ENERGY EFFICIENCY IN PHOTOSYNTHESIS 275 



experiments have been carried out with leaves of land plants placed in 

 the Warburg manometers. A limited number of experiments have been 

 carried out on land plants, beans, small elm trees, and even a young pine 

 tree. Within the limits of accuracy of experiment, the energy efficiencies 

 in photosynthesis are all about the same. This fact indicates that the 

 process of photosynthesis under investigation is a fundamental process 

 and is not a phenomenon connected with any one type of organism. The 

 experiments with algae and with water plants have the advantage that 

 the temperature control is simpler because the surrounding water is ther- 

 mostated. But these experiments are handicapped by the fact that the 

 carbon dioxide and oxygen from the organism have to obtain equilibrium 

 with a water phase before they can be measured in the gas phase. 



The algal cells are relatively simple morphological plant units in which 

 the whole process of photosynthesis and the later reaction products are 

 all enclosed together. In larger plants it is possible to store the chemi- 

 cals produced by photosynthesis in remote parts of the plant, and this 

 fact can alter the interpretations of the over-all photosynthetic process. 



The purpose of the experiments has been to try to find the maximum 

 efficiency in photosynthesis, and this calls for the optimum biological con- 

 ditions favorable to photosynthesis. The conditions of culturing algae 

 are rather special, but they have been easily duplicated in the different 

 laboratories. Algae go through the normal logarithmic growth and aging 

 that are common to all colonies of living cells. Usually the culture is 

 used in photosynthetic experiments after the algae have been growing in 

 a suspension for a few days or a week. After an algal suspension is more 

 than 2 weeks old, the photosynthetic efficiency drops markedly. Obvi- 

 ously, dead cells or inactive cells will reduce the efficiency, because they 

 absorb light without evolving any oxygen. There is a good deal of evi- 

 dence to show that the best results are obtained with algae that are very 

 green, with a high chlorophyll content. Several special conditions for 

 growth of the algae have been proposed by difTerent workers. They 

 include specifications regarding intensity of the light on successive days, 

 the temperature and the concentration of carbon dioxide in the gases 

 that are bubbled through the algal suspension, and the rate of bubbling. 

 These conditions have been duplicated in many laboratories, and algae of 

 different strains have been traded among different workers. 



Clearly it is necessary to have an ample supply of all the necessary 

 chemicals for proper growth. The different nutrient solutions developed 

 by Warburg, by Emerson, and by Stauffer have been described in the 

 literature. In order to supply all the necessary chemical elements, the 

 Wisconsin investigators used salts from sea water, soil extracts, and an 

 A-to-Z solution containing 28 different chemical elements. It is gener- 

 ally recognized that a city water supply may contain chlorine or other 

 added chemicals that are detrimental to growth, and water purified by 



