PHOTOSYNTHESIS 3 



of strong light, thus modifying their photosynthetic activity. 

 A thermometer enclosed in the plant-chamber gives no 

 correct indication of the temperature of the leaf, a difficulty 

 which Blackman attempted to overcome by embedding one 

 junction of a thermo-electric couple in the midrib of the 

 leaf. There are other drawbacks also which render the 

 method of the absorption of carbon dioxide unreliable for 

 the accurate estimation of photosynthesis. 



We are thus confronted with the necessity of devising 

 means for the determination of photcsynthetic activity 

 without recourse to the laborious chemical analysis and the 

 numerous corrections which complicate existing methods. 

 In physiological investigations we are chiefly concerned 

 with the study of the changes induced in the functional 

 activity of the plant under variations of external conditions. 

 We have to start with the constancy of the physiological 

 factor of an individual plant ; it is obvious that the statistical 

 method of obtaining average effects over a large number of 

 plants would not be suitable for our purpose. Again, for 

 the study of the effect of variation of any particular factor, 

 all the others should be maintained constant. This can be 

 assured only for a short time ; hence the period of the 

 experiment must be reduced to a minimum. There thus 

 arises the necessity for a sensitive method which would 

 indicate the normal rate of photosynthesis and its induced 

 variations in the course of less than an hour, preferably 

 within a few minutes. For the elimination of personal 

 error, it is also desirable to devise means for the automatic 

 record of the normal rate and its induced variations. 



A very sensitive, means of observing and measuring 

 photosynthesis is afforded by the evolution of oxygen by 

 water-plants ; it is a direct method which avoids the 

 necessity of laborious processes of chemical analysis. The 

 photosynthetic action is immediately observable, and any 

 change induced in the normal rate is visually demonstrated 

 by the quickening or slowing down of the evolution of 

 bubbles of gas. 



