Transport in plants - Principles of Horticulture - page 121

LEAF

Epidermis

Spongy mesophyll cell

Water passes between cells
through cell walls

Water evaporates into
air space of leaf

Water passes through cells
by osmosis

Water vapour

Water vapour passes out of
leaf (transpiration)

Stoma

Water pulled up xylem to replace
water lost from leaf (transpiration pull)

STEM

Xylem vessel Pericycle

Endodermis

Epidermis

Root cortex

Soil particle

Root hair

ROOT

Casparian strip prevents
water passing between cells

Figure 10.3 Pathway of water movement from the soil through the plant to the atmosphere

its individual cells stay upright like a stack of inflated
balloons. Turgor pressure is also the way in which new
cells enlarge, contributing to growth by causing the cell
to swell until the cell wall prevents further expansion.
Without the cell wall the cell would explode!
The pathway of water movement through the plant
falls into three distinct stages:
X water uptake from the soil by the roots
X movement up the stem in the xylem
X movement across the leaves and loss to the air by
transpiration.

osmosis . Whereas the cell wall is permeable to both
soil water and its dissolved inorganic minerals, the cell
membrane freely allows water through but is selective
about passage of other dissolved molecules, somewhat
like a sieve. Sucrose, for example, is too large to cross
the membrane. A greater concentration of dissolved
substances such as mineral and sugars (solutes) is
usually maintained inside the cell compared with the
soil water outside the cell. The water concentration
outside the cell will therefore be greater than inside
the cell so water will move in by osmosis (Figure 10.2).
The greater the difference in concentrations of
water, the faster water moves into the root cells.
If water is not available at the roots (see permanent
wilting point p. 154) or if the plant is losing water
from the leaves faster than it can be replaced, water

10

Water uptake from the soil

Soil water enters the root passing through the cell
walls then across cell membranes and into cells by