Another class of conditions which are mostly negative, 

 though often of use, are the hairy structures on the skins 

 of plants. Few plants are perfectly hairless. In Roses we 

 find a peculiar development of the hairs in prickles. These 

 are of obvious advantage, sometimes as protective organs 

 sometimes as in the case of the Bramble also to help to 

 support it amojigst undergrowth. At other times a copious 

 hairiness protects a surface from being wetted, from frost. 

 or from rapid evaporation. But the slight hairiness so 

 commonly met with has no useful purpose. It sometimes 

 means the survival of a previous more hirsute condition ; 

 at others a fortuitous development that may become of use. 



