Pollination - Encyclopedia

GEOGRAPHICAL NAMES

POLLINATION, in botany, the transference of the pollen from the stamen to the receptive surface, or stigma, of the pistil of a flower. The great variety in the form, colour and scent of flowers (see Flower) is intimately associated with pollination which is effected by aid of wind, insects and other agencies. Pollen may be transferred to the stigma of the same flower - self-pollination (or autogamy), or to the stigma of another flower on the same plant or another plant of the same species - crosspollination (or allogamy). Effective pollination may also occur between flowers of different species, or occasionally, as in the case of several orchids, of different genera - this is known as hybridization.

The method of pollination is to some extent governed by the distribution of the stamens and pistil. In the case of unisexual flowers, whether monoecious, that is, with staminate and pistillate flowers on one and the same plant, such as many of our native trees - oak, beech, birch, alder, &c., or dioecious with staminate and pistillate flowers on different plants, as in willows and poplars, cross pollination only is possible. In bisexual or hermaphrodite flowers, that is, those in which both stamens and pistil are present, though self-pollination might seem the obvious course, this is often prevented or hindered by various arrangements which favour cross-pollination. Thus the anthers and stigmas in any given flower are often mature at different times; this condition, which is known as dichogamy and was first pointed out by Sprengel, may be so well marked that the stigma. has ceased to be receptive before the anthers open, or the anthers have withered before the stigma becomes receptive, when crosspollination only is possible, or the stages of maturity in the two organs are not so distinct, when self-pollination becomes possible later on. The flower is termed proterandrous or proterogynous according as anthers or stigmas mature first. * The term homogamy is applied to the simultaneous maturity of stigma and anthers. Spontaneous self-pollination is rendered impossible in some homogamous flowers in consequence of the relative position of the anthers and stigma - this condition has been termed herkogamy. Flowers in which the relative position of the organs allows of spontaneous self-pollination may be all alike as regards length of style and stamens (homomorphy or homostyly), or differ in this respect (heteromorphy) the styles (From Strasburger's by permission of Gustav Fischer.)

FIG. I. - Long-styled, L, and short-styled, K, flowers of Primula sinensis. G, Level of stigma; S, level of anthers; P, N, pollen grains and stigmatic papillae of long-styled form; p, n, ditto of short-styled form.

and stamens being of different lengths in different flowers (heterostyly) or the stamens only are of different lengths (heteranthery). Flowers which are closed at the time of maturity of anthers and stigmas are termed cleistogamous.

Self-pollination is effected in very various ways. In the simplest case the anthers are close to the stigmas, covering these with pollen when they open; this occurs in a number of small annual plants, also in Narcissus, Crocus, &c. In snowdrop and other pendulous flowers the anthers form a cone around the style and the pollen falls on to the underlying stigmas, or in erect flowers the pollen may fall on to the stigmas which lie directly beneath the opening anthers (e.g. Narthecium). In very many cases the pollen is carried to the stigma by elongation, curvature or some other movement of the filament, the style or stigma, or corolla or some other part of the flower, or by correlated movements of two or more parts. For instance, in many flowers the filaments are at first directed outwards so that self-pollination is not possible, but later incline towards the stigmas and pollinate them (e.g. numerous Saxifragaceae, Cruciferae and others), or the style, which first projects beyond the anthers, shortens later on so that the anthers come into contact with the stigmas (e.g. species of Cactaceae), or the style bends so that the stigma is brought within the range of the pollen (e.g. species of Oenothera, Epilobium, most Malvaceae, &c.). In Mirabilis Jalapa and others the filaments and style finally become intertwined, so that pollen is brought in contact with the stigma. Selfpollination frequently becomes possible towards the end of the life of a flower which during its earlier stages has been capable only of cross-pollination. This is associated with the fact, so ably demonstrated by Darwin, that, at any rate in a large number of cases, cross-pollination yields better results, as measured by the number of seeds produced and the strength of the offspring, than self-pollination; the latter is, however, preferable to absence of pollination. In many cases pollen has no effect on the stigma of the same flower, the plants are selfsterile, in other cases external pollen is more effective (pre-potent) than pollen from the same flower; but in a very large number of cases experiment has shown that there is little or no difference between the effects of external pollen and that from the same flower.

