296 PHOTOSYNTHESIS 



(0.078 mm. thick) that its temperature quickly reaches that of the 

 surrounding hquid. This plant is the marine alga Ulva rigida (sea 

 lettuce) which was chosen because of its rapid rate of photosynthesis, 

 its hardiness under laboratory conditions, and its excellent mechanical 

 properties. 



The method^ (which seems to be more accurate than those hitherto 

 employed) consisted in placing the plant in sea water to which an in- 

 dicator had been added, the whole being contained in a tube which 

 was immersed to a depth of 2 or 3 inches in a water bath and exposed 

 to direct sunlight.'* Photosynthesis was allowed to proceed until a 

 definite amount of CO2 had been abstracted from the sea water, as 

 shown by the color of the indicator. This was repeated as often as 

 necessary. It has been shown in a previous article^ that the rate of 

 photosynthesis is not constant from the start, but steadily increases 

 until it reaches a constant value. It was therefore necessary to con- 

 tinue the exposure until the constant value was reached. After de- 

 termining the constant rate the temperature was changed and new 

 determinations were made. 



Since the amount of CO2 abstracted by the plant (and consequently 

 the amount of photosynthesis) was always the same, the rates at 

 different temperatures are inversely proportional to the time required 

 for the standard amount of photosynthesis. 



Since the object of the investigation was to ascertain whether the 

 temperature coefficients are those of light reactions or of ordinary 

 chemical processes, it was not necessary to extend the experiments 

 over a wide range of temperature. The temperatures chosen were 

 17° and 27°C., which are high enough to ensure rapid photosynthesis 

 but which do not produce the slightest injury. 



The results are shown in Table I. For convenience the rate of 

 photosynthesis at 17°C. is taken as 100 and other rates are expressed 

 as per cent of this. Thus the rate at 27°C. is found to be 169 ±1.^ 



^ The details of technique are fully explained in a previous article. See Oster- 

 hout, W. J. v., and Haas, A. R. C, /. Gen. Physiol., 1918, i, 1. 



* The tubes were inclined so as to receive the sunhght as nearly as possible 

 at right angles. 



^ This figure represents the average of eight experiments. The results were 

 first expressed as per cent and then averaged. 



