LEAVES 



573 



FIGS. 816, 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-gray scales (figs. 

 8 1 8, 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- 



FIGS. 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 ; 819, the terminal scale, 

 as seen from above ; highly magnified. 



