. 
SESSILE GLANDS.—STINGS. 69 
it secretes (figs. 160 and 161), or of two (fig. 164), or more ( fig. 
165) secreting cells placed at the end of a hair ; or they congiat 
of a mass of secreting cells (figs. 162 and 163). 
Sessile Glands present various appearances, and consist, like 
the former, of either one secreting cell (fig. 167), or of two, or 
more (fig. 166). Those with one secreting cell placed above the 
level of the epidermis are frequently termed papille (fig. 167) ; 
and it is to their presence upon the surface of the Ice-plant 
(Mesembryanthemum crystallinum) that the peculiar crystalline 
appearance of that plant is due. When sessile glands are com- 
posed of cells containing solid secretions so that they form 
hardened spherical or other shaped appendages upon the surface 
of the epidermis, they are termed warts ; these are now, however, 
commonly placed among the Emergences (see page 66). 
Fic. 168. Fia. 169. 
(> 
Fig. 168. Sting of a species of Malpighia. 
e. Epidermis. 06, 6, g. Glandular appa- 
ratus. Fig. 169. Sting of the common 
Nettle (Urtica dioica), consisting of a 
single cell with a bulbous expansion at 
its base, 6, and terminated above by a 
swelling, s, and containing a granular 
irritating fluid, 7, 7 we. Epidermal 
cells surrounding its base, 
When a sessile gland contains an irritating fluid, and is 
prolonged above into one or more hair-like processes, which 
are placed horizontally (jig. 168), or vertically (jig. 169), we 
have a sting formed. Stings are sometimes arranged under the 
head of stalked glands ; we place them here because their secre- 
ting apparatus is at the base, and not at the apex, as in stalked 
glands. 
In the Nettle (fig. 169), the sting consists of a single cell, 
enlarged at its base, b, by the irritating fluid f, Ff, which it con- 
tains, and tapering upwards. to near the apex, when it again 
expands into a rounded head, s. The enlarged base is closely 
invested by a dense layer of epidermal cells, we, which forms 
a kind of case toit. In touching a nettle lightly, the knob-like 
head, s, is broken off, and the sharp point of the sting then left 
enters the skin, while the irritating fluid is pushed up at the 
same time into the wound by the pressure occasioned by the 
