43 ° Vines.— The Suction-force of 
the transpiring surfaces. The former make the following 
definite statement of their conclusion 1 : — 5 6 Our theory is that 
this (the suction- force, Saugkraft , of the leaf) is the all- 
sufficient cause of the elevation of the sap, not however by 
establishing differences of gas-pressure, but by exerting a 
simple tensile stress on the liquid in the conduits ’ : and 
Askenasy says 2 , — £ the warmth of the sun causes evaporation 
at the external surface of the mesophyll-cells, the imbibition- 
force of the wall of these cells sucks water from their contents, 
thus increasing their osmotic capacity : this gives rise to a pull 
which, in virtue of the cohesion of the water (we assume for the 
moment the existence of continuous columns of water in the 
conducting tissues, which is certainly true in many cases), 
is propagated to the root, and thus affects the living cells 
of the root : here it is reconverted into osmotic force which 
promotes absorption by the roots, provided that they are in 
contact with water. 5 
Such being the current views on the subject, it becomes 
a matter of importance to study and measure this suction- 
force ; to determine the relation of the force to the transpiring 
leaf-area and to variations in external conditions ; and to 
analyse it into its ultimate physical factors. 
Experiments of this sort are by no means new. So long 
ago as 1726, Hales 3 instituted ‘experiments whereby to find 
out the force with which trees imbibe moisture’; and the 
same has been done since by Meyen 4 , Unger 5 , von Sachs 0 , 
von Hohnel 7 , Boehm 8 , and others. In all cases the method 
of experimentation has been essentially the same. A cut-off 
branch was attached air-tight to a glass-tube filled with water, 
1 Phil. Trans., p. 563. 2 Loc. cit., 1895, p. 11. 
3 Statical Essays, Vol. i, 4th ed., 1769, p. 84. 
i System der Pflanzenphysiologie, 1838, Vol. ii, p. 70. 
5 Beitrage zur Anatomie und Physiologie der Pflanzen, XIII ; in Sitzungsber. 
d. k. Akad. d. Wiss., Wien, Vol. 50, 1864. 
6 Experimental-Physiologie, 1865, p. 260. 
7 Ueb. den negativen Druck der Gefassluft, Inang. Diss., Wien, 1876. 
8 Ber. d. deut. bot. Ges., VIII, 1889; Bot. Centralblatt, 42, 1890; Ber^d. 
deut. bot. Ges., XI, 1893. 
