TRANSPIRATION 
105 
It is, however, very sensitive to slight changes in the 
environment. 
It was mentioned in an earlier part of this chapter that 
the force of transpiration is 
of considerable assistance in 
maintaining the upward flow 
of water from the roots. The 
apparatus shown in fig. 69 
enables this to be demon- 
strated. The cut end of a 
branch is connected by an 
air-tight joint with a glass 
tube filled with water, the 
lower end of which dips into 
a vessel of mercury. As the 
water is transpired, a certain 
quantity of mercury enters 
the tube, and is drawn up for 
some considerable distance 
by the suction. 
The evaporation from the 
cells takes place, as we have 
seen, not immediately into 
the external air, but into the 
intercellular passages of the 
plant. The force causing 
this suction, so far as it is 
due to evaporation, is there- 
fore localised in the surface 
film formed in the evapo- 
rating cell-walls. Such an 
evaporation has been shown 
by Strasburger to be capable 
Fic. 69.—ApPARATUS TO SHOW THE 
SucTION CAUSED BY ‘T'RANSPIRA- 
tion, (After Detmer.) 
of raising a current of water through pieces of dead wood 
which have been soaked and injected with water. 
Osmosis IN THE LEAvES.—There is reason to believe, 
however, that a third factor in the ascent of the stream is 
