2GG RADIATION BIOLOGY 



of short duration, in easily handled laboratory equipment using mono- 

 chromatic light. But precise measurements with monochromatic light 

 recjuire that the intensity of radiation be low, because the monochro- 

 mators or filters reduce the light intensity. High biological efficiency 

 also requires that the intensity of the light be low. The energy can be 

 measured precisely, but the chemical changes are so small that micro 

 methods must be used, and micro methods are apt to give results of low 

 accuracy. Much ingenious work has gone into the perfecting of micro 

 methods for determining the chemical change in photosynthesis. 



The most obvious way in which to follow photosynthesis and respira- 

 tion is to measure the change in concentration of oxygen and carbon 

 dioxide of the surrounding gas by standard methods of gas analysis. 

 The micro gas analysis for carbon dioxide and oxygen using alkaline 

 absorbents to remove the carbon dioxide and phosphorus or alkaline 

 pyrogallic acid to remove the oxygen has been perfected (Manning, 

 Stauffer, et al., 1938). The analysis can be carried out either in a flow 

 system in which the gases are absorbed and weighed or in an enclosed 

 space in which the gases are removed by absorption and measured volu- 

 metrically. However, the changes in composition of the gases in most 

 experiments of photosynthesis are so small that this method has not been 

 much used. 



The chemical determination of carbohydrates or of carbon dioxide is 

 difficult in a complex system such as a suspension of algae or a mass of 

 growing plants, but the chemical analysis for dissolved oxygen is quite 

 simple. The Winkler method, based on the oxidation of manganous 

 hydroxide by dissolved oxygen and subsequent titration for iodine, is 

 simple, and it has been used successfully (Magee et al., 1939; Petering 

 and Daniels, 1938; Petering et al., 1939). The algal suspension is exposed 

 to the measured light in square bottles with polished sides and titrated 

 under conditions that prevent errors from atmospheric oxygen during 

 analysis. Good results have been obtained even when the algae are 

 present in the titrating solution. 



The dissolved oxygen can be determined more easily by means of the 

 dropping-mercury electrode (Petering and Daniels, 1938; Petering et al., 

 1939; Dutton and Manning, 1941) by an empirical method in which the 

 differences in galvanometer current at 1.0 and 0.1 volt are measured and 

 converted to oxygen concentration by reference to a calibration curve in 

 which the currents are plotted against known concentrations of dissolved 

 oxygen, conveniently determined by the Winkler method. The mercur}^ 

 cathode is kept separate from the mercury that drops down from the 

 capillary anode. Experiments have shown that the metallic mercury has 

 no detrimental physiological effect on Chlorella, although some compli- 

 cation was observed in experiments with salt-water diatoms. The 

 response to dissolved oxygen is rapid, with a time lag of less than half a 



