EOOT-PEESSUEE 83 



quantities of liquid are continually being forced into the 

 axial stele. This rhythm, which is comparatively rapid, 

 must not be confused with another rhythm which is much 

 more gradual, and which constitutes what is called the 

 periodicity of the root-pressure. 



When transpiration is not taking place, the water may 

 accumulate in the vessels, and its presence can then very 

 readily be demonstrated, and the force of the root-pressure 

 measured. If a vine stem is cut through in the early 

 spring before its leaves have unfolded, a continuous escape 

 of water takes place from the cut surface, and the vine is 

 said to bleed. The phenomenon is not peculiar to the vine, 

 but is exhibited by most other terrestrial plants. 



In plants which have a large woody system the accu- 

 mulation of water in the vessels can only be demonstrated 

 while the absence of leaves renders transpiration impossible. 

 Many herbaceous plants show a similar phenomenon daily, 

 owing to the intermission of transpiration during the 

 night. In these cases it is not necessary to cut the axis 

 at all ; the accumulation of water extends to the whole of 

 the plant. In the early morning the plants show a certain 

 exudation of water from the tips or apices of the leaves, 

 drops accumulating on their surfaces. Alchemilla and 

 Tropceolum especially display this phenomenon, which is 

 due to' the over-turgescence of their tissues, brought about 

 by the pumping action of their roots. 



This phenomenon of setting up a hydrostatic pressure 

 causing an exudation of water is not confined to roots. 

 Whenever the active living cells of the stem, or even of 

 the leaves, force water into the vessels, the same exudation 

 can be noticed. It can be shown by burying the cut ends 

 of young stems of grasses in wet sand ; after a time drops 

 of water ooze out of their projecting upper ends. If the 

 leafy branches of some trees are immersed in water so that 

 only the cut ends project, the leaves can absorb water and 

 force it through the stem, so that an exudation after a time 

 can be noticed to take place from the cut surface which is 



G 2 



