TRICHOMES. 



69 



foot of the hair may differ somewhat in shape, size, and arrange- 

 ment from the other epidermal cells. They may form an emi- 

 nence upon which the foot rests, or they may be somewhat 

 sunken so that the body of the hair 

 hardl}" reaches the general surface of 

 the epidermis ; but usually the hair 

 projects for a considerable distance 

 above the border of the depression. 



Both simple and compound hairs 

 may be variously curved and 

 branched, giving rise to stellate and 

 many other forms. 



233. Scales are trichomes which 

 are mostly compound, and consist 

 of discs borne by their edges or cen- 

 tres, either with or without a short 

 foot or stalk. If the disc is com- 

 posed of radiating cells, the scale 

 becomes stellate, a form which re- 

 sembles or passes into the stellate 

 and tufted hairs common in Mal- 

 vaceae, etc. Well-marked stellate 

 scales are met with in Oleacea? and 

 Elaeagnaceae. 



234. Bristles^ prickles and epidermal spines are firmer or 

 stouter outgrowths. When such outgrowths are truly epidermal, 

 the}- come off with the epidermis. 



Hairs, scales, and prickles differ very greatly as to their per- 

 sistence, some being exceedingly short-lived, as, for instance, 

 the hairs which occur on roots ; while others, for instance the 

 prickles on the rose, last for long periods. 



235. In certain outgrowths from the edges of leaves or else- 

 where the structure is complicated by the presence of a portion 

 of the underlying framework. This is notably the case in the 

 fringe upon the leaves of Droseraceae. There are all degrees of 

 variation between such trichomatous outgrowths and spinulose 

 teeth, or lobes. 



236. The consistence of the cell- wall in trichomes varies 

 widely, from extreme tenuity to the density of a silicified wall. 

 The more delicate hairs are transparent, so that the contents 



Fro. 4!>. Branching unicellular hairs: a, from Huraulus (the hop); 6, stellate hair 

 of Deutzia. (Van Tieghem.) 