Cross-pollination may occur between two flowers on the same plant (geitonogamy) or between flowers on distinct plants (xenogamy). The former, which is a somewhat less favourable method than the latter, is effected by air-currents, insect agency, the actual contact between stigmas and anthers in neighbouring flowers, where, as in the family Compositae, flowers are closely crowded, or by the fall of the pollen from a (From Darwin's by permission.)

FIG. 2. - Diagram of the flowers of the three forms of Lythrum salicaria in their natural position, with the petals and calyx removed on the near side. (X 6 times.) The dotted lines with the arrow show the directions in which pollen must be carried to each stigma to ensure full fertility.

higher on to the stigmas of a lower flower. Anton Kerner has shown that crowded inflorescences such as those of Compositae and Umbelliferae are especially adapted for geitonogamy. Xenogamy is of course the only possible method in diclinous plants; it is also the usual method in monoclinous plants, owing to the fact that stamens and carpels often mature at different times (dichogamy), the plants being proterandrous or proterogynous. Even in homogamous flowers cross-pollination is in a large proportion of cases the effective method, at any rate at first, owing to the relative position of anther and stigma or the fact that the plant is self-sterile.

The subject of heterostyly was investigated by Darwin (see his Forms of Flowers) and later by Hildebrand. In the case of a dimorphic flower, such as Primula, four modes of pollination are possible, two distinguished by Darwin as legitimate, between anthers and stigmas on corresponding levels, and two so-called illegitimate unions, between anthers and stigmas at different levels (cf. fig. i). In a trimorphic flower such as Lythrum salicaria there are six possible legitimate unions and twelve illegitimate (see fig. 2). Experiment showed that legitimate unions yield a larger quantity of seed than illegitimate.

Many plants produce, in addition to ordinary open flowers, so-called cleistogamous flowers, which remain permanently closed but which notwithstanding produce fruit; in these the corolla is inconspicuous or absent and the pollen grows from the anther on to the stigma of the same flower. Species of Viola (see fig. 3), Oxalis acetosella (wood sorrel) and Lamium amplexicaule are commonly occurring instances. The cleistogamous flowers are developed before or after the normal open flowers at seasons less s-- favourable for cross-pollination. In some cases flowers, which open under normal circumstances, remain closed owing to unfavourable circumstances, and self-pollination occurs as in a

FIG. 3. - Cleistogamous typical cleistogamous flower - these flower of Viola sylvatica. have been distinguished as pseudoi, flower X 4.

cleistogamous. Instances occur in 2, flower more cu highly magnified and cut open. water plants, where flowers are un- a, anther; s, pistil; able to reach the surface (e.g. Alisma st, style; v, stigmatic natans, water buttercup, &c.) or surface.

where flowers remain closed in dull or cold weather.

Systems of classification of flowers according to the agency by which pollination is effected have been proposed by Delpino, H. Miller and other workers on the subject. Knuth suggests the following, which is a modification of the systems proposed by Delpino and Muller.

A. Water-pollinated plants, Hydrophilae. A small group which is subdivided thus: a. Pollinated under the water' e.g. Najas where the pollen grains are rather heavier than water, and sinking down are caught by the stigmas of the extremely simple female flowers.

b. Pollination on the surface, a more frequent occurrence than (a). In these the pollen floats on the surface and reaches the stigmas of the female flowers as in Callitriche, Ruppia, Zostera, Elodea. In Vallisneria (fig. 4) the male flowers become detached and float on the surface of the water; the anthers are thus brought in contact with the stigmas of the female flowers.

B. Wind-pollinated plants, Anemophilae. - In these the pollen grains are smooth and light so as to be easily blown about, and are produced in great quantity; the stigmas are brushlike or feathery, and usually long and protruding so as readily to catch the pollen. As no means of attraction are required the flowers are inconspicuous and without scent or nectar. The male inflorescence is often a pendulous catkin, as in hazel and many native English trees (fig. 5); or the anthers are loosely fixed on long thread-like filaments as in grasses (fig. 6).

FIG. 4. - Vallisneria spiralis. A, female flower; s, stigmas.

B, male flowers; I before; 2, after spreading of the petals... A male flower has floated alongside a female and one of its anthers, which have opened to set free the pollen, is in contact with a stigma. a, anther.

- C. Animal-pollinated plants, Zoidiophilae, are subdivided according to the kind of animal by agency of which pollination is effected, thus: a. Bat-pollinated, Chiropterophilae. - A Freycinetia, native of Java, and a species of Bauhinia in Trinidad are visited by bats which transfer the pollen.