STRUCTUEE OF LEAVES 



715 



The epiderm in many plants, especially those belonging to the 

 grass tribe, has its cell-walls impregnated with silex, like that of 

 Equisetum ; so that, when the organic matter seems to have been 

 got rid of by heat or by acids, the forms of the epidermal cells, hairs, 

 stomates. itc.. are still marked out in silex, and (unless the dissipa- 

 tion of the organic matter has been most perfectly accomplished) 

 arc most beautifully displayed by polarised light. Such silicified 

 epiderms are found in 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 rice, which contains .some curious elongated cells 

 with toothed margins. The hairs with which the joafece (chaff-scales) 

 of most grasses are furnished are strengthened by the like siliceous 

 deposit ; and in Festtn-n /n-titi-nxix. one of the common meadow- 

 grasses, the palese are also beset with longitudinal rows of little cup- 

 like bodies formed of silex. The epiderm and scaly hairs of Deutzia 

 'scabra also contain, a large quantity of silex, and arc remarkably 

 beautiful objects for the polariscope. 



In nearly all plants which possess a distinct epiderm, this is 

 perforated by the minute openings termed stoinuli'* (tigs. 557, 561), 

 which are bordered by cells of a peculiar form, the guard-cells, 

 differing from those of the epiderm. and more resembling in character 

 those of the tissue beneath. 

 They are further distinguished 

 by containing a larger number 

 of chlorophyll-grains than the 

 ordinary cells of the epiderm. 

 These guard-cells are usually 

 somewhat kidney-shaped, and 

 lie in pairs (fig. 5G1, ?>), with 

 an oval opening between them : 

 but by an alteration in their 

 form, the opening may be con- 

 tracted or nearly closed. In 

 the epiderm of Yucca, however, 



the opening is bounded bv two 



/ n i i , PIG. 561. Portion of epiderm of leat ot Lns 



of cells, and is somewhal 



' 



pairs 



quadrangular (fig. 550) ; and a 

 like doubling of the guard- 

 cells, with a narrower slit be- 

 tween them, is seen in the epi- 

 derm of the Indian corn (lig. 

 557). In the stomates of no 



i/rrniiiii/rit torn from its surface, and 

 carrying away with it a portion of the 

 parenchymatous layer iu immediate con- 

 tact with it: a, a, elongated cells of the 

 epiderm ; b, b, cells of the stomates ; c, c, 

 cells of the part-nchyme ; (1, (1, impressions 

 011 the epidermal cells formed by their 

 contact ; e, cavity in the parenchyme. cor- 

 responding to the stomate. 



phanerogam, however, do we 

 meet with any conformation at all to be compared in complexity 

 with that which has been described in the humble Marchantia. 

 Stomates are usually found most abundantly (and sometimes exclu- 

 sively) in the epiderm of the lower surface of leaves, where they open 

 into the air-chambers that are left in the parenchyme which lies 

 next the inferior epiderm ; in leaves which float on the surface of 

 water, however, they are found in the epiderm of the upper surface 

 only ; whilst in leaves that habitually live entirely submerged, as 



