33* 
PHYSIOLOGY 
water is sweet with sugar and often fragrant. Not all glands, however, 
secrete water and its solutes. There are glands whose secretion is an 
essential oil, 1 of which a great variety are formed. Still others secrete 
resin, which may be formed from an essential oil. 
Form of glands. The form of glands is various. A single epidermal 
cell may differ from its neighbors; it may be level with them, or sunk, or 
raised upon a shorter or longer stalk, like the glandular hairs (fig. 631). 
\ filament or a cluster of such cells may form a stalked gland (fig. 632); 
the gland cells 
may 
rather 
FIG. 632. Gland (g) from the upper surface of th *eaf of 
lilac (Syringa vulgaris) : e, epidermis ; c, cuticle ; p, f palisade 
cells ; i, intercellular space. 
form a 
indefinite 
mass, or they 
may line a shal- 
low cavity (fig. 
633), or a deep 
pouch, as in the 
nectary of the 
nasturtium (fig. 
634); or they 
may be the epi- 
thelium of a sim- 
ple or branched 
duct, as in the 
lilies (fig. 635). Nor do all glands pour out their secretion on the 
surface. The gland cells may part when young, forming intercel- 
lular spaces into which the secretion exudes to escape through water 
pores. Or a single intercellular space may develop in the center of the 
group (fig. 636) which receives the secretion; then as the gland and space 
grow, the secreting cells form an epithelium for a closed reservoir, 
larger or smaller, containing the secretion. Or, later, by the destruction 
of the gland cells loaded with the secretion, it finally occupies their place 
as well as the intercellular space, and reaches the surface only by 
mechanical rupture of enveloping tissues (fig. 637). 
Emission of secretions. Very little is known of the chemical pro- 
cesses by which the peculiar materials of the secretion are formed. 
Each sort of gland doubtless pursues a different course. Nor is it pos- 
sible to account for the emission of the various substances. Some, like 
1 Not true oils, from which they may be distinguished by making only a transient grease 
spot on paper. 
