374 
Experimental Researches on Vegetable Assimilation and Respr- 
ration. VIIL*—A New Method for Estimating the Gaseous 
Exchanges of Submerged Plants. 
By F. Frost Brackmay, D.Sc. F.RS., Fellow of St. John’s College, 
Cambridge, and Reader in Botany in the University ; and A. M. SMITH, 
M.A., formerly Senior Demonstrator in Botany in the University of 
Cambridge. 
(Received December 19, 1910,—Read February 9, 1911.) 
INTRODUCTION. 
The method in universal use for evaluating assimilation in submerged 
plants consists in counting or measuring the evolution of bubbles of gas. 
This method gives satisfactory results in medium conditions, but fails at 
both extremes of the conditions—hght, temperature, and CO2-supply— 
which chiefly control the magnitude of assimilation. Critical work over 
a wide range of conditions is therefore impossible with it. 
A really satisfactory method must take account of the alteration of the 
dissolved gases as well as of those that are liberated as gas-bubbles ; because, 
when assimilation is slight, the oxygen formed may all dissolve in the water 
and no bubbles appear. When temperature is high the bubbles consist 
partly of other gases physically liberated from the water, and when the 
CQO.-content of the surrounding water is very high the bubbles escaping 
will consist chiefly of this gas. 
Wishing to extend our researches from land-plants to water-plants, we have 
elaborated a method which is open to none of these objections, and which will 
be described in the present paper. In this method a continuous current of 
water containing dissolved CO, flows over the assimilating plant, and the 
difference in the COs-content of the water before and after contact with 
the plant is a measure of the assimilation taking place. Fewer bubbles 
are liberated under these conditions than when the water is stationary, but 
the gas given off is collected automatically, analysed and allowed for in 
estimating the total assimilation. 
The plant is contained in a glass chamber, and the conditions of 
illumination and temperature and COs-supply are completely under 
* The first six contributions of this series have appeared in the publications of the 
Royal Society ; No. VII, by Miss A. Irving, on “The Beginning of Photosynthesis and 
the Development of Chlorophyll,” will be found in the ‘Annals of Botany,’ October, 
1910. 
