FUNCTION OF LEAVES. 5 



by the plant. The carbonic acid being thus removed 

 from the cell sap, it becomes capable of dissolving a 

 fresh supply. All green parts of a plant share in this 

 power of decomposing carbonic acid, but it is pre- 

 eminently the function of the leaves. The decom- 

 position of carbonic acid does not proceed in darkness, or 

 at a very low temperature. The rays of light most active 

 in effecting the decomposition are the orange-red rays ; 

 the green, violet, and dark red rays of the spectrum 

 have scarcely any influence. The rays of light absorbed 

 by chlorophyll are, in fact, the ones which accomplish the 

 chemical work. 



The decomposition of carbonic acid by green plants 

 during daylight is of the utmost importance in maintaining 

 an atmosphere suitable for the respiration of animals. An 

 animal in breathing inspires atmospheric air ; it expires 

 air in which a part of the oxygen has been replaced by 

 carbonic acid; the result of animal life is thus to accumu- 

 late carbonic acid in the atmosphere. Such accumulation 

 would be injurious to the health of animals, but is pre- 

 vented by the growth of plants. It has been calculated 

 that an acre of forest, producing annually 5755 Ibs. of 

 dry matter, will consume the carbonic acid produced by 

 the respiration of 15'4 men. 



Besides carbonic acid, plants are apparently capable 

 of absorbing a small quantity of ammonia through their 

 leaves. The uncombined nitrogen of the atmosphere is 

 not appropriated by the leaves of green plants. When 

 rain occurs after severe drought, water may be taken up 

 to some extent through the leaf. 



Plants which have no chlorophyll cells, and possess 

 consequently no green colour, do not decompose carbonic 



