EPIDERMAL APPENDAGES. GLANDS. 



31 



secreting cell, there are occasionally two (fig. 85 c) or more (fig. 85 d). 

 Hairs sometimes serve as ducts through which the secretion of glands is 

 discharged; these are glandular hairs, with the secreting cells at the base. 

 Such hairs are seen in the nettle (fig. 86), in Loasa or 

 Chili nettle, and in Malpighia (fig. 84), and are commonly 

 called stings. In the nettle they are formed of a single 

 conical cell, dilated at its base (fig. 86 >), and closed at first 

 at the apex, by a small globular button placed obliquely 

 (fig. 86 s). This button breaks off on the slightest touch, 

 when the sharp extremity of the hair enters the skin, and 

 pours into the wound the irritating fluid which has been 

 pressed out from the elastic epidermal cells at the base. 

 When a nettle is grasped with violence, the sting is frac- 

 tured, and hence no injury is done to the skin. The 

 globular apex of glandular hairs sometimes forms a viscid 

 secretion, as in the Chinese primrose and sundew. The 

 hairs of the latter plant, by this secretion, detain insects 

 which happen to light on them. 



63. When glands are sessile, they consist of epider- 

 mal cells either surrounding a cavity, or enclosing small 

 secreting cells. In fig. 87, is represented a gland taken 

 from the flower-stalk of Dictamnus albus, cut vertically to 

 show the cavity surrounded by cells, and filled with a 6 



greenish oil; while in fig. 88, there is a gland with a short thick 

 stalk, full of cells, taken from Rosa centifolia. 

 These figures show the transition from sessile 

 to stalked glands. Some of the superficial cells 

 of the epidermis are sometimes slightly elevated 

 above the rest, and contain peculiar fluids. In 

 the Ice-plant, the appearance of small pieces of 

 ice on the surface is produced by cells contain- 

 ing a clear fluid, which is said to have an alka- 

 line reaction, while that of the tissue around 

 the vesicles is acid; in the Chick-pea, similar 

 superficial cells contain a subacid fluid. Glandular depressions or pits 

 occur, surrounded by secreting cells. At the base of the petals of 

 the Crown-imperial, for instance, cavities are seen containing a honey- 

 like fluid, secreted by what are called nectariferous glands. Cavities 

 containing saccharine matter, surrounded by small thin-walled cells, 

 are met with in the leaves of Acacia longifolia, also in Viburnum 



Fig. 86. Conical hair of Urtica dioica, or common nettle, ending in a button or swelling s, 

 with a dilatation or bulb at its base 6, which is surrounded by epidermal cells u e. In the hair 

 are currents of granular matter//. 



Fig. 87. Gland from flower-stalk of Dictamnus albus, cut vertically, showing central cavity I, 

 filled with greenish oil, and surrounded by a layer of cells c, which contain a red juice and are 

 connected with the epidermis e. 



Fig. 88. Gland from Rosa centifolia e, The epidermis. 





