90 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



into the absorbing tube, and the method of calculating the carbon 

 dioxid for each period was essentially the same as in the respira- 

 tion experiments. A curve of the specific resistance for concentra- 

 tions of 0.1001 to 0.0860 normal, similar to the one used for the 

 respiration work, was determined by means of the high-resistance 

 cell, and was used for establishing the amount of carbon dioxid 

 absorbed. 



The barium-hydroxide solution, after absorption of the carbon 

 dioxid, was transferred rapidly to narrow bottles; these were then 

 stoppered and sealed with paraffine. After all the barium carbonate 

 had settled out, the conductivity of the clear supernatant liquid 

 was determined bj^ means of the electrolytic cell shown in figure 23. 

 This is essentially a pipette form of cell with a glass stopcock, and 

 the solution is protected from the air by means of a soda-Hme tube. 

 The cell had a resistance of 1,950 ohms when filled with 0.1 normal 

 potassium-chloride solution. For making the conductivity deter- 

 minations, the cell was submerged slightly above the upper electrode 

 in a water thermostat kept at 25°. A difference of 1 ohm in observed 

 resistance represented about 0.00014 gram of carbon dioxid when 

 using 68.12 c. c. of the barium-hydroxide solution. For the deter- 

 mination of the carbon dioxid in the different periods of a single 

 experiment, differences of at least several ohms were observed, 

 which gives an indication of the degree of accuracy of the method. 



As a source of Ught there was used a 500 or 750 watt tungsten 

 Mazda lamp. The distance of the filament from the leaf was 35 

 cm., and the light traversed about 8 cm. of water in the thermostat. 

 The electric lamp was so placed that the light fell on the leaf at right 

 angles. Immediately above and fitting over the top of the glass 

 bulb of the lamp was a metal hood in the form of an inverted funnel. 

 This hood was connected by means of a 5-cm. pipe to an electric 

 suction fan. In this way the hot air surrounding the electric lamp 

 was drawn off. 



After each experiment the leaf was removed from the leaf-container 

 and placed in a photographic printing-frame with blue-print paper. 

 From the print thus produced the area of the leaf was determined 

 by means of a planimeter. 



EXPERIMENTAL RESULTS. 



The problems of photosynthesis must be met in succession. It is 

 questionable whether the ultimate or final causes can be discovered 

 before a clearer understanding is gained of the workings of the 

 immediate causes and relationships. The introduction by Blackman 

 of the conception of Hmiting factors into the experimentation on this 

 phenomenon, while it has contributed little to the ultimate causes 

 involved, has aided very materially in the experimental investigation. 



