STRUCTURE OF LEAVES 



713 



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

 (fig. 558, &, 6). Even here, however, the cells of that portion of the 

 epiderm (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 ordinary epiderm cells of monocotyledons has reference to 



FIG. 558. Portion of epiderm of lower surface of leaf of apple, 

 with layer of parenchyme in immediate contact with it : 

 a, a, elongated cells overlying the veins of the leaf; b, fe, 

 ordinary epiderm-cells, overlying the parenchyme ; c, c, 

 stomates ; d, d, green cells of the spongy parenchyme, 

 forming a very open network near the lower surface of the 

 leaf. 



that parallel arrangement of the veins which their leaves almost 

 constantly exhibit. 



The cells of the epiderm are colourless, or nearly so, having no or 

 but little chlorophyll in their interior ; and their walls are generally 



A B 



FIG. 559. Portion of epiderm of upper surface of leaf of 

 Rocliea falcata, as seen at A from its inner side, and at B 

 from its outer side: a, , small cells forming inner layer; 

 b, b, large prominent cells of outer layer; c, c, stomates dis- 

 posed between the latter. 



thickened by secondary deposit, especially 011 the side nearest the 

 atmosphere. This outermost hardened continuous wall of the 

 epidermal layer of cells is known as the cuticle. The deposit (cutiii) 

 is of a nature to render the membrane very impermeable to fluids, 

 so as to protect the soft tissue of the leaf from drying up. In most 



