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 chemical 

 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 re- 

 semblance 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 



