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CARNEGIE INSTITUTION OF WASHINGTON. 



Earlier experiments on the influence of sunlight and ultra-violet 

 light on important plant processes had given indication that light 

 acted not only directly by inducing chemical and physical changes of 

 physiologically important substances within the organism, but also 

 acted indirectly by affecting the air surrounding the plant. This latter 

 effect is plainly noticeable in the respiratory activity. An extended 

 series of experiments was carried out in which the only variable factor 

 to which the plants were exposed was the air of the day on the one 

 hand and that of the night on the other. A respiration apparatus was 

 so arranged that air was drawn directly from out-of-doors, freed from 

 the carbon dioxide by means of coarse soda-lime, then drawn over the 

 plants kept in the dark at constant conditions of temperature and 

 humidit}^, and finally through a standard barium-hydroxide solution 

 in Meyer's tubes. Thus it was found that the rate at which carbon 

 dioxide was evolved was regularly higher during the hours of day than 

 during the night. As a working hypothesis, it was assumed that these 

 differences in respiratory activity were due to variations in the chemical 

 activity of the atmospheric oxygen, as indicated by the values for 

 atmospheric ionization. Therefore the air was discharged by passing 

 it between the opposite poles of a series of electric batteries before 

 coming in contact with the plants. Thus the differences in respiratory 

 activity between day and night were considerably reduced. 



It was found that these variations in carbon-dioxide evolution were 

 sufficiently great to give reliable results only when the material used 

 is actively respiring, when the gaseous exchange with the atmosphere 

 is not difficult, and when there is a sufficient supply of carbohydrate 

 food material. For these and cultural reasons wheat seedlings proved 

 most convenient. However, due to the relatively rapid rise and fall of 

 their respiratory activity, a mathematical comparison between the day 

 and night rates becomes difficult. Nevertheless, the course of the 

 rate of carbon-dioxide evolution of successive 3-hour periods shows a 

 marked similarity with the variations generally given for atmospheric 

 ionization; while with the deionized air the rate of carbon-dioxide 

 evolution rises and falls in an even curve. The following summarizes 

 the experiments, which ran 12 to 15 days; the rate at which carbon 

 dioxide was produced is taken from 12-hour periods: 



