POLLEN. 



[ 515 ] 



POLLEN. 



two Nicol's prisms, or two plates of tourma- 

 line. Some artificially prepared crystals 

 exert a powerful polarizing action, and may 

 be used either as polarizers or analysers, or 

 as both ; among these the salt of quinine 

 occupies the first place. Others form in- 

 teresting analysers, some of which have been 

 noticed under Analytic Crystals and 



DiCHROISM. 



Numerous salts and other crystalline 

 bodies, which pow^erftdly depolarize the 

 already polarized light, and exhibit beautiful 

 colours, are mentioned under their respective 

 heads ; some of these may be enumerated 

 here ; as the oxalate of ammonia, of soda, 

 and of chromium and ammonia; the oxalu- 

 rate of ammonia, the acetate of copper, 

 chlorate of potash, the prismatic form of the 

 ammonio-phosphate of magnesia, the am- 

 monio-phosphate of soda, the sulphates of 

 cadmium and of magnesia, selenite, salicine, 

 lu-ic acid, &c. 



Many organic bodies and tissues also 

 possess considerable depolarizing power ; as 

 horse-hair, portions of feathers, sections of 

 quill, of hoof, horn, &c. ; the siliceous 

 cuticle of the Equiseta and various grasses, 

 starch-grains, &c. 



The examination of objects by polarized 

 Ught is often looked upon as a mere amuse- 

 ment, and the polariscope as a toy, and to 

 those who can perceive only the beautiful 

 colours, such they are. Their use, however, 

 consists in developing the optical properties 

 of bodies, the cause of which may be deter- 

 mined by histological analysis (Intro d. 

 p. xxxvi.) ; and for this purpose they are 

 invaluable and indispensable. 



Objects to be examined by polarized light 

 shoidd be immersed in turpentine or balsam, 

 to render them as transparent as possible. 



BiBL. Herschel, Encycl. Metropol. art. 

 Light; Pereira, hectures on Polarized Light, 

 by B. Powell; Woodward, On Polarized 

 Light ; Brewster, Optics ; Erlach, Mik. 

 Beobacht. iib. organ. Element, bei Polar. 

 Licht. Mailer's Archiv, 1847. 



POLLEN. — This name is applied to the 

 coloured pulverulent substance famihar to 

 every one as occurring scattered in the 

 interior of fuU-blowni flowers; it is produced 

 in the anthers, the (usually) stalked club- 

 shaped organs which stand in one or more 

 circles between the floral envelopes and the 

 pistils, and is discharged from them when 

 ripe, in order to fertilize the ovules. When 

 slightly magnified, the pollen of most flowers 

 appeai-s to consist of granules, of different 



size and colour in different plants; hence 

 the individual particles are called pollen- 

 grains or granules (PI. 32). Examination 

 under a sufficient magnifying power shows 

 that the simple or typical forms of pollen- 

 grains are single^ free cells, filled with fluid 

 matter ; more complex forms occur in many 

 cases, which, however, may be simply cha- 

 racterized as groups of simple pollen-grains, 

 permanently coherent into definitely-formed 

 groups. 



The pollen-grain may be examined as to 

 the form and structure, its contents and its 

 development. 



The forms of simple grains presented in 

 different plants are tolerably varied, spherical 

 (PI. 32. figs. 8-10, 22, 23, 25) and elliptical 

 (figs. 6, 11, 29), being perhaps those most 

 common ; but besides these, numerous geo- 

 metrical forms occur, such as tetrahedral 

 (fig. 14), polygonal (figs. 16, 27, 28), cubic 

 (fig. 19). But it must be noted here that 

 the forms frequently vary according as the 

 pollen is viewed dry or in fluid, since the 

 elliptical and allied forms often expand into 

 a spherical form, when they absorb hquid 

 (figs. 18 & 20 a, b, c). The explanation of 

 this wdll be given presently. The external 

 appearance is fm-ther greatly influenced by 

 minor peculiarities of form, such as ridges, 

 spines and processes of different kinds; 

 these, however, are referable to the structm'c 

 of the outer coat. 



The ordinary structure of the coats or the 

 cell-wall of the pollen-grain is that of a 

 delicate internal cell-membrane, with an 

 outer, thick and resisting layer, which may 

 be regarded as the Cuticle of the inner or 

 proper membrane of the cell. In a few 

 cases the inner membrane alone exists, as 

 in the cylindrical pollen-cells of Zostera, 

 and some other aquatic plants. In other 

 cases, the outer or cuticular coat presents a 

 more complex structure, and two or, it is 

 said, even three layers may be distinguished 

 in it ; these, however, seem to be merely a 

 lamination of the outer coat. The conditions 

 in some of the Coniferae are different from 

 this, and will be alluded to presently. The 

 inner membrane is exceedingly dehcate and 

 homogeneous, in ordinary spherical or oval 

 grains it accurately lines the outer coat ; in 

 some of those forms w hich present processes 

 of various kinds, such as (Enothera, it seems 

 to us that the inner coat does not extend 

 into these processes in the mature pollen. 

 The outer coat exhibits, as to sm'face, every 

 variety of appearance from smooth, through 



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