REPORT OF THE SECRETARY 67 



wheat is grown in an all-vitreous tubular container illuminated 

 symmetrically from four sides. The wheat is protected by a sheet of 

 copper-sulphate solution which serves the double purpose of main- 

 taining the temperature and absorbing the excessive infra-red from 

 the artificial sources. Carbon-dioxide concentration was measured by 

 means of a potassium-hydroxide conductivity cell. The chief diffi- 

 culties in this type of measurement have arisen from the high tem- 

 perature coefficient and the polarization of the conductivity cell. 

 These have been overcome, respectively, by improved thermostating 



— — P __ p — _ , 



.3 A .5 



CCKCEHTaaTESH 



ASSIMILATION CURVES FUK WHEAT 



Figure 1. — Assimilation as a function of concentration for light intensities 400 foot- 

 candlcs and 930 foot-candles 



which holds the solutions to within less than one one-hundredth of 

 a degree, and the installation of a commutator which reverses the 

 polarity of the conductivity cell without changing the direction of 

 the current through the galvanometer. By this method it has been 

 possible to secure carbon-dioxide measurements which are significant 

 to the order of one one-thousandth of a per cent. Typical curves of 

 assimilation as a function of carbon-dioxide concentration are shown 

 in Figure 1. Each of these curves represents the values for a single 

 light intensity. For a light intensity of 950 foot-candles photosyn- 

 thesis is proportional to the concentration of carbon dioxide from 

 to 0.04 per cent. A maximum rate is reached at a concentration of 

 0.140 per cent. Further increase in concentration up to one-half 

 per cent produces no further change in assimilation, light intensity 

 149571—33 6 



