VEGETABLE SECRETIONS. 47 



in both instances to be an acid, which here is 

 probably the oxalic, acting upon the carbonate 

 of lime, and producing the gradual excavation 

 of the rock. This view is confirmed by the ob- 

 servation that the same species of lichen, when 

 attached to rocks which are not calcareous, re-| 

 mains always at the surface, and does not pene- 

 trate below it. 



A caustic liquor is sometimes collected in 

 vesicles, situated at the base of slender hairs, 

 having a canal which conducts the fluid to the 

 point. This is the case with the Nettle. The 

 slightest pressure made by the hand on the hairs 

 growing on the leaves of this plant, causes the 

 fluid in their vesicles to pass out from their 

 points, so as to be instilled into the skin, and 

 occasion the well known irritation which ensues. 

 M. De Candolle junior has ascertained by che- 

 mical tests that the stinging fluid of the nettle is 

 of an alkaline nature. In some species of this 

 genus of plants, the hairs are so large that the 

 whole mechanism above described is visible to 

 the naked eye. This apparatus bears a striking 

 resemblance to that which exists in the poisonous 

 teeth of serpents, and which is hereafter to be 

 described. 



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 the injurious effects of moisture ; and 



