Preliminary Observations on the Transpiration Current 
in Submerged Water-plants. 
BY 
D. THODAY, M.A., 
Trinity College , Cambridge , Mackinnon Student of the Royal Society. 
AND 
M. G. SYKES, 
Girton College , Fellow of Newnham College , Cambridge. 
T HE growing tendency to doubt whether known physical forces are 
sufficient to bring about the ascent of water in tall trees, and to look 
to the agency of living cells adjacent to the conducting channels to supple- 
ment them, has suggested to us that the water current in submerged water- 
plants might be worth investigation. In this case evaporation from the 
leaves, an essential link in the chain of physical factors in the transpiration 
of land plants, is eliminated. 
Experimental evidence for the existence of a transpiration current in 
water-plants has been brought forward by Sauvageau , 1 Hochreutiner , 2 
Pond , 3 and others. Hochreutiner, by using a solution of eosin, obtained 
values for the rate of the current in cut branches of Potamogeton pectinatus , 
P. crispus , P. densus , and Ranunculus aquatilis , but his experiments were 
of many hours’ duration, and the eosin travelled not more than half a 
centimetre per hour. 
A similar eosin method was employed in some experiments with 
Potamogeton lucens , which were made by a class of students superintended 
by one of us during a course of demonstrations on the physiology of plants 
given in the laboratory of the Royal Holloway College. A glass cylinder 
was divided into two portions by a closely fitting flat cork covered with 
a soft wax mixture. The lower chamber was filled with a dilute solution 
1 Sauvageau: Feuilles des Monocotyledones aquatiques. Ann. des Sc. Nat., Bot., ser. 7, xiii, 
1891, p. 103. 
2 Hochreutiner, G. : Etudes sur les Phanerogames aquatiques du Rhone et du Port de Gen&ve. 
Rev. G&i. de Bot., viii, 1896, p. 158. 
3 Pond : The biological relation of aquatic plants to the substratum. Contributions to the 
biology of the Great Lakes, 1905. 
[Annals of Botany, Vol. XXIII. No. XCXI. October, 1909.3 
