STRUCTURE OF THE CUTICLE. 



389 



sinuous or crenated in the Indian Corn. In most Dicotyledons, 

 on the other hand, the cells of the cuticle 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 cuticle of the Apple 

 (Fig. 182, b, b). Even here, however, the cells of the portion of 



FIG. 182. 



Portion of the Cuticle of the inferior surface of the Leaf of the Apple, with the layer of parenchyma 

 in immediate contact with it: a, a, elongated cells of the cuticle overlying the veins or nerves of 

 the leaf; b, 6, ordinary cuticle cells overlying the parenchyma; c, c, stornata; rf, d, green cells of the 

 parenchyma, forming a very open network near the lower surface of the leaf. 



the cuticle (a, a) that overlies the veins of the leaf, have an elon- 

 gated form, approaching that of the wood-cells of which these 

 veins are chiefly composed ; and it seems likely, therefore, that 

 the elongation of the ordinary cuticle-cells of Monocotyledons 

 has reference to that parallel arrangement of the veins, which 

 their leaves almost constantly exhibit. The cells of the cuticle 

 are colorless, or nearly so, no chlorophyll being formed 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 mem- 

 brane very impermeable to fluids ; the retention of which within 

 the soft tissue of the leaf is obviously the purpose to be answered 

 by the peculiar organization of the cuticle. In most European 

 Plants, the cuticle contains but a single row of cells, which are 

 usually, moreover, thin-sided ; whilst in the generality of tro- 

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

 sided cells ; this last number being seen in the Oleander, the 

 cuticle of which, when separated, has an almost leathery firm- 

 ness. The difference in conformation is obviously adapted to 

 the conditions of growth under which these plants respectively 

 exist; since the cuticle of a plant indigenous to temperate 

 climates, would not afford a sufficient protection to the interior 

 structure against the rays of a tropical sun ; whilst the diminished 

 heat of this country would scarcely overcome the resistance 

 presented by the dense and non-conducting tegument of a 



