The Controlling Influence of Carbon Dioxide. 139 



Experiments were conducted in parallel series, each parallel series consisting 

 of a number of experiments set up at the same time with varying percentages 

 of CO2 present and a control. The respiring material was enclosed in pressure 

 flasks of about 1000 c.c. capacity closed by rubber corks or by specially con- 

 structed ground glass stoppers and a mercury seal. The desired atmospheres 

 were made up, after evacuating the flasks with material in place, from 

 cylinders of compressed gas, measured in by means of a manometer. Nitrogen 

 was used as the residual gas in order to conduct all the experiments at 

 atmospheric pressure. In the case of seeds a little wet sterilised sand was 

 put at the bottom of the flasks, the amount of sand and water being carefully 

 equalised for experiments of the same series, and the dry seeds were set on 

 this. Before the first gas estimations were made a short interval of time was 

 allowed for a physical equilibrium to be reached between the water in the 

 flasks and the partial pressures of CO2 introduced. In the case of leaves a 

 longer period was allowed before the first analysis or pressure reading. 



A difficulty encountered in the experiments was to obtain accurate 

 measurements of small increases of CO2 relatively to large partial pressures 

 of the same gas initially present. Two different methods were employed in 

 estimating the CO2 produced. It was calculated from analyses of small 

 samples withdrawn at intervals, or, in the case of the anaerobic experiments, 

 it was calculated from the increased pressures created in the flask. Mano- 

 meters are necessary in either case, since to obtain comparable results from 

 analyses the pressures of the gas mixtures must also be known. Open mercury 

 gauges were used and corrections made for the barometer variation. The 

 highest pressure changes recorded were not more than of an atmosphere, 

 i.e.. 76 mm. 



The gas analyses were made with Haldane's apparatus, 10 c.c. samples 

 being withdrawn, that is from 1 to 2 per cent, of the total gas present. The 

 withdrawal of these samples, though it does not alter the proportions of the 

 various gases present, decreases their active mass or pressure. With regard 

 to carbon dioxide this decrease is always more than compensated by the CO2 

 evolved in respiration. Only where large percentages of CO2 are present does 

 the amount lost in any analysis come near to equalising the amount produced 

 by seeds or leaves in the interval since the previous analysis. Generally 

 speaking, then, the pressures of CO2, the effect of which upon respiration we 

 wish to find, rise continually while an experiment lasts. This rise is deter- 

 mined by the same readings which give us the amount of CO2 produced in 

 respiration, so that the data obtained are sufficient for us to calculate figures 

 for the effect of any constant pressure of carbon dioxide. 



It is, of course, the value of the active mass or absolute pressure of carbon 



