PROTECTIVE ARMORS 



349 



It seems likely that in many plants the hairs are really related to 

 the moisture. The absence of moisture, or, rather, a shortage of 



Cortex 



Shaft 



ifedtiUa 



Horny 

 ^M: layer 

 'i^'^-^Dermis 

 ' \haceoui> 

 gland 



Papilla with 

 blood vessels 



b 



-~~1 



6 = 



3 



Fig. 164. Hair of mammals 



Branches 



of f^haft 

 formbuj 

 barbules 



Horny 

 layer 



r, human hair follicle, showing mode of growth (the dead shaft is pushed forward by 



the new growth about the papilla) ; 2, hair of horse ; s, hair of mouse ; 4, hair of marmot. 



a, base of hair ; ^, tip ; c, more highly magnified portion of shaft 



moisture, is known to bring about the production of hairs in species 

 of plants that ordinarily do not produce hairs when water is abundant. 



Hairs are also likely ^,,^^Shcath 



to protect many plants 

 against extremely high 

 or low temperature. 



The hairs familiar to 

 us in common animals 

 and on our own skin 

 are much more complex 

 in structure than are 

 plant hairs (Fig. 164). 



The feather of the 

 bird may be considered 

 as a highly complex 

 hair. In the manner of growth the feather resembles the hair 

 of mammals very much, but in its structure it is of course very 



Growing 

 region, 



Fig. 165. Feather structure 



The feather of a bird is a skin structure that 



grows in substantially the same way as a hair or 



a finger nail 



