CONDITION OF SOIL NECESSARY. 285 



and these results must be proportionably diminished. The 

 functions and powers of the roots have been already de- 

 scribed (p. 39). Let us now briefly inquire into those of 

 the leaves of plants. In the economy of vegetation, we 

 assign to the leaves the duty of obtaining and preparing 

 for the plant the largest proportion of its entire substance 

 its organic constituents. 1 They have the power of ab- 

 sorbing carbonic acid from the atmosphere, decomposing it, 

 fixing and appropriating the carbon, and setting free an 

 equal volume of oxygen, which is returned to the air in order 

 to sustain its power for the support of animal life. Leaves 

 both absorb and give out watery vapour, according to the 

 differing hygrometric conditions of the surrounding atmo- 

 sphere, and their own requirements. They have also, ac- 

 cording to Ville, the power of abstracting nitrogen from the 

 air, and also, according to Draper, of giving out nitrogen 

 to the air in variable proportion, and under certain con- 

 ditions. The first point, which involves considerations of 

 vast importance to agriculture, has been controverted by 

 other chemists. At present the balance of evidence is 

 against it ; science, however, is progressive, and to 7 morrow 

 may enable us to read characters which to-day are not 

 within our range. Our acquaintance with physiological 

 botany is still very imperfect ; we know that carbonic acid 

 is absorbed, and that oxygen is given out by the leaves; 

 and we are fairly entitled to believe that the proportion 

 of carbon thus retained is elaborated by the plant into 

 those carbon compounds starch, gum,' sugar, wax, oils, 

 cellulose, &c., which form so large a portion of all our 

 plants; but here our knowledge fails us, for we are in 

 ignorance of the manner in which these complicated ar- 

 rangements are effected. 



The turnip, therefore, through the agency of its roots 



1 The turnip, for instance, contains about 1 per cent, of inorganic and 99 

 per cent, of organic matter. See analysis, p. 232. 



