638 CONSERVATIVE ORGANS. [LECT. XI. 



ratively unyielding; the former are, properly 

 speaking, hairs (pili), the latter bristles (setce) ; 

 and, therefore, under these two heads every kind 

 of pubescence may be arranged. 



A. HAIRS, Pili, are fine, slender, cylindrical, 

 flexible bodies found on the surfaces of the herba- 

 ceous parts of plants. Some of them are the excre- 

 tory ducts of glands, a fact which was first de- 

 tected by Guettard ; and on which Linnaeus too 

 hastily formed his definition of the hair, which 

 he describes generally to be <f a bristle-like ex- 

 " cretory duct of the plant*;" but many of them 

 are not excretory ducts, and it is not easy to con- 

 ceive any satisfactory opinion of their use to the 

 plant. In some instances indeed they serve one of 

 the purposes which Linnaeus ascribes to them ; 

 that of defending the plant from external injuries*^, 

 as we find downy and hairy leaves are not so fre- 

 quently attacked by insects as smooth ones; but 

 they can be very imperfect safeguards against 

 heat, cold, or wind ^. 



When placed under the microscope, vegetable 

 hairs appear to be membranous tubes, articulated 

 in the majority of instances, often punctured, and 



* " Pilus est ductus excretorius plantae setaceus." Phil. 

 Bot. 84. 



f " Pubescentia est armatura plantae, qua ab ex'ternis in- 

 " juriis defenditur." Phil. Bot. 163. viii. 



t " Lana servat plantas ab cestu nimio. Toraentum servat 

 " plantas a ventis." Phil. Bot. 1. c. 



