KriDERMIS. 



25 



39. Stomata are always placed over, and communicate ^vith, 

 the intercellular 2}assages, that is, the spaces between the cells of 

 the tissue. They are never found on the midrib, or veins, of 

 the leaf, or over any ligneous part of the structure. They are 

 most abmidant over the soft, green tissue of the leaves, young 

 shoots, and the parts of the flower. 



a. These organs are of a size so minute, that more than 100,000 of them havo 

 been counted witliin the space of a square inch. The largest kno^\^l arc about 

 gX.^ of an inch in length. Tlieir function is intimately connected with respira- 

 tion. 



FIG. 3. — Hairs and glands ; a, c, simple hairs ; b, branched hair of the mullein ; rf, gland 

 Burmounted by a hair; e, gland at the top of a hair; _/", prickles of the rose. 



40. The surface of the epidei*mis is either smooth, or furnished 

 with numerous processes, originating from itself, or from the 

 cellular substance beneath it. These are of several classes, 

 namely, glaiids, hairs, prickles, stings, &c. 



41. Hairs are minute expansions of the epidermis, consisting 

 each of a single lengthened cell, or of a row of cells, placed end 

 to end, containing air. They are si7n2yle or bra?iched. (Fig. 3.) 



a. Hairs are occasionally found upon the leaves, stem, and indeed upon any 

 other part. In the cotton plant (Gossypium) they envelope the seed. They give 

 various names to the surfiice, to which they are appended, according to their 

 nature and appearance ; thus it is said to be downy, or pubescent, when clothed 

 with soft, short hairs; — hirsute, with longer hairs: — rough, with short, stiff 

 hairs; — tomentose, -when Uiey are entangled and matted; arachnoid, when like 

 cobwebs; — sericeous, when silky; — velvety, when they are short, soft, and 

 dense ; — dliate, when long and fringed, like the eyelasli. 



42. Stings are tubular and acute hairs, fixed ujion minute 

 glands in the cuticle, which secrete an acrid fluid. By the 



