CH. XVl] HAIRINESS 125 



similarly greasy feathers of Ducks and other birds, e.g. 

 shoots of Raspberry, Birches, Myrica, Alnus, &c. Simi- 

 larly waxy, or sometimes resinous, secretions are poured 

 out by peculiar microscopic glands or epidermal hairs in 

 other cases e.g. the bud-scales of the Horse-chestnut, 

 Black Poplar, and on the twigs of the Birch, Alder, and 

 Sweet Gale. 



This brings us again to the subject of hairiness. The 

 epidermis of the bud-scales and twigs of many species give 

 rise to the minute outgrowths which are termed hairs. 

 When these are sufficieutly numerous and long, or when 

 relatively large, they are so obvious to the touch or un- 

 aided eyesight as to affect the otherwise smooth surface, 

 and we say generally that the latter is hairy. But as 

 we have seen, hairs differ greatly in character, as any 

 suitable hand-lens will show. 



Sometimes they are expanded above into plate-like 

 discs, or scales, and the silvery lustre or coppery metallic 

 sheen of the twigs or buds of Hippopliae and Elceagnus 

 are due to the reflection of the broken light playing on 

 such scales (Fig, 55). 



In other cases the hairs are swollen at their ends into 

 little rounded heads containing semi-fluid, sticky excre- 

 tions, usually composed of resinous or gummy substances. 

 Such glandular hairs occur on the young shoots of the 

 Hazel and species of Bibes, Rosa, &c., the flower-buds of 

 Azalea, the bud-scales of Horse-chestnut, &lc. But in 

 most cases the hairs are simple, or occasionally branched 

 filamentous structures containing nothing but air, and 

 in the mass appearing white by reflected light, or, if 

 not sufficiently long and numerous to reflect much light, 

 modifying the smoothness and colour of the twig or bud 

 in other ways ; occasionally these hairs are coloured, 

 and the hue of the twig, &c., is altered accordingly, e.g. 



