RESPIRATION OF PLANTS 



85 



oxygen remaining after the experiment. The initial composition 

 of the air being known, it is not difficult to compute the amount 

 of oxygen absorbed during respiration. 



To obtain a more complete picture of the gas exchange during 

 respiration, at present it is found preferable to place the parts 

 of the plants subjected to investigation in hermetically sealed 

 containers, with tubes through which samples of air are taken 

 from time to time by means of a 

 special gas pipette. These samples 

 are analyzed in a special apparatus 

 permitting a very precise determina- 

 tion to 1 part in 10,000 on volumes 

 of gas not exceeding 1 cc. Recently, 

 McAllister at the Smithsonian 

 Institution has used the spectropho- 

 tometer to measure the infrared 

 absorption bands of carbon dioxide, 

 which permits changes of 1 part per 

 million to be quantitatively deter- 

 mined at 5-sec. intervals without 

 removing an aliquot of the gas 

 sample. 



All experiments with green plants 

 or those which are able to become 

 green must, of course, be conducted in 

 darkness, lest the opposite process of 

 absorption of carbon dioxide should 



superimpose itself on respiration. Fig. 1 7.— Study of respiratioa 



r\r. i 1 ,1 r • -• • of germinating seeds by means of 



(Jiten, the study oi respiration is. ^ eudiometer, 

 confined only to the determination 



of the carbon dioxide liberated, which of course, is more 

 readily made than is a complete gas analysis. For this purpose, 

 the respiring parts of a plant are placed in a container through 

 which is drawn an air current that has been freed of carbon 

 dioxide. Having passed over the plant, the air current leaves 

 the carbon dioxide, caught on its way, in a special absorbing 

 apparatus. This may be either a U-shaped tube containing 

 potassium hydroxide or soda lime, or one of the various kinds of 

 vessels containing potash. The quantity of carbon dioxide 

 formed per unit of time is estimated by the increase in weight 



- Wafer 

 '^^Merci/ry 



