THE ROOT 183 
Absorption of water. The cell wall of the root hair is lined with 
a thin layer of protoplasm, within which is a large vacuole that 
has sugar and other substances dissolved in it. The protoplasm 
acts as a semipermeable membrane around the vacuole, while 
the plasma membrane which bounds the cytoplasm is a semi- 
permeable membrane around the remainder of the protoplasm. 
The osmotic pressure in the root hair is greater than that of 
the surrounding soil water, and so water is drawn from the 
soil into the root hair. Osmotic pressure is also instrumental in 
causing the movement of water from cell to cell in the cortex of 
the root. 
Passage of water through the cortex. When water passes into 
a root hair, it goes from a solution with a low osmotic pressure to 
one witha high osmotic pressure; when it passes from the paren- 
chyma cells to the vessels, the reverse is the case; but the water 
is frequently forced into the vessels under considerable pressure. 
This is clearly shown in the bleeding, or exudation of water, 
from the stems of many plants after the upper part of the plant 
has been removed. The bleeding seems to be due to great tur- 
gidity and high turgor pressure in the parenchyma cells. When 
pressure within the cells becomes sufficiently great, water and 
other substances to which the protoplasm is permeable appear 
to be forced out of the cells under pressure; but this process is 
not fully understood. 
Turgor pressure. The pressure of bleeding is often designated 
as root pressure because when plants are turgid the roots usu- 
ally produce such a pressure. Turgor pressure is a more appro- 
priate name, however, as pressure of a similar nature may be 
produced in other parts of plants. 
Turgor pressure of roots cannot be considered as an explana- 
tion of the movement of water up the stem, as such pressure can 
occur only when the parenchyma cells of the root are turgid, 
and this is not usually the case when transpiration is active and 
the movement of water most rapid. At such times there is little 
or no turgor pressure, and water may be absorbed by the stump 
of a decapitated stem instead of being exuded from it. 
