ANGIOSPERMS
435
of powdery pollen, much of which never reaches its proper described when pollination is effected. But this is not duction destination, and the pollen-grains themselves are Methods always the case. The embryo-sac may not be formed, the smooth and light. The anthers as the distributing 0f crossovule itself may hardly be visible when the pollen-grain organs are freely exposed, and the outgo ol the p0iuna. reaches the style, and in these cases it is through the pollen-grains in suitable weather is facilitated by such tion. as the assemblage of the flowers in catkins, stimulus of the growth of the pollen-grain that the changes means their hanging from slender pedicels, long slender filaments to in the embryo-sac are initiated. the anther, explosive mechanisms in the flower, and similar The pollen-grains escape from the anther by its rupture arrangements. The reception of the pollen is aided by a free through differences in drying between its epidermal layer and extensive entangling stigmatic surface, produced by copious and the fibrous wall of the pollen - sacs within. branching of the style. Bright colours, nectar - glands giving Pollina- As a rule the pollen-grains are simply exposed honey odour, and generally conspicuousness, are more or less absent. Many flowers, however, which are anemophilous may tion. by this rupture, but in other cases the relations be also zooidiophilous. There is no doubt that the fertilization of the stamens and the flower-envelopes give rise to an of many flowers is effected by animals in their transit over the upon which the plants are growing. Such pollinations are, explosive mechanism, brought into action either from areas however, entirely fortuitous. On the other hand, the majority oi the inside by the natural expansion of the flower, or flowers are regularly visited by insects of many kinds, and these by mechanical disturbance from without, by which the insects effect pollination. Such flowers are entomophilous, and pollen-grains, which in such a case are always dry and are the chief zooidiophilous ones. Here conspicuousness oi kind is a primary requirement for the securing of insectpowdery, are shot out to some distance from the flower. some visits, and this may be effected in various ways, for example, by The destination of the pollen - grains is the stigmatic size and form of individual flower, by aggregation of flowers in surface of the carpel, to which they cohere partly through inflorescences, by coloured and glistening surfaces in the flower the viscid secretion exuded from it, partly through or in the neighbouring parts. Doubtless the most potent lure is and alike in connexion with it and with colour it is proentanglement amongst the stigmatic papillae, and to. this odour, bable our senses do not appreciate scents and colours which are outgrowths of the exine may contribute. The proximity recognizable by insects. The gain to the insect by its visit is of the stamens and carpels in a hermaphrodite flower, and the honey, which is often protected in elaborate fashion, so that the fact of their ripening, as is common, at the same only insects of special form can secure it, and the pollen itself. In for the honey the insect becomes dusted with pollen-grains time, render possible the direct application of the pollen seeking from the anthers—which are found in positions in relation to the to the stigma either by contact of anther and stigma, by whole form of the flower such that this dusting of the body of a its falling from anther upon stigma, or in other ways, suitable insect is readily effected, the pollen-grains themselves within the one flower. This is self-pollination and it having commonly more or less tuberculated surfaces, and being sticky—and it carries them to the stigma of flowers takes place undoubtedly with frequency in nature, and somewhat subsequently visited. Birds sometimes effect pollination ol may be said to be the raison d’etre of the hermaphrodite flowers (ornithophilous), snails are also sometimes the agents flower. It is often called self-fertilization or autogamy, (malacophilous), and other animals—in all of these cases the attracbut in using these terms and other terms of similar form, tion of food brings the animal to the flower. The relationships ol to animals, especially insects, are infinite in variety, and and they are common, it must be remembered that polli- flowers are the subject of a copious literature. Pollination by insects is nation is only a spore-distribution, and a step on the way a prevalent method of cross-pollination at the present epoch, and towards the accomplishment of the sexual act which stands in correlation with the evolution of the angiospermous is fertilization, and which indeed may not. result flower. It is manifestly a more certain and more economical one pollination by wind. Its hazard is the absence of the proper after pollination. When the flowers are umsexua', than insect, especially in cases where the form-development is such whether on the same plant (monoecism) or on separate that but few species of insects can effect it.. Pollination by wind ones (dicecism), such facile pollination is precluded, and is a more primitive method. Pollination in the ways described it must be effected by the conveyance of pollen from the must often result in the application of pollen-grains of one species the stigma of another. Such foreign pollen is frequently stamens of one flower to the stigma of another. . This upon sterile upon the stigma, and no result follows. There is also a is cross-pollination, often termed cross-fertilization or prepotency in its own pollen belonging to every.species, and the allogamy. Experiment has, however, shown that even application of one grain of it will render futile the germination in hermaphrodite flowers cross - pollination may be. the of any foreign pollen upon its stigma. If such pollination by pollen is followed by the completion of the sexual act, more effective process in its ultimate result after fertiliza- foreign and an embryo is formed, the offspring will be a hybrid. In tion has been effected, and that the progeny of a cross- nature such hybrids between allied forms are common, and they pollinated hermaphrodite flower is often better than that are readily produced in cultivation. They may in time be the of one self-pollinated. Cross-pollination is also frequently starting-point of specific forms. Generic hybrids are not so in nature, but in certain families of Angiosperms quite a preferred even when the flower is hermaphrodite and self- common number have been produced in horticultural practice. pollination might be possible, for there are arrangements Pollination having been effected, the pollen-grain of different kinds which may either altogether prevent moistened by the viscid secretion upon . the stigma self-pollination or at least promote cross-pollination m the first instance. Thus the relative position of anthers and resumes active growth. The intine investing Second stigma and the form of the pollen may be such as to the vegetative energid is protruded through germination of place a barrier to self-pollination {hercogamy), or the one of the germ-pores as the pollen-tube, and microapparently under chemiotactic influence grows sporeperiods of rupture of anthers and of maturation of stigma may be different {dichogamy, either protandrous or pro- downwards through the style, dissolving its togynous). At the same time, m many hermaphrodite tissue by the aid of an enzyme. The vegetative energid is flowers which are normally cross-pollinated, self-pollination found at the tip of the tube, and in dicotyledonous plants becomes possible before withering, and may take pface is followed out of the spore by the male gametes, in should cross-pollination have failed. On the other hand, monocotyledonous plants by the generative energid which there are flowers which are so constructed as entire y o then divides in the tube and forms a pair of gametes. exclude the chance of cross-pollination, inasmuch as they The pollen-tube reaches the cavity of the ovary and do not open, and self-pollination is therefore a necessity commonly passes along the placenta, guided by projections (cleistogamy). The agencies by which cross-pollination is or hairs, to the micropyle of the ovule; this it enters, and effected in Geophytes are wind and animals, and their flowers dissolving its way through any cells of the nucellus interhave been classed as anemophilous and zooidiophilous, m vening between it and the apex of the embryo-sac, it conformity with their adaptation to pollination by one or pierces the latter. This piercing, however, is not always necessary, for the egg-apparatus may have developed so as other of these agencies. Anemophilous flowers are characterized by the prodigal pro- to rupture the embryo-sac, and even the apex of the
