168 PHOTOSYNTHESIS 



Dried leaves can be kept for a long time, up to three months, without 

 completely losing the capacity for oxygen formation in the light. That 

 chlorophyll is essential for the formation of oxygen in the light by killed 

 leaves is shown by the fact that etiolated leaves do not possess this prop- 

 erty, although the amount of chlorophyll that is necessary is exceedingly 

 small. 



Molisch demonstrated this phenomenon with a wide variety of plants. 

 Some plants with a relatively high acid content on being dried and tested 

 for oxygen evolution gave very slight or negative results. When the acids 

 are neutralized during the preparation of the leaf material positive results 

 are obtained, and it is not improbable that the negative results in the first 

 case are to be attributed to the effect of the acid on the chlorophyll during 

 the process of drying. 



Molisch regards that these experiments on the oxygen production of 

 killed leaves in the light, demonstrate the existence of an enzyme which 

 is essential for the photosynthetic process ; that the life of the plant is not 

 necessarily essential for the photosynthetic activity any more than the 

 living yeast plant is essential for the fermentation process. While the 

 results of Molisch perhaps do not entirely remove the photosynthetic 

 process from the living activity of the plant to the same degree that the 

 isolation of zymase did for the process of alcoholic fermentation, they 

 nevertheless are a distinct contribution toward establishing the enzymatic 

 nature of photosynthesis, a trend which has already been fostered by the 

 results of Willstatter and Stoll, Pringsheim and others. 



Returning to the influence on photosynthesis of lack of oxygen, it is 

 apparent that this is largely one of degree; some plants are easily per- 

 manently inhibited while others withstand absence of oxygen for long 

 periods. All show some injury. In viewing these facts critically it is 

 well to bear in mind that it is extremely difficult to free any plant from 

 the last traces of a gas. Plants differ greatly in their rate of oxygen 

 consumption as well as in the rate of gaseous interchange with the sur- 

 rounding atmosphere. Also it is difficult to free a gas from the last traces 

 of oxygen; this is apparently impossible by the use of glowing copper. 

 Moreover, the injurious effects of lack of oxygen may well be due to 

 injury of the protoplasm caused by the accumulation of toxic substances. 

 It is well known that plants deprived of oxygen continue to consume 

 carbohydrates, but that the course of metabolism is greatly altered, usually 

 resulting in the formation of ethyl alcohol. Different plants vary in the 

 rate of alcohol formation. So also does the protoplasm of different plants 

 vary in resistance to toxic substances, such as alcohol, produced by the 

 plant itself, or administered from without. 



Apparently photosynthesis is more sensitive to protoplasmic disturb- 

 ances than is respiration. Thus Wurmser and Jacquot ^^* found that when 

 certain marine algae were subjected to higher temperatures (36° to 45°) 



'^^ Wurmser and Jacquot, Bull. Soc. Chim. biol, 5, 305 (1923) ; 6, 169 (1924). 



