154 



PHYSIOLOGICAL BOTANY. 



392. Hairs are outgrowths of epidermal cells, and 

 are composed usually of greatly elongated single cells 

 (Figs. 490-497), which fre- 

 quently branch ; others are 

 made up of a number of cells. 

 Hairs take on a number of 

 forms by branching. 



Scales are another form of 

 epidermal outgrowth, and ap- 



- _ _ .. . 489, Is a vertical section of a leaf of Bank- 



pear in the lOrm OI dlSKS. s\&\ a, a, show two layers of cells in the epi- 



dermis; c, hairs found in little cavities on the 



Bristles are hair-like proc- "under side of the leaf, 

 esses, the walls of whose cells are hardened. 



Prickles are outgrowths of a still firmer character. 



494 



490, Rootlet of Madder, showing cells expanded into hair-like processes. 491, Section of a glandular 

 hair of Fraxinella. 492, Hair of Brionia, composed of several cells. 493, Hair surmounted by a gland, An- 

 tirrhinum majus. 494, Stinging hair of Urrica dioica. 495, Jointed hair from a stamen of Tradescandia. 

 496, Star-shaped hair from the petiole of Nuphar advena (X200). 497, Branched hair of Arabis. 



393. Glands are processes consisting of a single 

 cell or an aggregation of cells, situated a little above, 

 at, or just beneath the surface, the function of which 

 is to secrete and discharge peculiar substances, as oils, 

 nectar, etc. Glands sometimes terminate in a hair-like 

 process (Fig. 493). 



Stinging hairs are usually setaceous and sufficiently 

 rigid to perforate animal tissue ; having entered, the 



