Plants absorb
water through their roots. Despite the fact that plants have adaptations
– such as waxy cuticles on their leaves – 95% of this water
will be lost by evaporation, mainly from the stomata. Some water is also
lost via evaporation through the cuticle and through pores in the stem
known as lenticels. Wherever it occurs, the evaporation of water from
the plant is called transpiration.
How is water transported up stems?
There are 3 hypotheses put forward to explain how water is transported
up stems.
The most important of these
is the cohesion tension hypothesis.
Water evaporates from spongy mesophyll cells into the air spaces of
the leaf. Water then diffuses out of the leaf via the stomata. The loss
of water means that the water potential of these cells decreases. Since
water always moves from a region of high water potential to a region
of low water potential, water now moves from into them from the adajacent
cells. This causes the water potential of these to decrease, and so
on, all the way back to the xylem. The loss of water from the xylem
causes a negative pressure or tension which lifts water up the xylem.
Within the xylem the columns of water are held together by cohesion
(the molecules are hydrogen bonded to each other) and by adhesion (the
attraction between a water molecule and the side of the xylem). Movement
of this column of water (often hundreds of columns because there are
hundreds of xylem vessels in the xylem of a tree) is known as the transpiration
stream.
Capillarity. This
results from the adhesion of water to the sides of the microscopic xylem
vessels and tracheids. The maximum upward movement of water by such
capillarity in vessels is about 1 metre.
Root pressure. This
can be summarised as follows:
1. Minerals are actively absorbed at night and pumped into the xylem
tissue.
2. Water potential of the xylem cells decreases.
3. Water, therefore, moves in by osmosis.
4. This increases pressure within the system since the stomata are closed.
5. Water is forced out of the leaves by guttation. Root pressures of
this kind can push water several centimetres up a plant.