MEASURING PHOTOSYNTHETIC ACTIVITY 235 



the bubble stream does not commence at once. In fact, the longer the 

 duration of the dark period the longer does it take for the bubbles to 

 appear. Thus, for example, Kniep found for Elodea canadensis: 



20 Bubbles in 5.4 seconds. 

 Time plant was kept 



in dark 20 20 40 40 80 300 300 600 



Time required for 



bubble stream to 



start after reillumi- 



nation 8.0 8.2 13.2 13.0 18.2 60.0 61.0 93.5 



The explanation for this phenomenon given by Kniep is that when 

 photosynthesis ceases there is a reduction of gas pressure in the inter- 

 cellular spaces. This permits the further entrance of water into these 

 spaces. When photosynthesis recommences a certain gas pressure must 

 be reestablished to force out the water-column before the bubbles can 

 be forced through the intercellular canals. The time required for the 

 bubble stream to start after reillumination is the time during which this 

 gas pressure is being developed. 



Kohl ^' has also called attention to the fact that an error is intro- 

 duced on account of the varying size of the gas bubbles. With a rapid 

 bubble stream the individual bubbles are smaller than when the rate is 

 slower and each bubble clings to the plant longer. Kohl devised there- 

 fore his volumetric bubble counting method. He used a single Elodea 

 leaf which is cut in such a manner that a small portion of the stem 

 is attached to the leaf. The leaf is held in the bottom of a shallow dish 

 of water by means of a small glass plate. • The bubbles emitted in the 

 light from the cut surface take on a practically spherical form so that 

 by measuring their diameter with the microscope micrometer the volume 

 of the gas emitted can be easily calculated. In this way an advantage 

 is gained in that the angle of the light falling on the leaf can be con- 

 trolled. However, the method has found very little application. 



It is evident then that the bubble counting method can rarely be used 

 as a strictly quantitative measure of photosynthetic activity. For pur- 

 poses of demonstration and for comparative observations it has certain 

 advantages. It is possible that for studying the effects of substances 

 in solution, different frequencies of light, of varying light intensity and 

 of temperature the method may be jxit to use. However, it is exceed- 

 ingly important to remember that tlie relative position of the plant to 

 the source of light must be maintained and that after changing any of 

 the external conditions some time must elapse before a constant rate of 

 bubble emission is attained. It is certain that under strictly constant 

 external conditions an increase or decrease in the rate of bubble emission 

 represents respectively an increase or decrease in the rate of photosyn- 

 thesis. This relation is not directly proportional however. In order 

 to determine the absolute rate of photosynthesis it is necessary to deter- 



"Kohl, F. G., Ber. hot. Ges., 15, 111 (1897). 



