RESPIRATION AND FERMENTATION 277 



balance. In deep shade, on the contrary, respiration usually 

 exceeds assimilation and, instead of accumulating organic sub- 

 stance, the plant is consuming them. Finally, there may be 

 obtained an intensity of light under which respiration will just 

 balance assimilation. In this case, the plant will produce no 

 change in the composition of the atmosphere, since all of the carbon 

 dioxide eliminated during respiration will be immediately used up 

 in assimilation, and all the oxygen eliminated during assimilation 

 will be consumed in respiration. This intensity of light is called 

 the "compensation point." Its position determines to a certain 

 degree the light requirement of a plant. The rate of respiration 

 of shade plants is in general considerably lower than in light-loving 

 plants. Still, in spite of a feeble intensity of light, their assimila- 

 tion proceeds at a comparatively higher rate. 



Light by itself has no perceptible influence on the respiration of 

 plants, without chlorophyll. When the intensity of light is very 

 great, however, there may be an effect as a result of the increasing 

 influence of a high temperature on respiration and the destructive 

 action of light on protoplasm. 



Within a fairly wide range, the amount of oxygen in the air has 

 no perceptible influence on the intensity of respiration. This is 

 in complete accord with the views on the chemistry of respiration, 

 according to which the first phase of respiration proceeds without 

 the participation of free oxygen. Only when the amount of 

 oxygen has decreased to as low a point as 1 to 2 per cent, the respi- 

 ration coefficient begins to increase perceptibly. Alcohol then 

 accumulates in the tissues of a plant, which is a sign that the oxi- 

 dation begins to lag behind the processes of anaerobic disintegra- 

 tion. Hence, no reduction in respiration is usually observed, even 

 in massive plant organs in the presence of sufficiently developed 

 intercellular spaces. In the case of seeds maturing within a 

 fruit or during germination of seeds having a very compact coat, 

 aeration may be insufficient, resulting in retardation of growth. 

 Even here, it is not the deficiency in oxygen that is of importance, 

 but the excessive accumulation of carbon dioxide which acts like a 

 narcotic. An excess of oxygen, for instance, with a considerable 

 increase of air pressure, as in the case of placing the plant into an 

 atmosphere of pure oxygen, may also prove detrimental, by 

 inducing excessive oxidation and the inactivation of the respiratory 

 chromogens. 



