the Ascent of Water in Trees. 653 
is injected with water and left standing in a vertical position, the lower 
end dipping in water, it is found, after some time, to dry back from 
the upper cut surface. Here imbibition of the cell-wall shows itself 
inadequate to draw up water against gravity and the viscous resistance 
of the liquid. How then is it able to functionate in the leaf? Again, 
a cut branch fed with a liquid injurious to osmotic action shows, first, 
diminished transpiration, ending in complete drying up of the leaf. 
The same effect may be brought about by exposing the leaf to a tem- 
perature sufficient to kill the protoplasm : a ‘ damp heat ’ of 8o° C. 
Lastly, we must consider that in some cases individual cells show 
themselves capable of effecting an actual extrusion of water by 
internal actions. The unicellular Fungus, Mucor , shows this in 
a marked degree. Pilobolus and Phycomyces may also be mentioned. 
The phenomenon of the extrusion of water by certain cells at the 
base of the petiole of the sensitive plant ( Mimosa pudica ), when 
responding to stimulation, is probably another case. 
All this suggests that the actions progressing in the turgescent cells 
actually assist the extrusion of water. Indeed, we may go further and 
state more positively that the inadequacy of imbibition when unaided 
to maintain the moisture at the surface of the cell-wall on the one 
hand, and the evidence for the existence of extrusive forces in the 
turgescent cell on the other, at once forbid us to close the question 
as settled, and suggest where to look for the answer. 
But what is the nature of the extrusive forces ? 
It would be a matter of profound interest if it turned out that ‘ vital ’ 
actions entered into this question. Or, if £ vital ’ processes in the 
primordial utricle are not responsible, can we refer the action to 
osmotic pressure alone ? If we do so, we certainly find ourselves in 
the difficulty of ascribing to this force the opposite effects of securing 
the entrance of the sap into the cell against the tension in the 
conduits, and its subsequent expulsion through the cell-wall. 
While this appears hardly admissible it is very certain that the 
osmotic pressure in the cell is effective in largely promoting the 
evaporation in so far as its action upon the flexible membrane results 
in the distension of this. This distension may amount to as much as 
from 10 per cent, to 20 per cent. 1 We may imagine the elastic 
shrinkage of the wall and its reduction in volume under the tensile 
1 Strasburger, Lehrbuch der Botanik, p. 138. 
