PHOTOSYNTHETIC EFFICIENCY OF HYDRILLA 22J 



of the sum-total of the fluctuating energy of light. There 

 were again the various losses of the incident energy which 

 were more or less indeterminate. Finally, no accurate 

 method was available for the determination of the energy 

 stored in photosynthesis. 



Determination of the Photosynthetic Efficiency 



of Hydrilla 



An ideal method for determining the efficiency should 

 include (i) a brief period of experiment ; (2) the practical 

 elimination of the indeterminate losses of the incident 

 radiation ; and (3) the simultaneous determination of the 

 energy absorbed and of the energy stored, the measurements 

 being carried out with the highest degree of accuracy. 



These objects have been attained by the method which 

 I have been able to devise for determining the photo- 

 synthetic efficiency of Hydrilla. The period of a complete 

 experiment has been reduced to a few minutes, so that 

 the external conditions can be maintained absolutely 

 constant. Very sensitive and accurate methods have been 

 adopted for the determination of the energy absorbed and 

 of the energy stored by the plant. The energy absorbed is 

 determined by two independent methods rapidly following 

 each other, and the agreement of the results gives assur- 

 ance of their accuracy. Finally, the determinations of the 

 absorption and of the storage of energy are made under 

 an identical intensity of light. 



Elimination of loss by reflection, emission and transpira- 

 tion. — These losses are eliminated when the plant under 

 experiment is immersed in water, as in my investigation 

 with Hydrilla verticillata. The sap contained in the cells of 

 Hydrilla has an index of refraction not very different from 

 that of water in which the plant is immersed, hence the loss 

 by reflection is practically negligible. The loss by emission 

 depends on rise of temperature in the plant above its surround- 

 ings ; but the plant and the water in which it is immersed 



