100 THE LEAF. 



277. Pvespiration appears to be going on constantly, by day 

 and by night, dviring the hfe of the plant, even while it is act- 

 ively engaged in the contravening process of the fixation of car- 

 bon. The result of it is, the removal of a certain superfluous 

 portion of carbon, in a state of combination with oxygen,* from 

 the nutritive substances of the plant, just as the same deleteri- 

 ous acid is removed from the blood of animals by brea tiling. 



278. Let a few healthy plants be placed under a bell-glass containing air from 

 which all the carbonic acid has been previously removed. After a few hom'S 

 let the air be tested by sliaking it with lime-water, and it will be found to contain 

 carbonic acid, rendering the lime-water turbid. This effect will be produced, 

 whether the bell-glass stand in the sunshine or in darkness, but the quantity of 

 acid evolved will be found to be much greater in the darkness. 



279. Respiration is carried on with peculiar activity during 

 the two periods of germination and flowering. 



a. In germination pure oxygen is absorbed, either ft-om the air or water, or 

 both, in the absence of light (133, d), and returned to the air combined with the 

 superfluous carbon of the starch, which thus is converted into sugar for the nour- 

 ishment of the young plant. 



b. It is also equally active at the time of flowering, a large quantity of oxygen 

 being converted into carbonic acid by the flower. By this process it seems that 

 the starch previously contained in the disk (107), or receptacle (59), is changed 

 into saccharine matter for the nutrition of the pollen and ovules (70,81), the 

 superfluous portion flowing off" in the form of honey. And it has been ascer- 

 tained that the quantity of oxygen evolved bears a direct proportion to the devel- 

 opment of the disk, t 



280. The life of the plant depends upon the continuance of respiration, for if it 

 be suiTOunded by an atmosphere with too great a proportion of carbonic acid, or 

 in a confined portion of air, which has become vitiated by its own action, and ex- 

 cluded from the light, its respiration is necessarily soon suspended, and it speedily 

 perishes. J 



281. Digestion, in plants, consists properly of all those 

 changes effected by the leaves in rendering the crude sap fit for 

 the purposes of nutrition. But that process wliich is more par- 



* Carbonic acid is composed of parts (by weight) of carbon, combined with 16 parts of 

 oxygen. 



t Thus Sausaure found that the flower of the Arum, wliile in bud, consumed 5 or 6 times its 

 own volume of oxygen in 2t hours ; during the expansion of the flower, 30 times, and during 

 its withering, 5 times. When the floral envelopes were removed, he found that the quantity 

 of oxygen consumed by the stamens and pistils in 24 hours, was, in one instance, 132 times 

 their own bulk. 



