble bees, leaf-cutting bees, alkali bees, carpenter bees) are specially adapted for gathering pollen and nectar from flowers. Many other insects also do so some beetles, flies, moths, thrips. In fact, any of the thousands of insects that visit flowers purposely or accidentally can be agents for carrying pollen grains from the anther to the stigma. But of all of them the most important by far is the honey bee, Apis meffifera, whose existence depends on pollen and nectar from plants. We estimate that bees accomplish more than 8o percent of the pollination by insects. Yields of fruits and legumes and vegetable seed often have been doubled or trebled simply by providing adequate numbers of bees.
THE HONEY BEE was introduced into the United States from Europe. Unlike the native wild bees, it is a colonial insect throughout the year and therefore is available in force at any season. Semi-domestication in man-made hives makes it available for placement wherever needed for pollination service. ( The native bumble bee is also colonial in summer, but only the queen survives the winter to establish a new colony in spring.)