ROOT-PRESSURE 83 
quantities of liquid are continually being forced into the 
axial stele. This rhythm,” which ig 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 
Tropeolum 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 
