,378 



THE MICROSCOPE AND ITS REVELATIONS. 



-and present an approach to a rectangular contour; their margins being 

 ^straight in the Yucca and Iris, but minutely sinuous or crenated in the 

 Indian Corn. In most Dicotyledons, on the other hand, the cells of the 

 epiderm depart less from the form of circular disks; but their margins 

 usually exhibit large irregular sinuosities, so that they seem to fit together 

 like the pieces of a dissected map, as is seen in the epiderm of the nipple 

 (Fig. 270, b,b). Even here, however, the cells of that portion of the epi- 

 derm (a, a) which overlies the ' veins ' of the leaf, have an elongated 

 form, approaching that of the wood-cells of which these veins are chiefly 

 composed; and it seems likely, therefore, that the elongation of the ordi- 

 nary epiderm-cell of Monocotyledons has reference to that parallel arrange- 

 ment of the veins which their leaves almost constantly exhibit. 



378. The cells of the epiderm are colorless, or nearly so, having no 

 chlorophyll in their interior; and their walls are generally thickened by 

 secondary deposit, especially on the side nearest the atmosphere. This 

 -deposit is of a waxy nature, and consequently renders the membrane very 

 impermeable to fluids, so as to protect the soft tissue of the leaf from dry- 

 ing up. In most European plants the epiderm contains but a single row 

 of cells, which, moreover, are usually thin-sided; whilst in the generality 

 of tropical species, there exist two, three, or even four layers of thick-sided 

 cells; this last number being seen in the Oleander, the epiderm of which, 



Portion of Epiderm of upper surface of Leaf of Rochea falcata, as seen at A from its inner 

 .-side, and at B from its outer side: a, a, small cells forming timer layer; 6, 6, large prominent cells 

 of outer layer; c, c, stomata disposed between the latter. 



when separated, has an almost leathery firmness. This difference in con- 

 formation is obviously adapted to the conditions of growth under which 

 these plants respectively exist; since the epiderm of a plant indigenous to 

 temperate climates, would not afford a sufficient protection to the inte- 

 rior structure against the rays of a tropical sun; whilst the less powerful 

 heat of this country would scarcely overcome the resistance presented by 

 the dense and non-conducting tegument of a species formed to exist in 

 tropical climates. 



379. A very curious modification of the epiderm is presented by the 

 Rochea falcata, which has the surface of its ordinary epiderm (Figs. 271, 

 272, a, a, nearly covered with a layer of large prominent isolated cells, 

 b, b. A somewhat similar structure is found in the Mesembryanthemum 

 crystallinum, commonly known as the Ice-plant; a designation it owes to 

 the peculiar appearance of its surface, which looks as if it were covered 

 with frozen dewdrops. In other instances, the epiderm is partially 

 invested by a layer of scales, which are nothing else than flattened cells, 

 often having a very peculiar form; whilst in numerous cases, again, we 

 nd the surface beset with hairs, which occasionally consist of single 



