VEGETABLE SECRETION. 41 



such as the Patellaria immersa, are observed, in 

 process of time, to sink deeper and deeper beneath 

 the surface of the rock, as if they had some mode 

 of penetrating into its substance, analogous to that 

 which many marine worms are known to possess. 

 The agent appears in both instances to be an acid, 

 which here is probably the oxalic, acting upon the 

 carbonate of lime, and producing the gradual ex- 

 cavation of the rock. This view is confirmed by 

 the observation that the same species of lichen, 

 when attached to rocks which are not calcareous, 

 remains always at the surface, and does not pene- 

 trate below it. 



In many stinging plants, as in the Dolichos pni- 

 riens, or Cowhage, and the Urtica dioica, or Nettle, 

 the fluid circulating in their tubular hairs which 

 penetrate into the skin, is found to be highly acrid. 

 The extremities of the hairs, which are often curi- 

 ously barbed, or terminating in bulbs, or hooks, are 

 extremely hard and brittle, so that after entering 

 the skin they easily break off, and being retained 

 there, produce violent mechanical irritation ; an 

 effect which, in the above-mentioned instances, is 

 much exasperated by the caustic liquor that is at 

 the same time instilled into the punctured part, 

 and which, in the case of the nettle, has been 

 ascertained by M. De Candolle, junior, to be of an 

 alkaline nature. The entrance of the sting is often 

 facilitated by its surface having spiral grooves, 

 giving it the effect of a screw.* 



As the resinous secretions resist the action of 

 water, we find them often employed by nature as a 

 means of effectually defending the young buds from 



* Ch. Morren, Bulletin de I'Acad. de Bruxelles, vi. 239. 



