THE WALLFLOWER 63 



granules. The outer layer of the superficial cell-wall 

 of the epidermis forms the cuticle, and does not con- 

 sist like the other walls of cellulose, but of a substance 

 (cutiri) resembling cork, which is extremely impervious 

 to water and watery vapour, and which is chemically 

 a remarkably stable body, resisting the action even 

 of strong sulphuric acid, and of other substances 

 which easily dissolve the cellulose. The inner and 

 lateral walls of the epidermal cells consist of ordinary 

 cellulose, and the same is the case with the innermost 

 layer of the superficial wall. 



The epidermis of the Wallflower bears very large 

 and characteristic hairs. Each hair is a single cell. 

 It has the form of a spindle, and is attached at the 

 middle by a short f stalk, which is inserted between 

 the ordinary epidermal cells (see Figs. 26 and 27, 

 which show similar hairs on the leaf). These hairs 

 are always so placed that the spindle lies close to 

 the surface of the epidermis, and parallel to the axis 

 of the stem. They have an external cuticle, like 

 other epidermal cells. Their surface is rough, with 

 projecting knobs, which contain calcium carbonate 

 (chalk). Experiments have shown that these rough 

 and sharp hairs are of great importance to the plant, 

 as a protection against the attacks of slugs and snails. 



It has already been mentioned that the ordinary 

 epidermal cells have no intercellular spaces between 

 them. There are, however, intercellular spaces in 

 the epidermis, but they only occur at definite points, 

 between specially modified cells. Such a space, 

 with the cells which enclose it, is called a stoma. 



