40 THE VITAL FUNCTIONS. 



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 excavation of the rock. This view is 

 confirmed by the observation that the same species of lichen, 

 when attached to the rocks which are not calcareous, re- 

 mains always at the surface, and does not penetrate 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 grow- 

 ing on the leaves of this plant, causes the fluid in their vesi- 

 cles to pass out from their points, so as to be instilled into 

 the skin, and occasion the well known irritation which en- 

 sues. M. De Candolle, junior, has ascertained, by chemical 

 tests, that the stinging fluid of the nettle is of an alkaline na- 

 ture. In some species of this genus of plants, the hairs are 

 so large that the whole mechanism above described is visi- 

 ble 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 effec- 

 tually defending the young buds from the injurious effects 

 of moisture; and for a similar purpose we find the surface of 

 many plants covered with a varnish of wax, which is another 

 secretion belonging to the same class: thus, the Ceroxylon, 

 and the Iriartea have a thick coating of wax, covering the 

 whole of their stems. Sometimes the plant is strewed over 

 with a bluish powder, possessing the same property of re- 

 pelling water: the leaves of the Mesembryanthemum, or 

 Fig-marigold, of the Jltriplex, or Orache, and of the Bras- 

 sica, or Cabbage, may be given as examples of this curious 

 provision. Such plants, if completely immersed in water, 

 may be taken out without being wetted in the slightest de- 

 gree; thus presenting us with an analogy to the plumage of 

 the cygnet, and other aquatic birds, which are rendered com- 

 pletely water-proof by an oily secretion spread over their 



