TBANSPIKATION 



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 



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 



FIG. 69. APPARATUS TO SHOW THE 

 SUCTION CAUSED BY TRANSPIRA- 

 TION. (After Detmer.) 



