637 
the Ascent of Water in Trees. 
walls of the evaporating cells,’ is therefore preferable. But Askenasy 
seems to me to put the matter more conveniently by using the term 
‘imbibition 1 / The force with which vegetable membranes, e. g. the 
thallus of Laminaria, absorb water, has been demonstrated by Reinke 
and others, and the existence of such a force is familiar to botanists. 
Both Askenasy (loc. cit.) and Dixon and Joly 2 have pointed out 
that the force of imbibition, or the surface tension forces, as the case 
may be, can exert a tractional effect on the water in. the tracheals, 
when the turgescence of the mesophyll has been destroyed. But 
Askenasy in his original paper (1895), Dixon in the January 1896 
paper, and again Askenasy in his second paper (March, 1896) have 
also considered the imbibitional or surface tension forces in con- 
nexion with the turgescent cell. In his 1896 paper Dixon in fact 
gives up the view published in the Phil. Trans, and adopts the view 
given by Askenasy in his original paper, that the tractional force is 
supplied by the osmotic suck of the leaves. It must clearly be under- 
stood that this does not remove imbibition from the problem. It is 
one of the chief merits of Askenasy’s work that he clearly sees and 
states the important relation between these forces 3 . The sun’s heat 
causes the evaporation of the water with which the walls of the 
mesophyll cells are imbibed ; this water is replaced by imbibition from 
the cell-sap. The concentration of the cell-sap so produced maintains 
the osmotic force of the cell, which again exerts suction on the water 
on the tracheals 4 . 
I have now given, in its simplest form, the modern theory of the 
rise of water. Apart from the main idea, it combines the points of 
several familiar views. Imbibition becomes a factor of paramount 
importance, though not in the way that Sachs employs it. The 
suspended threads of water remind us of Elfving’s capillary theory, 
while the living-element factor is represented by the turgescent 
mesophyll cells. 
Resistance . — It is not possible to discuss the question whether the 
tractional forces in the leaf are sufficient for the work imposed on 
them until we know what is the resistance to the passage of water 
through wood. For it is clear that the work done by the leaf includes 
1 Loc. cit., 1895, p. 10. 2 Annals of Bot., Sept. 1895. 
3 Askenasy, 1895, p. 11. 
4 Sachs’ Text Book, edit, iv, Eng. Tr., p. 679, describes evaporation taking place 
in the cell-wall, which makes good the loss by imbibition. 
