7 EPIDERMIS. 293 
39. Stomata are always placed over, and communicate with, 
the intercellular passages, 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 abundant 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 have 
been counted within the space of a square inch. The largest known are about 
sto of an inch in length. Their function is intimately connected with respira- 
FiG. 3.— Hairs and glands ; a, c, simple hairs; 6, branched hair of the mullein; d, gland 
surmounted by a hair; e, gland at the top of a hair; 7, prickles of the rose. 
°40. The surface of the epidermis is either smooth, or furnished 
with numerous processes, originating from itself, or from the 
cellular substance beneath it. These are of several classes, 
namely, glands, 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 s¢mple or branched. (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 surface, 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 they are entangled and matted; arachnoid, when like 
cobwebs ;—sericeous, when silky;—velvety, when they are short, soft, and 
dense ; — ciliate, when long and fringed, like the eyelash. 
42. Strives are tubular and acute hairs, fixed upon minute 
glands in the cuticle, which secrete an acrid fluid. By the 
