284 Plant Physiology 



a control vessel lacking seed ; (2) by drawing air deprived 

 of CO 2 through the respiration chamber, and catching 

 the CO 2 given off in a wash-bottle of baryta water. For 

 quantitative work it is absolutely necessary to study stand- 

 ard methods of CO 2 determination and to employ these 

 with all the chemical precautions required ; especially 

 important is a gravimetric method in which the CO 2 is 

 caught in potash bulbs. 



The rapid inhibition of growth in the absence of oxygen 

 may be taken as indicative of the use of this atmospheric 

 constituent. The effect upon growth of an atmosphere 

 deprived of oxygen may be demonstrated by a compara- 

 tively simple experiment in which germinating seeds are 

 placed in a tube containing normal air, and this is compared 

 with another from which the O 2 (and CO 2 ) are absorbed, as 

 explained in the laboratory instructions. 



163. Respiratory phenomena in aerobic respiration. In 

 the type of respiration thus far particularly considered 

 oxygen has free access to the respiring cells, and it is used 

 in promoting chemical changes. This is called aerobic 

 respiration, as distinguished from anaerobic respiration 

 (subsequently discussed), which proceeds in the absence 

 of free oxygen. 



The following may be given as a concise summary of 

 the aerobic respiratory phenomena including the accom- 

 panying gas exchange in green plants : 



(1) Along with other gases, oxygen diffuses into the 

 tissues, it is absorbed by the cell-sap, and it reaches all 

 parts of the protoplasm of the cell. 



(2) Oxygen promotes catabolic processes, and whether 

 through the protoplasm and its constituents directly, or 



