ACID CONDITION OF HYDRILLA IN SUMMER 123 



evolution of oxygen was found to be as high as 60 c.mm. 

 per hour in the one case and 71 c.mm. in the other. This 

 evolution of oxygen in the absence of C0 2 -supply was by 

 no means transitory, for it persisted at a uniform rate during 

 the whole period of observation, extending over two hours. 

 The bubbles appeared only under exposure of the plant to 

 light, the rate increasing with the intensity of light. The 

 bubbling took place even in freshly boiled distilled water, 

 but the appearance of bubbles in that case was delayed for 

 about half an hour, for the escape of the gas is only possible 

 after saturation of the water with the evolved oxygen. 

 The bubbles appeared quickly in distilled water which had 

 previously been made to absorb a small quantity of pure 

 oxygen. 



A few specimens exhibited this peculiarity early in April, 

 but by the end of the month every specimen showed it : the 

 complete transformation occurred in the course of a few 

 weeks. 



It is at first difficult to imagine how photosynthesis could 

 take place at all in the absence of a supply of carbon dioxide. 

 It must have been due to some internal change brought 

 about by the change of season. Further investigation 

 showed that, while the juice of the plants was practically 

 neutral in winter and spring, it was very strongly acid in 

 summer. The acid condition of the plant therefore appeared 

 to be connected with its power of photosynthetic evolution 

 of oxygen in the absence of C0 2 . The acid condition in 

 summer is probably associated with the prevailing high 

 temperature, which, in contrast with the average tempera- 

 ture of 24 C. in spring, rose to 43° C. (no F.) in summer. 

 High temperature means increased respiration and greater 

 production of organic acid. The acidity of the plants was 

 found to be due to the presence of malic and oxalic acids, 

 the latter in small quantities. 



The results given in Table XVI. (p. 124) relate to the 

 reactions of Hydrilla in an acid condition. 



Comparing the results of non-acid plants in spring with 



