466 MANUAL OF BOTANY 



CO., by having a vessel containing a solution of caustic potash in 

 the receiver M^ith the plant. The potash will absorb the CO., of 

 the original atmosphere admitted, as well as any CO.j given off 

 by the plant during the experiment. If a capillary tube be fitted 

 into the receiver and its other end be made to dip into a vessel 

 of mercury, the latter will slowly and gradually rise in the tube, 

 indicating a diminution of the volume of the included air. If 

 the experiment be continued till the mercury ceases to rise in 

 the tube and the gas remaining in the receiver be measured at 

 the ordinary atmospheric pressure, it will be found that its volume 

 has been diminished by about twenty per cent, and that what is 

 left consists of nitrogen. The oxygen will have been completely 

 removed by the green plant, even when in sunlight. If the 

 caustic potash be examined, it will be found to have gained 

 considerably in weight and to contain a quantity of carbonate 

 of potassium. Thus there has been proceeding an absorption of 

 oxygen, attended as before by an exhalation of CO.,, the latter 

 having been combined with the potash. 



This process of gaseous interchange constitutes what is 

 known as respiration.^ Though more easily detected when the 

 absorption of CO., and its decomposition by the chloroplasts are 

 suspended, it is nevertheless constantly going on so long as the 

 plant is living. It is observable in all living parts of the plant, 

 whatever be their environment. 



If the plant be carefully weighed at the beginning and end 

 of the experiment, it will be found to have lost weight during its 

 stay in the receiver, so that respiration is associated with a loss 

 of weight to the plant. Not only is respiration attended with 

 the emission of CO.,, but there is also a certain exhalation of 

 watery vapour, which takes place quite independently of anj- 

 supply from the root or the cut end of the stein. The nature of 

 the metabolism is such that the living substance gives off both 

 water and C0._, while it coincidently absorbs oxygen. This is 

 quite independent of any constructive processes, for it can be 

 observed when no nutritive material of any kind is supplied to 

 the plant. 



We may inquire what is the relation of the absorption of 

 oxygen to the elimination of CO., and water. It is conceivable 

 that the oxygen may unite in the plant with carbon and with 

 hydrogen to produce at once the exhaled compounds. A study 

 of the living organism at work, however, soon shows us that the 

 process is not of this simple nature. If a study be made of 

 the germination of starchy seeds, the volume of oxygen absorbed 



