100 THE LEAF. 



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

 and by night, during the life 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 " 

 ])ortion 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 breatliing. 



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

 ^vliich all the carbonic acid lias been previously removed. After a few hours 

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

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

 Y'hether the ])ell-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. E,espiration is carried on with peculiar activity during 

 the two periods of germination and fioioering. 



a. In gennination pure oxygen is absorbed, either from the air or water, or 

 both, in the absence of light (1-33, rf), and returned to the air combmed -with the 

 si'.pei-fluous carbon of the starch, wliich thus is converted into sugar for the nour- 

 ishment of the young plant. 



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

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

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

 into .«;accharinc matter for the nutrition of the pollen and ovules (70,81), the 

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

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

 cpment of the disk, t 



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

 be surrounded by an atmosphere with too gi-eat a proportion of carbonic acid, or 

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

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

 ])erishes. 



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

 changes efteeted by the leaves in rendering the cmde sap fit for 

 tiie purposes of nutrition. But that process which is more par- 



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

 oxygen. 



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

 own volume of oxygen in 24 hours ; during the expansion of the flower, 30 times, and during 

 i-a withering, 5 limes. 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 

 I'leir own bulk. 



