STRUCTURE OF CUTICLE AND LEAVES. 417 



scarcely be conceived save by those who have made a special study 

 of these "wonderful structures. Even the commonest Trees, Shrubs, 

 and Herbaceous Plants, yield specimens that exhibit a varied 

 elaboration of design, which cannot but strike with astonishment 

 even the most cursory observer ; and there is none in which a 

 careful study of sections made in different parts of the stem, and 

 especially in the neighbourhood of the 'growing point,' will not 

 reveal to the eye of the scientific Physiologist some of the most 

 important phenomena of Vegetation. — Fossil Woods, when well 

 preserved, are almost invariably Silicified, and require, therefore, 

 to be cut and polished by a Lapidary. Should the Microscopist be 

 fortunate enough to meet with a portion of a Calcified stem in 

 which the Organic structure is preserved, he should proceed with it 

 after the manner of other hard substances which need to be 

 reduced by grinding (§§ 138-40). 



308. Structure of the Cuticle and Leaves. — On all the softer 

 parts of the higher Plants, save such as grow under water, we find 

 a surface-layer, differing in its texture from the parenchyma be- 

 neath, and constituting a distinct membrane, known as the Cuticle. 

 This membrane is composed of Cells, the walls of which are flat- 

 tened above and below, whilst they adhere closely to each other 

 laterally, so as to form a continuous stratum (Figs. 225, 227, a, a). 

 Their shape is different in almost every tribe of Plants ; thus 

 in the Cuticle of the Yucca (Fig. 221), Indian Corn (Fig. 222), 

 Iris (Fig. 226), and most other Monocotyledons, the cells are 

 elongated, 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 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 Apj>le (Fig. 223, b, b). 

 Even here, however, the Cells of that portion of the Cuticle (a, a) 

 which overlies the Veins of the leaf, have an elongated form, ap- 

 proaching 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. 



309. The Cells of the Cuticle are colourless, or nearly so, no 

 Chlorophyll being formed in their interior ; and their walls are 

 generally thickened by secondaiy deposit, especially on the side 

 nearest the atmosphere. This deposit is of a waxy nature, and 

 consequently renders the membrane 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 



K K 



