POLLINATION. The pollen of seed plants are really microspores in a more or less advanced stage of germination. They are developed within anthers and, at the time of shedding, form a powdery mass which usu ally has a distinctly yellowish color. The pol len is the male product of the plant. The transfer of the pollen to the female structures — to the ovule of Gymnosperms and to the stigma of the ovary in Angiosperms — is called Pollination (Fig. 1).
The terms pollination and fertilization are often confused, but they are very different processes; for pollination is merely the trans fer of the pollen grains from the anther to the ovule or stigma, while fertilization is the union of the sperms and egg, or many would define the term more closely as union of the sperm and the egg nuclei. The confusion in the use of terms is particularly persistent because the earlier writers used both terms indiscriminately and because a widely read book by Darwin on 'The Fertilization of Orchids by Insects' deals entirely with pollination, there being no refer ence whatever to the real process of fertili zation.
The interval between pollination and fertili zation varies in different species. In some cases it is only a few hours; in the majority of our familiar flowering plants the interval is probably less than a week; in some of the Gymnosperms it is several months, and in others, like the pine, it is a year; in the oaks, the interval is even longer.
Pollen is carried from the male to the fe male by the wind, by various animals, such as insects, snails and birds, or by water. The most primitive method is wind pollination; plants pollinated by wind are said to be anemophilous. Probably all Gymnosperms are pollinated by the wind, although claims have been made that in a few species the pollen is carried by insects. Many of the lower Angiosperms like the grasses, oaks, chestnut, beech and birch are wind pollinated; but the great majority of the group are pollinated by insects, and so are said to be entomophilous. In a few Angiosperms, growing in the water and with flowers resting on the surface of the water, the pollen floats on the water from the anther to the stigma. Elo
dea, a very common submErged water weed, is a familiar example.
The relative position of the pollen and the ovule or stigma to which it must be carried is various in different plants. In most of our familiar flowers, like the lily, the anthers with their pollen are in the same flower with the stigma. Such flowers are °bisporangiate) or °hermaphrodite.) In many plants, as in corn, the male structures (the tassel with the an thers) and the female (the ear with the stig mas — called the °silk)) are borne on different parts of the same plant but not in the same flower. Such plants are amonozcious.) Many Gymnosperms bear male and female cones on the same tree and so belong in this category. In a smaller number of cases the male and female flowers are on different individuals, as in the willows. Such plants are °dicecious.) In case of wind pollination, the pollen is produced in immense quantities. In Dioon, a Gymnosperm in the cycad family, the male cone produces about 10,000,000,000 pollen grains. The female cone produces 200 to 300 ovules, each of which exudes at its tip a small sparkling Mucilaginous droplet, which. catches any pollen grains that chance to fall upon it. The com bined surface of the 300 droplets would be less than an inch square, and yet, if a female cone is within 100 feet of a male cone, the ovules are likely to be thoroughly pollinated. As the droplet dries, the pollen grains are drawn down to the place where they begin to germinate and form pollen tubes. The pollen may be car tied for miles, but at such distance very few ovules of an ovulate cone are likely to be polli nated. In pine forests, the pollen falls in such abundance that people call it °sulphur showers.) The comparatively few plants in which the pollen is carried by water are mostly dicecious. The pollen simply floats upon the surface of the water until it reaches the sticky, mucilagi nous secretion of the stigma.