NO. l6 CARBON DIOXIDE ASSIMILATION HOOVER ET AL. 9 



tance of the cell is given by 1/26450, and for a concentration of .782 

 per cent the conductance is 1/545 13. Intermediate concentrations 

 will l)e proportional to the change in the reciprocal of the resistance. 



EXPERIMENTAL PROCEDURE 



Wheat ' of the Marquis variety was selected for this work. It grows 

 well in nutrient solutions, and by the use of such a medium a more 

 accurate control of the nutritional phases of the work can be obtained. 

 The nutrient solution used and the general cultural methods have been 

 described in a previous publication.^ The wheat was germinated be- 

 tween layers of moist filter paper in a covered glass dish at a tempera- 

 ture of 25° C. When the roots had grown to a length of 2 to 4 cm 

 the young plants were transferred to a germination net stretched 

 over a glass dish through which tap water flowed. The plants were 

 illuminated by a 200-watt Mazda lamp placed 30 cm above the netting. 

 When the seedlings were approximately 4 to 5 cm in length, four 

 individuals selected for uniformity of size were transferred to the 

 growth chamber. The plants were supported by means of cotton in 

 the four small holes in a paraffined flat cork stopper which fitted into 

 the lower end of the tubular growth chamber. The growth chamber 

 was then placed over the Erlenmeyer flask containing the nutrient 

 solution and held in place by means of a rubber annulus. 



From the work of Johnston ° with tomato plants grown under 

 Mazda lamps it appears that the large proportion of infra-red radia- 

 tion found in this type of illumination is somewhat injurious. More 

 nearly normal growth and physiological response were obtained where 

 heat absorbing filters were used. Nearly normal wheat plants were 

 produced when the excessive infra-red radiation from the lamps was 

 absorbed by a solution of copper sulphate. This was easily accom- 

 plished by using a solution of copper sulphate (1.046 sp. gr.) in place 

 of water in the circulating system for controlling the temperature of 

 the growth chamber. Plate 2 shows four wheat plants grown in the 

 growth chamber for a period of 20 days. 



In the following tables the light intensities are expressed in ar- 

 bitrary units, and the carbon dioxide concentrations and assimilation 

 rates expressed as changes in the reciprocal of the resistance of the 

 conductivity cell. To express the concentration in terms of volume 



' The wheat used in these experiments was obtained through the courtesy of 

 H. H. McKinney, Jr., of the United States Department of Agriculture. 



* Johnston, Earl S., The functions of radiation in the physiology of plants. 

 II. Some effects of near infra-red radiation on plants. Smithsonian Misc. Coll., 

 vol. 87, no. 14, pp. 1-15, 1932. 



' Idem. 



