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 Tropwolum 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 
