AND CARBONIC ACID DURING THE NIGHT. 33 



The most general and probable expression, therefore, for the function 

 of the leaf, now under consideration, appears to be that in the sunshine 

 the leaves absorb from the air carbonic acid, and at the same time 

 evolve oxygen gas, the bulk of the latter gas given off being nearly 

 equal to that of the former which is taken in — the relative bulks of the 

 two gases varying more or less with the species of plant, as well as 

 with the circumstances under which it is caused or is fitted to grow.* 



4°. Such is the relation of the leaf to the oxygen and carbonic acid 

 of the atmosphere in the presence of the sun. During the night their 

 action is reversed, they emit carbonic acid and absorb oxygen. This is 

 proved by experiments similar to those above described. For if the 

 plant which has remained under the bell-glass for 12 hours in the sun- 

 shine — during which time the oxygen has sensibly increased, and the 

 carbonic acid diminished in bulk — be allowed to remain in the same air 

 through the following night, the oxygen will be found to have decreased, 

 while the carbonic acid will be present in larger quantity than in the 

 evening of the previous day. 



The carbonic acid thus given off during the night is supposed to be 

 partly derived from the soil through the roots, and partly from the sub- 

 stance of the plant itself. The oxygen absorbed either combines with 

 the carbon of the plant to form a portion of the carbonic acid which is 

 at the same time given off or is employed in producing some of the 

 other oxidixed [containing oxygen in considerable quantity] compounds 

 that exist in the sap. 



As a general rule, the quantity of carbonic acid given off during the 

 night is far from being equal to that which is absorbed during the day. 

 Siill it is obvious that a plant loses carbon precisely in proportion to the 

 amount of this gas given off. Hence, when the days are longest, the 

 plant will lose the least, and where the sun is brightest it v/ill gain the 

 fastest; since other things being equal, the decomposition of carbonic 

 acid proceeds most rapidly where the sky is the clearest, and the rays 

 of the sun most powerful. Hence we see why in Northern regions, 

 where spring, summer, and autumn are all comprised in one long day 

 — vegetation should proceed with such rapidity. The decomposition of 

 the carbonic acid goes on without intermission, the leaves have no night 

 of rest, but nature has kindly provided that, where the season of 

 warmth is so fleeting, there should be no cessation to the necessary- 

 growth of food for man and beast. 



This comparison of the functions performed by the leaf, during the 

 day and night respectively, explains the chemical nature of the blanching 

 of vegetables practised by the gardener, as well as the cause of the pale 

 colour of plants that grow naturally in the absence of light. 



When exjxtsed to the sun, the leaves of these sickly vegetables evolve 

 oxygen, and gradually become green and healthy. Woody matter is 

 formed, and the stems become strong and fibrous. 



The light of the sun, in the existing economy of nature, is indeed 

 equally necessary to the health of plants and of animals. The former 



* As the oxygen given off by the leaves is always the result of a chemical decomposition, 

 by which the carbonic acid or other compound is deprived of a portion, at least, of its oxy- 

 gen or de-oxidized, this function of the leaves in the presence of tlie sun is often spoken of 

 as their de-oxidizing power. 



5 



