EEPORT OF THE SECRETARY 69 



centration of carbon dioxide be brought in contact with all the sur- 

 faces of the leaves. Nevertheless, the apparatus has been so designed 

 as to minimize these difficulties. The fact that the sources of radia- 

 tion are symmetrically placed on all four sides not only reduces the 

 fluctuations of intensity as a function of direction, but, owing to the 

 fact that the leaves are exposed from both sides, reduces to a mini- 

 mum the variation of intensity through the leaf. A method of recir- 

 culation reduces the variation of concentration over the plant to 

 about one-thirtieth part of the difference between the input and the 

 output concentrations. 



In view of these precautions it does not seem likely that the whole 

 of this transition range can be accounted for by variations in the 

 environment. It is beyond the scope of this report to enter upon a 

 more critical and detailed discussion of these points and the wide 

 literature bearing upon them. A number of considerations are of 

 particular interest: First, that for an intensity of approximately 

 one-tenth of maximum sunlight the carbon-dioxide concentration of 

 air is the limiting factor. As one goes to lower light intensities, 

 intensity becomes first partially limiting and then wholly limiting, 

 so far as the actual conditions controlling the growth of higher plants 

 are concerned. On the other hand, by the extrapolation of the linear 

 portion of this curve for the range where carbon dioxide is the lim- 

 iting factor, together with the addition of the value a for the transi- 

 tion range, one may arrive at the concentration of carbon dioxide 

 which would be required to give a maximum assimilation for avail- 

 able light intensity. Assuming for such a noonday intensity 7 X 

 our experimental condition of 950 foot-candles one obtains 



Yx0.0G9+0.081=0.564 per cent. 



Such an increase in available carbon dioxide would yield an 

 increased assimilation rate amounting at times to tenfold or more. 



This experiment has been chiefly conducted by Mr. Hoover. Doc- 

 tor Johnston has worked with him upon some of the physiological 

 phases, and Doctor McAlister upon the light-intensity measure- 

 ments. 



PLANT GROWTH 



A set of individual plant-growth chambers has been completed 

 which enables one to make comparative observations upon the ef- 

 fects of different wave length distributions of light. The four 

 chambers have been so constructed as to permit of both lateral and 

 overhead illumination. They are controlled by a central circulating 

 system which maintains the same temperature and humidity in the 

 four chambers. Rate of recirculation is maintained constant by 

 flow meters. The same nutrient solution is used throughout. With 



