678 REPORT—1896. 
which was followed by that of Askenasy.' The leading idea common 
to these works is now well known, namely, that the raising of water 
to the tops of trees depends on the quality which water possesses of 
resisting tensile stress. To most botanists the existence of this quality 
is a new idea. To believe that columns of water should hang in the 
tracheals like solid bodies, and should, like them, transmit downwards the 
pull exerted on them at their upper ends by the transpiring leaves, is 
to some of us equivalent to believing in ropes of sand. 
Askenasy has earned the gratitude of his botanical readers by giving 
some of the evidence which demonstrates the existence of this property of 
water.?, A tube a meter in length was filled by Donny with water, and the 
remaining space was as far as possible freed from air. When the tube was 
placed vertically the water-column at the upper end hung there, and could 
not be made to break or free itself from the glass by violent shaking. 
Berthelot filled a thick-wall capillary tube completely with water at 
28°-30 C.° ; it was allowed to cool to 18°, so that the space left by the 
shrinking of water was filled with air. It was then sealed up and again 
warmed to 28°-30°, so that the air was dissolved in the water. When it 
was allowed to cool again it retained its volume, filling the tube com- 
pletely. A slight shake, however, allowed the water to break and return 
to its proper volume at 18° with the appearance of a bubble of air. In 
this experiment the water contained air, yet it seems to have been until 
recently assumed by some physicists that, to show cohesion, water must 
be air-free. If this were the case the application of the principle to 
plants would be impossible. Dixon and Joly have, however, proved that 
this is not so, and this forms an important part of their contribution to 
the subject. 
They also? investigated the amount of tension which water under 
these circumstances will bear, and found it about equal to seven atmo- 
spheres. If, therefore, the leaves at the top of a tall tree can exert the 
requisite upward pull on the water in the trunk, it seems certain (if no 
other conditions in the problem interfere) that the pull can be transmitted 
to the level of the ground. This opens up the question whether the leaves 
can exert this traction on the water in the tracheals, and what is equally 
important, Are there any factors in the problem incompatible with the 
theory ? 
1. Lhe sucking force of the leaves.—In Dixon and Joly’s first paper 4 
they assume that tractional force is given by the meniscuses ‘formed 
inthe membranous réseau of the evaporating cell walls,’ as well as pos- 
sibly by the osmotic action of the cells of the mesophyll. We shall take 
these theories in order. Our knowledge of the cell wall does not allow us 
to believe in the existence of pores visible with even the highest powers of 
the microscope. Dixon’s more general expression, ‘surface tension forces 
developed in the substance of the walls of the evaporating cells,’ is there- 
) Verhand. d. naturhist. med. Vereins Heidelberg, N.F., Bd. v. 1895; and N.F., 
Bd. v. 1896. 
* He gives references to Donny, Poggendonff’s Annalen, 67. Bd. (143. Bd. d. g. R.), 
1846, p. 562; Berthelot, Annales de Chimie et de Physique, 8. 3, t. 30, 1850, p. 232; 
Worthington, Proc. Roy. Soc. vol. 1. 1892, p. 423. 
° Phil. Trans. vol. 186, p. 570. With ethyl alcohol Worthington records a ten- 
sion of 17 atmospheres. See Proc. R. Soc., vol. 1. 
4 Phil. Trans. pp. 563, 567. 
> Proc. Roy. Irish Acad. Jan. 13, 1896, p. 767. 
