POLARIZATION OF LIGHT. [ G13 ] 



POLLEN. 



most important matter is the revelation, by 

 the use of the selenite plates, of the exist- 

 ence of positive and negative characters, 

 like those of positive and negative crj-stals, 

 in the chemically distinct constituents of 

 vegetable tissues. 



Let us suppose that between the lower 

 prism and the object is placed a plate of 

 selenite giving a red field ; the plate is then 

 rotated so that its neutral axes are at an 

 angle of 45° with the prisms. A section 

 of a cylindiical vegetable cell will be seen 

 to be divided into four quadrants : the two 

 alternate quadrants, whose middle lines cor- 

 respond to the neutral axes of the selenite, 

 are either blue or green, the other two 

 yellow or red : if the selenite is then rotated 

 so that its neutral axes are perpendicular to 

 the prisms, the colours will be all lost ; but 

 on continuing the rotation, they reappear in 

 the reverse order — what was blue appearing 

 yellow, and vice versa. When the walls are 

 rectilinear, all the ceU-walls perpendicular 

 to one of the prisms will give the colour of 

 the field, all those which run parallel with 

 one of the neutral axes of the selenite plate, 

 or form no great angle with it, will be blue, 

 those parallel with the other axis yellow. 



It is found that vegetable structures fall 

 into two classes in reference to these colours, 

 in one of which classes all layers lying ob- 

 liquely in the direction of a right-wound 

 screw are tinged blue and yellow, those 

 oblique in the opposite direction yellow or 

 red ; in the other class, the colours under 

 the same conditions are just the reverse ; so 

 that one class are optically positive, the 

 other optically negative. 



The optically negative are the ordinaiy 

 cell-membranes of the internal organs of 

 plants, whether in their natural condition 

 or cellidose purified by the help of nitric 

 acid and chlorate of potash : collenchyma, 

 horny endosperm-cells, the gelatinous ceUs 

 of Algae, &c., all agree in this property. 

 Optically positive colours are given by cell- 

 membranes of periderm and the cuticular 

 layers of epidermal cells. The contrast of 

 the positive and negative colours of the 

 cuticle and other parts of the cell-wall is 

 well seen in the epidermis of Aloe. The 

 diversity of colouring under polarized light 

 here corresponds to the diverse behaviour 

 under treatment with iodine after macera- 

 tion in solution of potash (Secoxdary 

 deposits). 



The longitudinal sections of all behave 

 like the cross sections j but the appearances i 



are not so clear. When side views of the 

 surface of cells ai'e obtained, the phenomena 

 are very varied ; but these are best seen in 

 vessels or ducts when the thickening layers 

 are in the form of spiral bands. Tluis, if 

 one of the spiral vessels of Musa is placed 

 (its spiral somewhat drawn apart) with its 

 long axis perpendicular to one of the prisms, 

 the fibres on the upper side turn to the left, 

 those on the under side towards the right ; 

 and when the selenite plate is interposed, 

 they exhibit the complementary colours. 

 When the side walls of cells have obscure 

 striation, as in the cells of Conifers, the 

 liber-cells of Apocyneas, kc, the membrane 

 gives evidence of its fibrillar structure by 

 the yellow or blue colour developed with 

 the selenite plate. If fibres of a spiral 

 vessel cross at right angles, and they are 

 pressed together, they neutralize one another 

 where they cross : when the prisms are 

 used alone, the crossing points are black, 

 the rest of the fibres white ; when the 

 selenite plate is interposed, the crossing 

 points exhibit the colour of the field, and 

 the uncrossed portions of the fibre are blue 

 or yellow according to position. 



The vicinity of a round bordered pit, as 

 in the wood-cells of Finns, exhibits a black 

 cross when seen perpendicularly by polarized 

 light. The black cross and the colours ex- 

 hibited by starch are well known. Chloro- 

 phyll does not seem to act on polarized 

 light, nor the primordial utricle of cells, 

 except a trace when contracted by weak 

 alcohol. 



The polarization apparatus is exceedingly 

 useful for the detection of ciystals (Raphi- 

 DEs) in vegetable tissues, when thej'^ are so 

 small as to be easily overlooked ; and the 

 larger kinds form beautiful objects with, 

 and often without the selenite plate. 



BiBL. Herschel, Encycl. Metrop. art. 

 Light ; Pereira, Lec-t. on Pol. Light, hy B. 

 Powell; Brewster, Optics; Erlach, Beobacht, 

 iib. organ. Element, bei polar. Licht, Miiller's 

 Archiv, 1847; Valentin, Untersuch. 1801; 

 Lobb, Qu. Mic. Jn. viii. 107; Carpenter, 

 The Microscope ; Beale, How 8fc. ; JDavies- 

 Matthews, Mounting ^-c. 



POLLEN.— This name is applied to the 

 coloured pulverulent substance familiar to 

 every one as occurring scattered in the in- 

 terior of full-blown 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 



