RATE OF BUBBLING 



29 



This uniformity in the rate of bubbling for an hour 

 shows how possible it is to maintain the external conditions 

 constant during that period. We find that the interval 

 between any two successive bubbles is practically the same 

 throughout; the mean period of five successive bubbles 

 would, however, be found to be still more accurate. Hence 

 three minutes' observation is enough for determination of 

 the normal period. Should we wish to determine the 



Table III. — Giving Bubbling- Periods at the Beginning, after 

 Twenty Minutes and after Forty Minutes 



effect of an external change, such as that of intensity of 

 light, we allow two minutes for adjustment to the new 

 condition and then determine the new rate that has been 

 induced. The first observation takes three minutes, the 

 second and subsequent observations five minutes each. 

 About seven different observations would be sufficient to 

 afford data for a complete photosynthetic curve. The whole 

 investigation can be completed in the course of about forty 

 minutes, during which the other factors are maintained 

 quite uniform. The time for obtaining a complete curve 

 for a single plant is so short that more than half a dozen 

 determinations with different plants can be easily made 

 in the course of a single day. 



