THE NATURE OF PHOTOSYNTHESIS 165 



Willstatter and Stoll ^^^ found that various plants exhibit a wide varia- 

 tion in their resistance to lack of oxygen. It is perhaps not without 

 significance that the mosses which are resistant to the absence of oxygen 

 have no stomata and hence the gaseous interchange with the atmosphere 

 is more difficult. According to Willstatter and Stoll the partial pressure 

 of oxygen can be reduced to one hundredth of that in air without disturb- 

 ing photosynthesis if the rest of the atmosphere is nitrogen. After com- 

 plete displacement of oxygen for two hours the leaves were no longer 

 capable of photosynthesis on illumination. This is quite in agreement with 

 the older observations. Under these conditions the leaves show no visible 

 signs of injury. After exposures to an oxygen-free atmosphere of 

 shorter duration (one hour) some plants as Cyclamen curopacuni, poly- 

 trichum juniperinmn, the photosynthetic activity is decreased but not 

 entirely inhibited. Of special interest are the observations that the plants 

 just mentioned when kept in an oxygen-free atmosphere for 15-24 hours, 

 on illumination, show no photosynthesis, but after about 30 minutes begin 

 to evolve oxygen and increase in this activity to a high rate. The long 

 continued exposure to an oxygen-free atmosphere is not without permanent 

 injurious effect on the photosynthetic mechanism ; the plant does not again 

 attain its original photosynthetic rate. The longer the exposure to an 

 oxygen-free atmosphere, the lower is the subsequent rate of photo- 

 synthesis and the more incomplete is the recovery. Lack of oxygen there- 

 fore produces a permanent injury of the protoplasm; the degree of this 

 injury depending upon the length of time the plant is deprived of oxygen 

 and the individual constitution of the plant. 



Willstatter and Stoll conclude that oxygen is absolutely essential for 

 photosynthesis, but that a very small quantity of oxygen suffices for sup- 

 plying the photosynthetic apparatus. Moreover, they consider that free 

 oxygen is not necessary but that oxygen held in an easily dissociable form 

 can do the work. The conception of oxygen loosely bound in the plants 

 is largely based upon those cases which were kept in an oxygen-free 

 atmosphere and on illumination again recovered their photosynthetic 

 activity. It is suggested that oxygen is removed from the plant in two 

 stages: 1, the replacement of the free oxygen by an oxygen-free at- 

 mosphere, and 2, the removal of loosely bound oxygen, through the dis- 

 sociation of an oxygen compound when the oxygen tension of the atmos- 

 phere becomes less than that of this hypothetical compound. The second 

 stage is slower than the first. Hence in plants which have lost oxygen 

 chiefly by the first stage, a recovery of photosynthesis is possible on illumi- 

 nation. Only the more resistant plants can withstand the second stage. 



On the basis of their work on the relation of chlorophyll-content to 

 photosynthetic rates, Willstatter and Stoll came to the conclusion that 

 there is no direct ratio between these two. Besides chlorophyll as an 

 internal factor influencing the rate of photosynthesis, there is, according 



"* Willstatter and Stoll, "Untersuchungen iiber die Assimilation," Berlin, 1918. 

 p. 349. 



