DETERMINATION OF PHOTOSYNTHETIC ACTIVITY 269 



These remarkable similarities in the modification of 

 different modes of response to definite external changes 

 prove that the protoplasmic reaction of the protoplast is 

 a most important factor in photosynthesis. 



I will now give a short summary of the more important 

 results obtained concerning photosynthesis. 



Determination of Photosynthetic Activity 



The methods of investigation in photosynthesis with 

 Hydrilla which I have been able to devise may be regarded 

 as ideally simple, and as nearly as possible, perfect. The esti- 

 mation of the activity of photosynthesis from the rate of 

 evolution of oxygen is direct and requires no prolonged 

 chemical analysis. Complications arising from the presence 

 of stomata do not arise in Hydrilla. The oxygen method is, 

 moreover, free from many sources of error unavoidable in 

 the method based on the absorption of carbon dioxide. 



The inaccuracies in the method of counting the bubbles 

 from the cut end of the stem have been completely removed 

 by the new method which I devised for determination of the 

 frequency of successive bubbles representing equal volumes 

 of pure oxygen given out by the special Bubbler. The 

 Automatic Method of record by the Electro-magnetic Writer 

 eliminates all personal error of observation. The entire 

 investigation can be completed in the course of about 

 forty minutes, during which the external conditions can be 

 maintained absolutely constant. The temperature of the 

 plant remains the same as that of the mass of water in which 

 it is immersed. The difficulty in the maintenance oi con- 

 stant light has been completely removed by the employment 

 of special devices. 



As regards artificial source of light, Pointolite has been 

 found most suitable, since it produces little heat, which is 

 completely absorbed by an interposed stratum of water. 

 The intensity of light can be varied from ioo to 4000 

 lux. 



