4 CHAP. I. INTRODUCTION 



The other advantages of experimenting with water- 

 plants are : (i) that the plant may be maintained under 

 normal conditions in a vessel of water free from unnatural 

 restraint ; (2) that there is no transpiration to modify the 

 normal activity ; (3) the absence of complicating stomata, 

 the carbonic acid in water diffusing readily through cell- 

 walls saturated with water ; and (4) the facility of escape of 

 the excreted oxygen from the intercellular spaces. 



The well-known bubble-counting method, so promising 

 at first sight, has many serious defects which are as follows : 



(1) The photosynthetic activity is measured by the rate 

 of evolution of gas which is assumed to be pure oxygen. 

 But the evolved gas is seldom pure. The water in which 

 the plant grows contains oxygen, nitrogen and carbon 

 dioxide in solution. These gases 'penetrate into the inter- 

 cellular spaces of the plant and may be given out along with 

 the evolved oxygen. 



(2) An unknown proportion of the evolved oxygen is 

 liable to be absorbed and retained by the water. 



(3) A portion of the oxygen is also lost through respira- 

 tion by the plant ; and, finally — 



(4) A very serious defect is that the volume of successive 

 bubbles given out is liable to irregular variation. 



These defects, though formidable, are however not 

 insurmountable ; I describe various means and appliances 

 by which they have been eliminated. The results obtained 

 show that no other method can rival that of the time- 

 record of the evolution of equal volumes of oxygen. The 

 quantitative results obtained possess such a high degree 

 of accuracy that it becomes possible to attack with success 

 many new and important problems of photosynthesis. The 

 new method secures (1) the certainty that the gas evolved 

 is actually pure oxygen ; (2) the determination of the times 

 required for the evolution of equal volumes of oxygen ; and 

 (3) the elimination of personal error by automatic record. 



Photosynthesis is admitted to be a vital phenomenon ; 

 we have, however, to enquire if it belongs to the same 



