DROPPING f ROM LEAVES. 31 



yades the enquiry — ^Whence plants, under peculiar circumstances, 

 derive the water essential to vegetation V 



The constituents of water are oxygen and hydrogen ; the con") 

 *stituent8 of ammonia are hydrogen and nitrogen ; the constituents of 

 carbonic acid are carbon and oxygen ; and the principal constituent 

 of atmospheric air are nitrogen and oxygen ; with these is combined 

 or intermixed carbonic acid ; carbonic acid and ammonia are likely to 

 be absorbed largely by water, and so carried into the plant ; carbon is 

 fixed, being the principal constituent of wood, and thus oxygen is set 

 free; a smaller quantity of nitrogen is fixed, but enough to indicate a 

 decomposition of ammonia to have taken place ; and it is not un- 

 reasonable to suppose that all the ammonia taken up by the plant 

 may have been decomposed, the nitrogen combining with oxygen, set 

 free by the decomposition of the carbonic acid yielding material for 

 the woody structure, and appearing as atmospheric air, and the 

 hydrogen combining with oxygen and forming water. 



With legard to the second statement by Dr Livingstone, I would 

 require to be in possession of much more information than I am 

 to enable me to form an opinion on the subject ; but I may state 

 that I find nothing in the narrative incompatible with the supposition 

 of the moisture in this case having been obtained from the tree ; and 

 I avail myself of the interest which such a narrative may have 

 excited to direct attention to the similar copious supplies of moisture 

 flowing from leaves and from incisions in the trunks of trees. 



Sect. III. — On the Dropping of Water from the Leaves of certain Trees. 



Qi the dropping of water from trees there are numerous instances ; 

 but while in some of the more remarkable cases the popular opinion 

 is in favour of the supposition that this was one of the primary 

 phenomena of their growth, the consideration of the conditions under 

 which it occurs leads to the conclusion that it is one of the secondary 

 effects of vegetation. 



I have spoken of some of the more remarkable cases. In an old 

 work, entitled Historia de la Conqicista de las siete islas de Gran 

 Canaria de Juan de Abreu Gal indi, published in 1832, at page 47, 

 mention is made of a celebrated laurel, in Ferro, which is said to 

 have furnished drinkable water to the inhabitants of the island. 

 According to the statement, every morning the sea breeze drove a 

 cloud towards the wonderful tree, which attracted it to its huge top, 



