EPIDERMIS. 31 



CHAPTER IV. 



VEGETABLE ORGANS. 



THE vegetable elements and tissues which have been 

 described form, either separately or by their com- 

 bination in various ways, the organs of plants. To 

 these we shall now pass, and consider the structure 

 of the principal organs of the members of the vege- 

 table kingdom. 



Leaves. Leaves in their simplest form consist of 

 a single sheet or layer of parenchymatous cells or 

 cellular tissue, an example of which may be found 

 in almost any moss (PI. III. fig. 30). The granules 

 of chlorophyll will often be very distinctly seen in 

 these cells. The first addition to this form of leaf is 

 a row or two of prosenchymatous cells running longi- 

 tudinally down the middle of the leaf, so as to form 

 a rudimentary vein or nerve. In other and more 

 highly developed leaves, the layers of cells are nume- 

 rous, and traversed by bundles of wood- cells, vessels, 

 and ducts (fibro-vascular tissue), forming the veins, 

 the entire surface being covered with a skin or mem- 

 brane, called the epidermis. 



Epider'mis (eVl, upon, Se/^a, skin). This mem- 

 brane is composed of one or more layers of colourless, 

 closely packed cells (PI. I. figs. 13&28), the colour 

 it occasionally exhibits usually arising from some of 

 the underlying cells of the leaf being seen through 

 it, or remaining adherent to it when stripped from 

 the leaf. It is easily separated, by making a cut in 

 a soft leaf, and peeling it off with a fine pair of for- 

 ceps, or by soaking a leaf for some time in water 

 and then stripping it off. It must be remarked that 



