308 Mr. A. J. Wilmott. Experimental Researches on 



It was demonstrated that the bubbles have a larger volume when the vessel 

 contains osmotically active salt solutions than when water is used. In a 

 whole shoot of Elodea carrying on assimilation in a salt solution the effect 

 would be recorded as a smaller number of bubbles in unit time, which is 

 what Treboux found in his Experiments 1 to 6. On replacing the salt 

 solution by distilled water, the cells, now containing salt, absorb water and 

 swell round the aperture, reducing its size and also that of the bubbles, and 

 sometimes (after strong solutions) completely stopping bubbling by closing up 

 the aperture. 



With this apparatus was also demonstrated the effect of changed surface- 

 tension. Thus, on adding methylated spirit to the vessel the bubble size is 

 decreased, which in natural experimentation would express itself as an 

 increased rate of bubbling, such as Darwin and Pertz observed. This smaller 

 size of bubbles on adding methylated spirit was produced (to about 10 per 

 cent.) also when a glass bubbler was fixed to the free end of the Elodea stem, 

 and air forced through the whole system. 



The mechanism of bubble-initiation, swelling, and liberation with the 

 glass bubbler, as described above, was followed carefully with this apparatus 

 at various rates, and as produced by different heads of water. In some cases 

 a slight head of water produced no bubbles, but increasing pressure caused 

 a sadden rapid rush of bubbles, which quickly slackened and then died 

 completely away. Further increase caused a repetition of this without any 

 regular bubbling occurring. This irregularity was evidently caused by the 

 formation of water films across all the intercellular spaces, bubbling occurring 

 only when the pressure became sufficient to burst them, after which they 

 re-formed. These same " rushes of bubbles " occurred also with material used 

 in these experiments, seeming to be most frequent during very dull weather. 

 Probably during very dull spring days the respiration may exceed the assimi- 

 lation, the volume of the internal air system being reduced until water films 

 are formed. Sometimes a plant which at first refused to bubble would begin 

 in the middle of an experiment (two shoots side by side were used to ensure 

 one of them finishing the experiment) and either bubble regularly or in 

 " rushes." 



Adopting the bubblers and bubbling cup described in this section, we have 

 every reason to believe that all the sources of error in bubble-counting which 

 are due to variation in size of bubbles have been successfully removed, so 

 that simple counting becomes a direct measure of volume of gas leaving the 

 shoot, even when different plants are used in a series of experiments. There 

 is now another, more subtle source of complications to be considered. 



