LEAVES 



573 



Figs. 8i6, 817. — Multicellular filamen- 

 tous hairs from a leaf of the cineraria 

 (Senecio cruentus): 816, a general view, 

 as seen in cross section, showing several 

 hairs with their whip-like ends, which spread 

 out horizontally, forming cham- 

 bers between the basal portions 

 of the hairs ; note the great length 

 of the hairs in proportion to the 

 leaf diameter ; considerably mag- 

 nified; 817, a single hair; highly 

 magnified. 



ings, Ledum, etc.). Leaves 

 frequently are hairier 

 when young than when 

 mature, many of the hairs 

 soon breaking at a more 

 or less definite weak spot. 

 While some leaves are 

 equally hairy on both 

 surfaces, many leaves are 

 hairy mainly or only on 

 the under (stoma-bearing) 

 surface (as in the silver 



the leaf hairs take the form of 

 brown or silvery-giay scales (figs. 

 818, 819). In scabrous leaves 

 the surface is papillate or warty, 

 as in many composites. 



Variations in hair distribution. 

 — "Protective" epidermal hairs 

 are most abundant in xerophytes, 

 especially in sandy and rocky 

 regions and in deserts, where 

 they often give a characteristic 

 grayish aspect to the vegetation, 

 as in sage-brush deserts. In 

 alpine and arctic regions and in 

 bogs and salt marshes, hairs are 

 less abundant though by no 

 means absent (as in the everlast- 



FlGS. 818, 819. — Multicellular shield-shaped scale 

 hairs from the leaf of Elaeagnus: 818, a general view as 

 seen in cross section, showing the hairs, each of which 

 consists of a vertical stalk surmounted by a horizontal 

 scale; considerably magnified ; Srg, the terminal scale, 

 as seen from above; highly magnified. 



