STRUCTURE OF CUTICLE AND STOMATA. 391 



246. The Cuticle in many Plants, especially those belonging 

 to the Grass tribe, has its cell-walls impregnated with silex, like 

 that of the Equisetum (§ 220) ; so that, when all the organic 

 matter has been got rid of by heat or by acids, the forms of the 

 cuticle-cells, hairs, stomata, &c., are still marked out in silex, and 

 are most beautifully displayed by Polarized light. Such silicified 

 cuticles are found on the husks of the " grains" yielded by these 

 plants : and there is none in which a larger proportion of mineral 

 matter exists, than that of Bice, which contains some curious 

 elongated cells with toothed margins. The hairs with which 

 the palece (chaff- scales) of most Grasses are furnished, are 

 strengthened by the like siliceous deposit ; and in the Festuea 

 pratensis, one of the common meadow grasses, the palese are also 

 beset with longitudinal rows of little cup-like bodies formed of 

 silica. The cuticle and scaly hairs of Beutzia also contain a lal-ge 

 quantity of silex ; and are remarkably beautiful objects for the 

 Polariscope. 



247. Externally to the cuticle, there usually exists a very deli- 

 cate transparent pellicle, without any decided traces of organiza- 

 tion, though occasionally somewhat granular in appearance, and 

 marked by lines that seem to be impressions of the junctions of 

 the cells with which it was in contact. When detached by 

 maceration, it not only comes ofl" from the surface of the cuticle, 

 but also from that of the hairs, &c., which this may bear. This 

 membrane, the proper Epidermis (p. 388, note), is obviously 

 formed by the agency of the cells of the cuticle ; and it seems to 

 consist of the external layers of their thickened cellulose walls, 

 which have coalesced with each other, and have separated them- 

 selves from the subjacent layers, by a change somewhat analogous 

 to that which occui-s in the Palmellese (§ 194), the outer walls of 

 whose original cells seem to melt away into the gelatinous in- 

 vestment, that surrounds the "broods" which have originated in 

 their subdivision. 



248. In nearly all Plants which possess a distinct Cuticle, this 

 is perforated by the minute openings termed Stomata (Figs. 182, 

 183, c, c) ; which are bordered by cells of a peculiar form, dis- 

 tinct from those of the cuticle, and more resembling in character 

 those of the tissue beneath. These boundary-cells are usually 

 somewhat kidney-shaped, and lie in pairs (Fig. 185, b, b), with 

 an oval opening between them ; but by an alteration in their 

 form, the opening may be contracted or nearly closed. In the 

 cuticle of Yucca, however, the opening is bounded by two pairs 

 of cells, and is somewhat quadrangular (Fig. 180) ; and a like 

 doubling of the boundary-cells, with a narrower slit between 

 them, is seen in the cuticle of the Indian Corn (Fig. 181). In 

 the stomata of no Phanerogamic Plant, however, do we meet 

 with any conformation at all to be compared in complexity with 

 that which has been described as existing in the humble Mar- 



