EPIDERMAL APPENDAGES — GLANDS. 35 



obliquely (fig. 91 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 crushed, 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 (fig. 88). The 

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

 happen to alight on them. The hairs gradually close on the insects, 

 electrical phenomena taking place during the movement. Some think 

 that in this case the insects are used as food by the plant. 



When glands are sessile, they consist of epidermal cells either 

 surrounding a cavity or enclosing small secreting cells. In fig. 92 

 is represented a gland taken from the flower-stalk of Dictamnus albus, 

 cut vertically, to show the cavity surrounded by cells, which is fiUed 

 with a greenish oil ; while in fig. 93 there is a 

 gland with a short thick stalk, fuU 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 containing m m" km 



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

 line reaction ; in the Chick-pea, similar superficial cells contain an 

 acid fluid. Clear glands are also seen on the under surface of the leaf 

 of Passiflora lunata. Eesinous glands are seen in the Hop and Hemp 

 plants. Glandular depressions or pits occur, surrounded by secreting 

 cells. At the base of the petals of the Crown-imperiaJ, for instance, 

 cavities are seen containing a honey-like fluid, secreted by what are 

 called nectariferous glands. Cavities containing sac- 

 charine matter, surroimded by small thin-walled cells, 

 are met with in the leaves of Acacia longifolia, also 

 in Viburnum Tinus, and Clerodendron fragrans. The 

 cavities communicate with the surface of the leaves 

 by means of canals. Peculiar glands are found at the 

 inner side of the base of the petioles of Cinchona and 

 Ipecacuan plants (fig. 94). ^^' **' 



Glands are occasionally sunk in the epidermis, so as merely to have 



Fig. 92. Gland from flower-staUi; of Dictamnus albus, cut vertically, showing central 

 cavity, I, flUed with greenish oil, and surrounded by a layer of cells, c, which contain a red 

 juice, and are connected with the epidermis, «. Fig. 93. Gland from Eosa centifolia ; e, 

 the epidermis. Fig. 94. Cluster of ovate-oblong ceUular glands from the base of the 

 stipule of the Ipecacuan plant (Cephaelis Ipecacuanha). 



