266 TRANSPIRATION CURRENT [CH. 



the supply of this gas which is given off from soil containing 

 organic matter. The divergence of these workers' views indi- 

 cates a direction in which further experimental work of a critical 

 nature is markedly needed. 



A piece of indirect evidence, which confirms, though it does 

 not actually prove, the existence of a transpiration current in 

 submerged vegetation, has recently been obtained in connexion 

 with certain studies on the relative osmotic strength of the cell- 

 sap in the leaves and roots of the same plant. In terrestrial 

 species, the osmotic pressure in the leaves has been shown to be, 

 as a general rule, less than that in the root, a result which is 

 entirely in harmony with the known facts relating to root- 

 pressure. In submerged plants (Ehdea, etc.), the same osmotic 

 relation has also been found to exist, a difference of as much as 

 four atmospheres being recorded, in one case, between the 

 pressures in leaf and root 1 . It seems impossible to explain these 

 results on the hypothesis that the transpiration current in such 

 plants is non-existent 2 . 



If it be granted that a transpiration 3 current occurs, even in 

 plants which are entirely submerged, and that this current is, 

 at least to some extent, dependent on the leaves 4 , we are at once 

 confronted with the problem of how the leaves eliminate the 

 water, since the discharge of water-vapour obviously cannot 

 occur in the manner characteristic of land plants. For a large 

 number of submerged plants, though by no means all, the 

 question has now been elucidated by the work of Sauvageau, 

 von Minden and other observers 5 . In many cases the mecha- 



1 Hannig, E. (1912). 2 Snell, K. (1912). 



3 The word " transpiration " is deliberately used throughout this 

 chapter, in lieu of " guttation," suggested by Burgerstein, A. (1904) as 

 more appropriate for submerged plants. The expression, "transpiration," 

 is not likely to cause any confusion, and the word " guttation," though 

 perhaps more strictly accurate in many cases, is too awkward and ugly 

 to be readily admitted into our language. 



4 Thoday, D. and Sykes, M. G. (1909). 



5 Oliver, F. W. (1888), Schrenk, J. (1888), Sauvageau, C. (iSgi 1 ), 

 Wachter,W. (i 89 y 1 ), Minden, M. von (i 899),Weinrowsky, P. (i 899), etc. 



