462 PLANTS LIBERATE OXYGEN. 



are unacted upon by rays of a more moderate brilliancy, for the general 

 rule under which the chemical action of light occurs is, the amount of 

 chemical change is as the quantity of light absorbed. 



These facts are of importance in all discussions respecting the prim- 

 itive formation of organic matter by plants. Guided by them, we infer 

 that, though vegetation may greatly differ in its luxuriance in different 

 climates of the globe, the manner of action of the light is always the 

 same. Nothing is gained under the brilliancy of the tropical skies be- 

 yond a shortening of the time required for the accomplishment of a given 

 amount of work. No substances are there decomposed, even in the or- 

 ganisms of plants, which could not equally well be decomposed by the 

 feebler light of more temperate climates, only in these it would demand 

 more time. The oils and other substances, almost or quite free from 

 oxygen, which abound in the plants of the torrid zone, are not excep- 

 tions to, but illustrations of, the doctrine here set forth. 



It is proper here to correct the statement which is usually made by 

 it is not the vegetable physiologists, that the decomposition of carbonic 

 green parts of ac i(j fty plants is accomplished by their srreen parts. A 



plants which J , * j j I.- i. t, 



decompose car- plant which has been etiolated, or, indeed, one which has 

 bonic acid. been raised from a seed in total darkness, when brought into 

 the sunshine, decomposes carbonic acid, liberates oxygen, and its pale 

 and sickly leaves presently turn green. This, therefore, demonstrates 

 that the green portions are not the seat nor the origin of the decomposi- 

 tion, but are, properly speaking, its effect. 



Thus, under the influence of sunshine, the leaves of plants decompose 

 Plants liberate carbonic acid, liberate its oxygen, which, for the most part, 

 oxygen. escapes into the atmosphere, the amount of gas decomposed 



depending primarily on the quantity of light supplied, and therefore, 

 among other conditions, on the surface of exposure of the leaves, and 

 not upon their thickness or' mass. But I found, on an examination of 

 the gas thus evolved, that it is never pure oxygen, but always contains 

 a certain though variable proportion of nitrogen. From this it follows 

 that a part of the oxygen appertaining to the carbonic acid is appropri- 

 ated for the uses of the plant. 



Such, in a general way, is what takes place in the daytime, but at 

 night the process is to a certain degree inverted, a plant absorbing oxy- 

 gen from the air, and yielding carbonic acid. The explanation which 

 Liebig offers of this state of things is doubtless correct, that the evolu- 

 tion of carbonic acid is a purely physical process, and the absorption of 

 oxygen due to the chemical action of the various deoxidized bodies 

 which have been accumulating during the day. 



As respects the sources from which the various constituents of the 

 plant organism are derived, they are sufficiently obvious. The carbon 



