806 Vegetable Physiology. 



combines with the fluids which constitute the sap. In the lat- 

 ter case, it is again ready to be presented to the leaf when day- 

 light returns, and a fresh decomposition is then effected. This 

 reversal at night of what was done during the day, mav at 

 first sight appear to be at variance with the unity of pl an 

 which we should expect to find preserved in the vegetable 

 economy ; but a more attentive examination of the process will 

 show that the whole is in perfect harmony, and that these con- 

 trary processes are both necessary, in order to produce the re- 

 sult intended. 



Thus the great object to be attained by this vegetable eera- 

 tion is just the converse of that which is effected by the respi- 

 ration of animals. In the former it is that of adding carbon 

 in an assimilated state, to the vegetable organization ; in the 

 latter il is that of discharging the superfluous quantity of car- 

 bon from the animal system. The absorption of oxygen, and 

 the partial disengagement of carbonic acid, which constitute 

 the nocturnal changes effected by plants, must have a tendency 

 to lessen the capability of the atmosphere to support animal 

 life ; but this effect is much more than compensated by the 

 greater quantity of oxygen given out by the same plants during 

 the day. On the whole, therefore, the atmosphere is con- 

 tinually receiving from the vegetable kingdom a large acces- 

 sion of oxygen, and is, at the same time, freen from an equal 

 portion of carbonic acid gas ; both which effects tend to its 

 purification and to its remaining adapted to the respiration of 

 animals. Nearly the whole of the carbon accumulated by 

 vegetables is so much taken from the atmosphere, which is the 

 primary source whence they derive that element. At the sea- 

 son of the year when vegetation is most active, the days are 

 longer than the nights ; so that the diurnal process of purifica- 

 tion goes on for a greater number of hours than the nocturnal 

 process by which the air is vitiated. 



The oxygen given out by plants, and the carbonic acid re- 

 sulting from animal respiration, and from the various processes 

 of combustion, which are going on in every part of the world, 

 are quickly spread through the atmosphere, not only from the 

 tendency of all gases to uniform diffusion, but also from the 

 action of the winds, which are continually agitating the whole 



