THEORY OF VEGETATION. 39 



absorb water. The experiments of Hales prove, that 

 the weight of plants is increased in wet and dimin- 

 ished in dry weather ; and that, in the latter, they 

 draw from the atmosphere, by means of their 

 leaves,* the moisture necessary for their well-being. 

 Du Hamel, and, after him, Sennebier, has shown, 

 that the filaments that surround the roots of plants, 

 and which have been called their hair, perform for 

 them in the earth the office which leaves perform in 

 the atmosphere ; and that, if deprived of these fila- 

 ments, the plants die. 



It would be easy, but useless, to multiply facts 

 of this kind, tending to establish a doctrine not con- 

 tested, but which, after all, does not assert that water 

 makes any part of the food of plants. On this point 

 two opinions exist ; the one, that this liquid is a 

 solvent and conductor of alimentary juices; the 

 other, that it is itself an aliment, and, at the same 

 time, a purveyor of vegetable food. The first opin- 

 ion is abundantly established. Water, when char- 

 ged with oxygen, supplies to germinating seeds the 

 want of atmospheric air ; and, saturated with animal 

 or vegetable matter in a slate of decomposition, or 

 slightly impregnated with carbonic acid, very per- 

 ceptibly quickens and invigorates vegetation. The 

 second opinion is favoured by some of De Saussure's 

 experiments. On these Chaptal makes the follow- 

 ing remark, which expresses very distinctly an ap- 

 probation of the doctrine they suggest : " The enor- 

 mous quantity of hydrogen, which makes so,large a 

 part of vegetable matter, cannot be accounted for 

 but by admitting, in the process of vegetation, the 

 decomposition of water, of which hydrogen is the 

 principal constituent ; and that, though there is no- 

 thing in the present state of our experience that di- 

 rectly establishes this doctrine, yet that its truth 



* Bonnet's experiments show, that it is the under surface of 

 the leaf that performs this function. The upper surface has a 

 different office. 



