AN ENCYCLOPEDIA OF HORTICULTURE. 



179 



Pollen continued. 



beyond genera ; nor does similarity of form of grain 

 necessarily indicate affinity. Zostera possesses one of 

 the most curious forms, the grains in this plant being 

 long, and extremely slender and thread-like. The colour, 

 in most plants, is some shade of yellow, but in some it 

 is deep orange (Lilium tigrinum), or red (Verbascum), 

 or bine (Scilla), or deep purple, approaching black. 



The contents of the grain are known as the fovilla. 

 They consist of viscid protoplasm, full of small starch 

 granules and oil-drops. Amidst this mass, in general, 

 lie two bodies, like nuclei, the nature of which has been 

 made clear, by the researches of Elfving and of Stras- 

 burger, within the past few years, and is most easily 

 understood if we look to the Pollen of Coniferce. In the 

 Scotch Fir, the very light Pollen has the outer coat 

 prolonged into two outgrowths containing air, which 

 render the grain light. There is comparatively little 

 difficulty in making out that there are three cells con- 

 tained within the large cell seen in the middle, and the 

 multicellular nature of the grain remains evident through- 

 out its existence in the Fir. 



In other Coniferce, e.g., the Tew, the Pollen is egg- 

 shaped, and there is a small part cut off by a partition 

 at the smaller end, rendering the grain two-celled; each 

 cell has a nucleus. In Monocotyledons and Dicotyledons, 

 the structure is less easily traced. In some (e.g., in 

 Pollen grains of Orchids), a small part at one angle of 

 the cell contents becomes separated from the rest, (though 

 a cell wall does not form between), and is called the 

 "vegetative cell." It draws itself away from the side 

 wall, and becomes imbedded in the contents of the large 

 cell. For a time, it remains different in form from the 

 nucleus of the large cell, but ultimately it becomes 

 quite like that nucleus, so that there seem to be two 

 nuclei. The vegetative cell, in many Pollen grains, 

 breaks up into two or more cells, and, in some (e.g., 

 Scirpus palustris), the process becomes quite complex. 

 When the Pollen tube is formed, the nucleus and the 

 vegetative cell, or cells, pass into it, and have been 

 traced into the end of it that passes down the micro- 

 pyle and comes into contact with the helper cells (see 

 Ovule). It is supposed that they perform some very 

 important function in the formation of the embryo. 

 The formation of the vegetative cells in the interior of 

 the Pollen grains is generally regarded as representing 

 the formation of the male prothallium in such Crypto- 

 gams as Selaginella. See Prothallium. 



POLLICAE.IS. The length of the terminal joint 

 of the thumb ; lin. 



FOLLICHIA. A synonym of Trichodesma (which 

 see). 



POLLINATION. The dusting of the stigma of a 

 flower with pollen grains, as distinguished from fertili- 

 sation, or the action of the pollen upon the ovule, 

 which gives rise to the development of the seed con- 

 taining an embryo. Pollination must precede fertilisa- 

 tion. It is effected in very different ways in different 

 flowers, and the agents by which it is effected are 

 manifold. The more important of these are here indi- 

 cated; but the account must be brief, though volumes 

 have been written upon the subject of Pollination. 

 Full information of the present state of our know- 

 ledge of the matter will be found in Dr. H. Muller's 

 "Fertilisation of Flowers," translated by Prof. W. 

 D'A. Thompson ; Darwin's " Fertilisation of Orchids ;" 

 and Kerner's "Flowers and their Unbidden Guests," 

 translated by Dr. Ogle. Sir John Lubbock's "British 

 Wild Flowers in Eelation to Insects " contains much 

 interesting information regarding the subjects indicated 

 in the title. There are also many shorter works and 

 articles upon Pollination published in most European 

 languages of late years. A list of these, compiled by 



Pollination continued. 



Prof. Thompson, is included in Muller's work referred 

 to above, and includes almost all published up to 1883 



The method of Pollination varies with the structure 

 of the flower. In those plants in which the pollen and 

 the ovules of the same flower ripen simultaneously, 

 the pollen may be transferred directly from the anthers 

 to the stigma either by the parts lying in contact, or 

 by their lying in such a position that the pollen falls 

 from the anthers upon the stigma. In cleistogamous 

 flowers, or those (e.g., in Sweet Violets, Wood Sorrel) 

 which, in certain plants, are formed in summer and 

 autumn, and never open, but yet are often more pro- 

 ductive of seeds than the conspicuous flowers, the pollen, 

 while in the anthers, pushes pollen tubes to the stigma, 

 which is thus Pollinated. In by far the greater number 

 of hermaphrodite flowers, even of those in which the 

 pollen and the stigma mature simultaneously, the 

 pollen is prevented from falling upon or reaching the 

 stigma by the arrangement of the parts. Necessarily, in 

 those hermaphrodite flowers in which the pollen and 

 the stigma do not mature simultaneously, the pollen 

 cannot reach the stigma of the same flower (see Pro- 

 terandrous and Proterogynous). In all these 

 cases, the pollen must be transferred from the anthers 

 to the stigmas by some agency from outside the 

 flower; and this must be so yet more evidently in the 

 case of unisexual flowers, whether male and female 

 flowers be on the same plant or on different ones. The 

 chief agents that effect Pollination in British plants 

 are wind and insects. A few plants are adapted for 

 conveyance of pollen by currents of water ; and, in the 

 tropics, humming-birds, and certain other birds, probably 

 aid materially. It is unnecessary*to refer further to these 

 latter agencies, since they would probably not come 

 under notice in gardening operations at all in the British 

 Islands. Plants suited for fertilisation by wind are 

 usually called " anemophilons," or wind - lovers (from 

 anemos, the wind, and phileo, I love). Those adapted 

 to have the stigmas Pollinated by insects, are called 

 " entomophilous," or insect-lovers (from entomon, an in- 

 sect, and phileo, I love). They differ from one another 

 so widely that a practised observer can conjecture almost 

 with certainty to which group any flower would belong, 

 though previously quite unacquainted with the flower. 

 The more distinctive characters of the two are as 

 follows : Anemophilous flowers are seldom large or 

 conspicuous individually ; the sepals and petals are 

 small, usually regular, often absent, or reduced to one 

 row of small, scaly bodies (e.g., Oak) ; they seldom 

 contain nectar, or afford other attractions for insect 

 visitors; the stamens have long filaments, with versa- 

 tile anthers, that turn with the least breath of wind, 

 and thus shed readily the loose, powdery, smooth pollen, 

 which is often produced in very great amount. The 

 grains are very light, and are occasionally (e.g., in 

 Firs) rendered relatively lighter by means of dilatations 

 of the outer coat filled with air. The stigma in such 

 plants is usually furnished at the end (Pellitory) or along 

 the sides (Grasses, &c.) with a quantity of long, simple 

 or branched hairs, which frequently hang out beyond the 

 perianth, or other coverings, e.g., beyond the glumes in 

 Grasses, and entangle the pollen grains when these are 

 carried against them by the wind. Anemophilous plants 

 are often social. Many trees under this group produce 

 their flowers in spring, before the leaves, thus prevent- 

 ing great loss of pollen among the leaves, and favouring 

 Pollination. Entomophilous flowers are the reverse of all 

 this. They are almost always more or less individually 

 conspicuous, with well-developed, coloured petals, and 

 often also coloured sepals, or are crowded in showy 

 masses. They are sometimes regular, but more generally 

 are only bilaterally symmetrical i.e., they have the two 

 sides alike, us in most Orchids and Leguminosce (see 



