RATE OF BUBBLING 1 5 



a deposit of mucilage at the cut end causes a slowing down. 

 This is specially evident in Vallisneria ; the vigorous 

 bubbling from a fresh cut leaf becomes slowed down and 

 is completely arrested in the course of an hour. This is, 

 however, an extreme case ; other water-plants exhibit this 

 retarding effect, though to a less extent. 



(4) Again, the normal rate of bubbling is liable to under- 

 go spontaneous variation ; this will be understood from the 

 account of the following experiment, where the rate of 

 bubbling after exposure to constant light was measured at 

 intervals of intervening darkness. The bubbling at first 

 took place at intervals ot nineteen seconds ; after a period of 

 darkness the plant was again subjected to the same intensity 

 of light as before. On the attainment of an uniform rate, 

 the frequency of bubbling was now found to have undergone 

 a sudden increase, the successive intervals being now reduced 

 to five seconds. It thus appeared as if previous exposure to 

 darkness had enhanced the activity -U times or nearly four- 

 fold. But independent measurement of the volumes of 

 gas given out during equal periods in the two cases showed 

 that they were equal. 



Wilmott 1 has attempted to obviate the difficulty of the 

 changing size of the bubblets by fitting a glass tube at the 

 cut end of the water-plant, the other end of the tube being 

 drawn out into a capillary from which the gas is delivered 

 into an isolated bubbling-cup of distilled water. 



For the determination of the photosynthetic activity 

 it is, however, necessary to measure the absolute rate of 

 evolution of oxygen and its modification under definite 

 external variations. In order to eliminate personal error 

 a device has, moreover, to be adopted for the automatic 

 record of the rate of evolution of oxygen. I shall presently 

 describe the Bubbler and the Electromagnetic Recorder by 

 which these special requirements have been fulfilled. 



1 Wilmott — Assimilation of submerged plants in dilute solutions of 

 bicarbonates and of acids ; an improved bubble-counting technique — 

 Proc. Roy. Soc, B., vol. 92, 1921. 



