636 Report of a Discussion on 
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 2 8°-30° C. ; it was allowed to cool to 18 0 , 
so that the space left by the shrinking of water was filled with air. It 
was then sealed up and again warmed to 2 8°-30°, so that the air was 
dissolved in the water. When it was allowed to cool again it retained 
its volume, filling the tube completely. A slight shake, however, 
allowed the water to break and return to its proper volume at 18 0 
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 
atmospheres. 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. The sucking force of the leaves . — In Dixon and Joly’s first paper 2 
they assume that tractional force is given by the meniscuses £ formed 
in the membranous re'seau of the evaporating cell-walls,’ as well as 
possibly 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 ex- 
pression 3 , ‘surface tension forces developed in the substance of the 
1846, p. 562 ; Berthelot, Annales de Chimie et de Physique, S. 3, t. 30, 1850, 
p. 232 ; Worthington, Proc. Roy. Soc., Vol. 1 ., 1892, p. 423. 
1 Phil. Trans., Vol. 186, p. 570. With ethyl alcohol Worthington records a tension 
of 17 atmospheres. See Proc. R. Soc., Vol. 1 . 
2 Phil. Trans., pp. 563, 567. 
3 Proc. Roy. Irish Acad., Jan. 13, 1896, p. 767. 
