304 



Mxperimental Researches on Vegetable Assimilation and Resjnra- 

 tion. Xiy. — Assimilation by Submerged Plants in Dilute 

 Solutions of Bicarbonates and of Acids : an Imjyi^oved 

 Bu bble- Counting Techniqu e. 



By A. J. WiLMOTT. 



(Communicated by Dr. F. F. Blackman, F.R.S. Eeceived June 28, 1921.) 



The procedure of couuting the bubbles given off from the cut stem of 

 A submerged water plant, to obtain a measure of the magnitude of the plant's 

 assimilation, was introduced by Dutrochet in 1837 and matured by Sachs in 

 1864. It has been of great use in demonstrations and class work, and has 

 also been seriously employed in a number of researches. From time to time 

 it has been subjected to a good deal of criticism, as giving a faulty measure 

 of the true rate of photosynthesis. It is, however, a striking fact that 

 nothing has been done to improve the technique of the procedure since the 

 method was originally introduced. 



The first part of the present paper describes a simple device which removes 

 at once two of the very serious defects of the method, and renders it much 

 more suitable for research work. The second part applies this method to an 

 elucidation of the extraordinary effect of dilute mineral acids upon bubble 

 rate brought forward by Treboux. In the third part the relation of bubbling 

 in bicarbonates to bubbling in carbonic acid is investigated, and it is shown 

 that Angelstein's statement that water plants can actively split bicarbonates 

 in solution is erroneous. 



Part I. — The Bubble-Counting Method of Measuring Carbon- 

 Assimilation IN Submerged Plants. 



During the course of the investigations described in the later parts of this 

 paper, which were carried out in the Botany School, Cambridge, in 1911, at 

 the suggestion of Dr. F. F. Blackman, the bubble-counting method was 

 carefully examined, and improved in several respects. 



Various defects of this method had been pointed out before the present 

 work, and, since that date, Kniep, in 1915, has destructively criticised it 

 None of its critics seems, however, to have attempted to improve it. Yet 

 the method is so extraordinarily sensitive to influences of all sorts that it 

 cannot be lightly abandoned as a research instrument. As will be shown 

 immediately, some of its most serious defects are quite easily remedied in 

 simple ways. These may be considered under two headings : (A) Methods of 



