FOOD OF PLANTS 131 



plant. Sir H. Davy, placed a plant in water, containing car- 

 bon in an impalpable |>o\vdcr, and not a '^arlicle entered the 

 roots. Lindiey says, '• tiic carbonic acid enters the root, be- 

 ing decomposed to a certain extent as il passes alon*;:, and giv- 

 ing apparently its oxygen to the spiral vessels, which convey it 

 into the other parts of the system-, when it reaches the leaves, 

 it liberates its oxygen completely, and leaves its carbon to 

 unite with tiie tissue of vegetation, or to enter into new com- 

 binations with water, atmospheric air, or other elements it 

 finds itself in contact with ; whence proceed the gummy, am- 

 yl ' -. resinous, oily, and other products, peculiar to the 

 veii*. V....C kintrdom. 



170. Water also is a necessary and important substance in 

 the economy of vegetables. Some have considered it almost 

 the only article ot nourishment of the plant ; N\hile others 

 have assigned it no other importance tiian that of forming a 

 medmm, by which the appropriate t'ood is enabled to pass into 

 the plant. Both we believe are wrong. It does form a me- 

 dium lor the passage of other tbod, and that a part, that enters 

 the plant, becomes food also, has been abundantly proved, but 

 not to the extent perhaps, that some would seem to suppose. 

 Theodore de Sausure remarks, that if we calculate with the 

 utmost care all the weight which a plant can gain by fixing 

 carbon, by depositing earthly, saline, alkaline, and metallic 

 matters, which it borrows from the soil, by respiring oxygen, 

 or from the soluble matter of the soil, we shall not be able to 

 account for more than one twentieth part of the rcjil weight 

 of such a plant. The other nineteen twentieths must there- 

 fore be fixtd water. This may be an overestimate, and prob- 

 ably is, but that water is decomposed and solidified, is placed 

 beyond a doubt when the composition of vegetable products is 

 considered ; tor from no other source could the plant obtain 

 the elements of its various |)roducts, but from the decomposi- 

 tion of tliis tluid. The large amount of Hydrogen, found in 

 the various volatile and essc.itial oils, must come from the de- 

 composition of water. But besides the water which is decom- 

 posed in vegetable digestion, much of it is, no doubt, solidified 

 without decomposition. The elements. Oxygen and Hydro- 

 gen, of Sugar, Starch, Gum &c., are in that exact proportion 

 in which these elements unite to form water. The atomic 

 composition of these substances seems to indicate the simple 

 union of carbon directly with water, and the conversion of 

 one into the other, by the simple loss of carbon confirms this 

 supposition, and we are unable to see the force of tiie argu- 

 meots, which have been offered to support a contrary opinion. 



