FACT
Capillary action
occurs when the
adhesion of water
molecules to the
walls of the vessel is
stronger than the
cohesive forces
between the water
molecules. Have
you ever seen fluid
in a drinking straw
move higher than
the level of the fluid
in the glass? This
happens due to
capillary action.
The narrower the
straw, the greater
the capillary action,
and therefore, the
higher the fluid will
rise in the straw.
FACT
Cohesionrefers to
the intermolecular,
attractive forces that
hold molecules in
solids and liquids
together. Imagine a
drop of water on a
waxy surface like
wax paper. Even if
the drop slides and
rolls around, the
water molecules
will stay together
due to the cohesive
forces.Adhesionis
the ability of a
substance to stick to
an unlike substance.
If you were to take
the same piece of
wax paper and turn
it upside down,
some water droplets
would still adhere to
the paper. This
indicates that there
must be an
attraction between
layer in the leaf to the atmosphere, through the stomata. This results in water being drawn
up the xylem vessel.Root pressure: results from the water potential of the root cells being more negative than
that of the soil so that water moves by osmosis from the soil into the root. This results in root
pressure which pushes water up the xylem.Capillary actionpulls water from the roots to the leaves. Evaporation of water from the
leaves surface causes a negative pressure in the xylem that pulls water from the roots and
soil. The primary forces that create the capillary action areadhesionandcohesion. Adhesion
is the attraction that occurs between water and the surface of the xylem, and cohesion is the
attraction between water molecules.Factors affecting the rate of transpiration DUMMY
There is a close inter-relationship between transpiration and leaf structure. The rate at which
transpiration occurs refers to the amount of water lost by plants over a given time period.
Plants regulate the rate of transpiration by opening and closing of stomata (Figure 6.15).
There are, however, a number of external factors that affect the rate of transpiration, namely:
temperature, light intensity, humidity, and wind.Figure 6.15: The opening and closing of stomata. Different environmental conditions trigger both the
opening and closing of stomata.TemperatureTemperature affects the transpiration rate in two ways. Firstly, at warmer temperatures water
molecules move faster, and the rate of evaporation from stomata is therefore much faster.
Secondly, the water-holding capacity of warm air is greater than that of cold air. Assuming
that cold air and warm air contain the same amount of water, the cold air may be saturated,
and therefore have a shallow water concentration gradient, while the warm air may will
be able to hold more water vapour, and will therefore have a steeper water concentration
gradient.170 6.3. Transpiration