88 VEGETABLE PHYSIOLOGY 



over-distension, and we get an intermittent pumping action 

 set up, which has a certain rhythm. By it large quan- 

 tities 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 accu- 

 mulates 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 accumu- 

 lation 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 accumulat- 

 ing on their surfaces. Alchemilla and Tropceolum especially 

 display this phenomenon, which is due to the over-tur- 

 gescence 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 



