Capillarity is the action that
causes water to rise in narrow tubes. Xylem vessels have a narrow lumen which
makes it possible for water to rise in them by capillarity.
Capillarity is made possible by cohesion
and adhesion forces. Cohesion is the attraction between like
molecules. It makes the water molecules Stick to each other. Adhesion is
attraction between different molecules. It causes water molecules to adhere to
the xylem vessels.
Root pressure
Root pressure
pushes water and dissolved mineral salts
upwards from the root. This happens because the cells of the endodermis push
mineral It’s into the xylem. This increases osmotic pressure in the xylem
thereby creating a force that moves the water and dissolved minerals up the
xylem vessel. When a plant is cut, fluid oozes out the remaining stem (Stump).
This is proof of root pressure in plant.
TRANSIPIRATION
Transpiration is the process by which plants lose water through the
stomata in the leaves.
Water flows from the roots to the
leaves through the xylem vessels. It enters the spongy mesophyll by osmosis.
The spongy mesophyll has substomatal air spaces where water enters as water
vapor. As a result, the concentration of water vapor in the substomatal air
spaces becomes higher than the concentration of water vapor in the air. This
causes water to diffuse into the atmosphere through the stomata.
Movement of water through leaves
Note: Another process known as guttation also occurs in
plants. It is the process by which plants lose water as droplets through
special glands found where veins are in contact with the leaf margin. Guttation
is different from transpiration in that transpiration is the loss of water
vapor mainly through the plant's stomata. Guttation occurs mostly at night or
in plants growing in wet areas
Types of transpiration
There are three types of
transpiration:
- Stomatal transpiration occurs through the stomata on
the leaves. It accounts for approximately 90% of the water lost by plants.
- Cuticular transpiration happens through the cuticle of
leaves. The cuticle is a waxy layer that covers the surface of leaves. A
thick cuticle prevents excessive loss of water.
- Lenticular transpiration takes place through the lenticels. Lenticels are pores
found on the bark of stems or roots in woody plant
Factors affecting the rate of
transpiration
The rate of transpiration is
affected by plant features as well as environmental factors.
Plant features
Plant features include the
following:
(a) The size of
leaves; a large leaf has more stomata than a small leaf. Therefore,
plants with large leaves lose more water than those with smaller leaves,
(b) An extensive root
system:Plants that have extensive roots absorb more water and can therefore
lose more water than those with few roots.
(c) Leaf
cuticle: A thick cuticle resists water loss by transpiration while a
thin cuticle makes water loss by transpiration easier.
(d) Number of stomata:
The more stomata a leaf have, the faster the rate of transpiration and vice
versa.
(e) Position of
stomata:Stomata on the upper surface of the leaf lose water more
easily than those on the lower surface. If a plant has leaves with more stomata
on the upper surface, the rate of transpiration is faster than in
a plant that has Leaves with more stomata on the lower leaf
surface.
(f) Size of
substomatal air spaces: Larger air spaces allow for a faster rate of
transpiration because the leaves can hold more water vapor. Smaller substomatal
air spaces slow down the rate of transpiration.
(g) Sunken stomata:
Sunken stomata occur in pits. They are not exposed to moving air so they
slow down transpiration rate.
(h) "Epidermal
hairs: Epidermal hairs trap water on the surface of the leaves, thus
preventing water
Environmental factors
(a) Temperature:
Transpiration rates go up as the temperature goes up. Higher temperatures
cause the stomata to open and release water into [the atmosphere. Lower
temperatures cause the stomata to close.
(b) Relative humidity:As
the relative humidity of the surrounding air rises, the transpiration rate
falls. It is easier for water to evaporate into dry air than into air saturated
with moisture.
(c) Wind and air
movement: Increased movement of the air around a plant results in a
higher transpiration rate. As water transpires from a leaf, the water saturates
the air surrounding the leaf. If there is no wind, the air
does not move, thus 11raising the humidity of the air
around the leaf. Wind moves the air causing the more saturated air close to the
leaf to be replaced by drier air.
(d) Availability of
soil moisture:When moisture is lacking in the soil, plants begin to senesce
(age prematurely) resulting in leaf loss and reduced transpiration. Also, less
water is absorbed by the roots when the soil is dry.
(e) Light: Increased
sunlight increases the rate of photosynthesis in the guard cells, causing them
to become turgid and open the stomata. Higher light intensity also increases
the plant's internal temperature and hence
increases the rate of transpiration.
(f) Atmospheric
pressure: When atmospheric pressure is low, for example at high
altitudes, plants lose water more easily. The rate of transpiration is reduced
in areas with high atmospheric pressure.
Significance of transpiration
- It helps to maintain transpirational pull which is
important for maintaining a constant stream of water between the roots and
the leaves.
- Transpiration enables the loss of excess water from the
plant,
- It helps to cool the plant and enables absorption and
distribution of water and mineral salts.
Summary:
- The vascular system in plants is made up of xylem and
phloem tissues.
- Xylem transports water and" mineral salts from the
roots to all parts of the plant.
- Phloem transports manufactured food from the site of
photosynthesis to all parts of the plant.
- The distribution of vascular bundles is different in
roots and stems and in dicotyledonous and monocotyledonous plants.
- Root hairs are extensions of the epidermal cells of the
root. They absorb water and mineral salts from the soil.
- Water is absorbed from the soil by osmosis.
- Mineral salts are absorbed from the soil by active
transport.
- Water and dissolved minerals move up thexylem by
transpiration pull, capillarity and root pressure.
- Transpiration is the process by which plants lose
excess water through their leaves. Transpiration is important because it:
- Helps to maintain the transpirational
stream.
- Enables the loss of excess water.
- Enables absorption and distribution of water and
mineral salts in a plant.
- Helps to cool the plant.
- Transpiration is affected by the features ofthe plant
and environmental factors. The features of the plant include: leaf size,
size of root system, size of leaf cuticle, size of air spaces, number and
position of stomata and whether the stomata are sunken or not, and the
presence of epidermal hairs.
- Environmental factors include the amounts of moisture
in air, temperature, and air movement, availability of soil moisture,
light and atmospheric pressure.
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