ELEMENT AEY STEUCTUEE. 



59 



sessile glands are composed of cells containing solid secretions so 

 that they form hardened spherical or other shaped appendages 

 upon the surface of the epidermis, they are termed verrucce or 

 ivarts. 



When a sessile gland contains an irritating fluid, and is elon- 

 gated above into one or more hair-like processes, which are 

 placed horizontally ( ig. 144), or vertically {fig. 145), we have a 

 sting formed. Stings are sometimes arranged under the head of 

 stalked glands ; we place them here because their secreting ap- 

 paratus is at the base, and not at the apex, as in stalked glands. 



In the Nettle {fig. 145), the sting consists of a single cell, en- 

 larged at its base h, by the irritating fluid /, which it contains, 

 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, w e, which forms a kind of case 

 to it. In touching a nettle lightly, the knob-like head is broken 

 off, and the sharp point of the stmg enters the skin, while the 

 irritating fluid is pushed up at the same time into the wound by 

 the pressure thus occasioned, and by the elastic force of the sur- 

 rounding epidermal cells. If a nettle, instead of being thus 

 touched lightly, be grasped firmly, the sting becomes broken, 

 and as the sharp point does not then enter the skin, no irritation 

 is produced. 



IP- ,.. Fig. 145. 



Fig. 144. ^ 



I 



Fig. 144. Sting of a species of Malpighia. 

 e. Epidermis, b, b, g. Glandular appa- 

 ratus. Fig. 145. Sting of the com- 

 mon Nettle iUrtica dioica), consisting 

 of a single cell with a bulbous expan- 

 sion at its base, b, and terminated 

 above by a swelling, s, and containing 

 a granular irritating fluid, /. /. \ce. 

 Epidermal cells surrounding its base. 



b. Internal Glands. — These are cavities containing secretions 

 situated below the epidermis, and surrounded by a compact layer 

 of secreting cells {figs. 146 and 147). They are closely allied in 

 their nature to receptacles of secretion (see page 62) from 

 which, in fact, in many cases, it is difficult to distinguish them. 



