146 



Elementary Botany 



water it gannot, as a rule, obtain a sufficiency of the needed 

 liquid, owing to the fact that cutting the stem in the air acts 

 upon the cut surface in such a way as to diminish its absorbing 

 power. If, however, the stem is cut under water, absorption 

 takes place as usual. That the current does take place through 

 the wood can be shown — (i) If a ring of bark is cut off 

 the stem of a growing plant, as long as the wood is uninjured 

 the leaves will not droop ; and (2) if the plant is watered 

 with some coloured solution it is found to rise in the wood, 

 staining it. The amount of the flow of water in the wood de- 

 pends upon the amount of transpiration. It is greatest in the 

 summer, when the transpiration is most rapid. At this time 



the wood cells and vessels are 

 full of air, hence the water 

 must pass not through the cell 

 cavities, but in the cell walls. 

 Trees can be much more 

 readily transplanted in the 

 late autumn and winter than 

 in the summer, because at the 

 former times they are not 

 so full of sap as when the 

 leaves are fully expanded, and 

 when they are transplanted 

 at first the roots do not take 

 up much water. 



3 . Root-pressure. — We 

 may regard both of the move- 

 ments of water of which we 

 have been speaking as due to 

 suction. There is, however, a 

 third movement which is due 

 to root-pressure, and which 

 depends neither upon growth 

 nor evaporation. If the stem 

 of a plant is cut, and the surface is protected from evaporation, 

 water is found to ooze out. This is caused by the pressure 

 of the root. The amoufit can be measured by means of a, 



Fig. 293. 



-Apparatus for measuring root- 

 pressure. 



