POLLINATION. Transference of the pollen (q.v.) from the stamen of a flower to the stigma of the same flower (self or close pollination) or to that of another flower (cross-pollination). Several terms associated with these processes may be here defined: .lutogamy, self-pollination of flowers; alloganty,cros,;-pollination; gritonogamy, cross-pollination between two flowers upon one plant ; xenoymny, between flowers of different plants. Flowers containing both stamens and pis tils are called monoclinous, or hermaphroditic: if stamens are in one flower and pistils in another, but both kinds are upon the same plant. the plants are said to be diciinous and momrcious (Fig. I) if upon different plants, direcions ( Fig. 2). Anthcsis is the period during which pollina tion is possible; flowers which remain closed dur ing this period exhibit dristoganoy (q.v.). those which open all or part of the I ime exhibit chas mogamy. Flowers whose stamens or pistils Or both are of unequal lengths are said to show hit r011ifirrh ism if two lengths occur. phism. (Fig. 3) ; if three are present. trimor phism. In hontogwnons flowers the stigmas ma ture when the pollen grains arc shed: in dirhog amons, these two periods differ. If the stigmas mature first, the flower exhibits protogyny or pro frrogYitn: if the pollen grains are shed first. vrotondry nr limit Emir/ it/ ( Fig. 4 ).
In general, cross-pollination seP111, to he some %dial advantageous to plants because it is sup posed to prevent close inbreeding. which in both plants and animal, seems to result in de generacy. The reason for this is not known. There are various devices by which cross-pol lination is obtained. The most perfect device is the so-called diweious habit illustrated by the 2). In the so-called namovious habit tFig. 1). cross-pollination is less certain, transferred by insects. The structure of the insect-pollinated flowers is commonly more or less complicated and differs somewhat widely though it is often facilitated by the pistillate flowers !wing uppermost on the tree. In the ease of insect pollinated flowers cross-pollination is sometimes secured by prota miry (Fig. 4). or pro togyny. In many eases the stigma when mature is higher than the stamens and hence self-pollina tion is diffietalt. Perhaps the most effective de vice to insure cross-pollination is the impo tence of the pollen upon the stigma of the same flower and even of other flower, on the Caine plant. It must lw remembered that in Marie
plants self-pollination is not only possible, but even common. It is invariably the case in cleistogammis flowers. (See CLE:tsTou.v M Y. ) In the subterranean flower of the violet self-pollina tion is necessary. and the rich development of seeds shows that it is effective.
Pollination is effected in various ways in plants, among which the most important are the follow ing: (I) Insect pollination. In a vast number. perhaps even the majority of plants. pollen is from that of flowers pollinated in other ways (Fig. 5). For example, the corollas are com monly prominent and highly colored, and it is be lieved that insects are more likely to visit flowers if their corollas are colored. Some experiments, however. would seem to show that color may not serve conspicuously as an attraction. Many flowers are attractive to insects because of their fragrance. which is a device to secure cross-polli nation. The pollen of insect-pollinated flowers is often comparatively sticky and heavy. A large number of structures are found in these flowers which seem to favor certain insects and oppose others. Among the more striking of these devices t is winged (Fig. 6. a). It is so abundant that t .sometimes the so-called sulphur showers. (4) Water-pollinated flower., A small number is a long corolla tube which favors insects with a long proboscis. In general. insect-pollinated flowers are rich in nectar. which is often the lure for the visit of insects. Other flowers are visited for their pollen. of which many insects are fond. (2) Bird pollination. A comparatively small and in North America unimportant number of flowers are pollinates by birds, especially bum ming-birds. (3) Wind pollination is common with a large number of plants, especially trees. grasses, and sedges, in which the flowers differ from insect-pollinated flowers in the relative absence of showiness, nectar, and odor, etc. These flowers are also comparatively small. The pollen is usually light, easily carried by the wind, and is also produced in immense quanti ties. Pine pollen is particularly interesting in that of aquatic plants are pollinated by means of water currents. In some cases. as in a somewhat complex mechanism is present.