The seed is set free from the parent plant and serves as the means of dissemination.
The pollination, of flowers and the dispersal of seeds by various animals are biological factors; but pollination and dispersal by the wind cannot be so regarded.
It is also probable that the various forms of the angiospermous flower, with its many specialized mechanisms for pollination, may be the result of insect-visits, the flowers becoming adapted to certain kinds of insects, and the insects having undergone corresponding modification.
Pollination is effected by aid of insects.
Owing, however, to the close proximity of stigma and anthers, very slight irregularity in the movements of the visiting insect will cause self-pollination, which may also occur by the dropping of pollen from the anthers of the larger stamens on to the stigma.
There is a close relation between the pollination of many yuccas and the life of a moth (Pronuba yuccasella); the flowers are open and scented at night when the female moth becomes active, first collecting a load of pollen and then depositing her eggs, generally in a different flower from that which has supplied the pollen.
The flowers are hermaphrodite and regular, with the same number and arrangement of parts as in the order Liliaceae, from which they differ in the inconspicuous membranous character of the perianth, the absence of honey or smell, and the brushlike stigmas with long papillae-adaptations to wind-pollination as contrasted with the methods of pollination by insect agency, which characterize the Liliaceae.
POLLINATION, in botany, the transference of the pollen from the stamen to the receptive surface, or stigma, of the pistil of a flower.
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 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.
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.
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.
Self-pollination is effected in very various ways.
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.
Cross-pollination may occur between two flowers on the same plant (geitonogamy) or between flowers on distinct plants (xenogamy).
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.
In some cases flowers, which open under normal circumstances, remain closed owing to unfavourable circumstances, and self-pollination occurs as in a FIG.
Systems of classification of flowers according to the agency by which pollination is effected have been proposed by Delpino, H.
Pollination on the surface, a more frequent occurrence than (a).
- C. Animal-pollinated plants, Zoidiophilae, are subdivided according to the kind of animal by agency of which pollination is effected, thus: a.
Many are highly specialized so that pollination can be effected by a few species only.
- Pollination I, Flower visited by a humblebee, showing the projection of the curved connective bearing the anther from the helmetshaped upper lip and the deposition of the pollen on the back of the humble-bee.
1806) was the first to study the pollination of flowers and to draw attention to the necessity of insect visits in many cases; he gave a clear account of cross-pollination by insect aid.
Sprengel came very near to appreciating the meaning of cross-pollination in the life of plants when he states that "it seems that Nature is unwilling that any flower should be fertilized by its own pollen."
The study of the fertilization, or as it is now generally called "pollination," of flowers, was continued by Darwin and taken up by other workers, notably Friedrich Hildebrand, Federico Delpino and the brothers Fritz and Hermann Muller.
Muller also suggested a modification of the Knight-Darwin law, which had left unexplained the numerous instances of continued successful self-pollination, and restated it on these terms: "Whenever offspring resulting from crossing comes into serious conflict with offspring resulting from selffertilization, the former is victorious.
An increasing number of workers in this field of plant biology in England, on the Continent and in America has produced a great mass of observations, which have recently been brought together in Dr Paul Knuth's classic work, Handbook of Flower Pollination, an English translation of which has been published (1908) by the Clarendon Press.
Arrangement self-pollination is prevented and cross-pollination ensured by the visits of bees which come for the honey secreted by the glands at the base of the inner stamens.
Self-pollination is rendered possible, since the divisions of the stigma begin to separate before the outer stamens have shed all their pollen; the nearness of the stigmas to the dehiscing anthers favours self-pollination.
In the second case the outer series (calyx of sepals) is generally green and leaf-like, its function being to protect the rest of the flower, especially in the bud; while the inner series (corolla of petals) is generally white or brightly coloured, and more delicate in structure, its function being to attract the particular insect or bird by agency of which pollination is effected.
Advance has been along two lines, markedly in relation to insect-pollination, one of which has culminated in the hypogynous epipetalous bicarpellate forms with dorsiventral often large and loosely arranged flowers such as occur in Scrophulariaceae, and the other in the epigynous bicarpellate small-flowered families of which the Compositae represent the most elaborate type.
In the polypetalous forms progression from hypogyny to epigyny is generally recognized, and where dorsiventrality with insect-pollination has been established, a dominant group has been developed as in the Leguminosae.
Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.
Pollination in cycads has always been described as anemophilous, but according to recent observations by Pearson on South African species it seems probable that, at least in some cases, the pollen is conveyed to the ovules by animal agency.
The ovule is usually surrounded by one integument, which projects beyond the tip of the nucellus as a wide-open lobed funnel, which at the time of pollination folds inwards, and so assists in bringing the pollen-grains on to the nucellus.
In function the perianth may be compared with a unilocular ovary containing a single ovule; the projecting integument, which at the time of pollination secretes a drop of liquid, serves the same purpose as the style and stigma of an angiosperm.
The flowers contain honey, and attract flies, short-lipped bees or other small insects by the agency of which pollination is effected.
Pollination of Salvia Pratensis.
Stratiotes has similar flowers which come above the surface only for pollination, becoming submerged again during ripening of the fruit.
After pollination the female flower becomes drawn below the surface by the spiral contraction of the long stalk, and the fruit ripens near the bottom.