338 



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). 

 A 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 the leaf of 

 lilac (Syringa vulgaris): e, epidermis; c, cuticle; p, p, 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. 



