MOVEMENT OF WATER AND DISSOLVED MINERAL SALTS

When water is absorbed by the root hair, it dilutes the contents of the cell sap

vacuole. As a result, the cells of the cortex, which are adjacent to the epidermis,

have less water than the root hair cells, hence hypertonic.

▪

Therefore, water moves from the root hair cells to the cortex cells by osmosis. It

moves the same way into the cells of the endodermis, then into the pericycle and

then into the xylem vessel.

Once in the xylem, the water and the mineral salts dissolved in it move up the xylem

vessel by transpiration pull, capillarity and root pressure.

Figure 4.7: Movement of Water from the root hair to the xylem

Factors / Forces Accounting for Water Movements in Plants

The factors or forces which cause a continuous flow of water in xylem are; Transpiration

Pull, Capillarity and Root pressure.

(i)

Transpiration Pull. This is the biological process by which plants draw water in the

upward direction as a result of transpiration. Transpiration occurs when water

evaporates from the plant through the stomata in the leaves. As the water is lost, the

mesophyll cells draw water from the xylem in the leaf which in turn draws water from

the xylem in the stem. This creates a tension called transpiration pull which draws

water from the roots.

(ii)

Capillarity. Capillarity is the ability of water to flow through narrow tubes. This is

the force which raises the water in narrow tubes or capillaries and xylem vessels

against the force of gravity. 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 force of attraction between similar molecules. Water molecules

are cohesive; thus, they stick to each other.

Adhesion is the force of attraction between different molecules. It causes water

molecules to adhere to the walls of xylem vessels.

(iii) Root pressure. This is the force that pushes water and dissolved mineral salts

upwards from the root. When a plant is cut, fluid oozes out the remaining stem or

stump. This is a proof that root pressure drives the fluid upward through the xylem

vessels.

TRANSPIRATION

Transpiration is the process by which a plant loses water to the atmosphere through

stomata in form of vapour. About 90% of water is lost from plants occurs in this way.

Types of Transpiration

1. Stomatal transpiration: This is a type of transpiration in which water vapour is lost

through the stomatal pores on the leaves. It accounts for approximately 90% of the

water lost by plants.

2.

Cuticular transpiration: This is the type of transpiration in which water is lost

through the cuticles of the leaves. It contributes about 10% of the whole process.

Lenticular transpiration: This is the type of transpiration in which the loss of water

takes place through the lenticels of the stem. Lenticels are pores found on the bark

of stems or roots in woody plant.

3.

Movement of Water Through Leaves

▪

Water flows from the roots to the leaves through the xylem vessels. It enters the spongy

mesophyll by osmosis.

The spongy mesophyll has air spaces where water enters as water vapour. As a result,

the concentration of water vapour in the air spaces becomes higher than the

concentration of water vapour in the air.

▪

This causes water to diffuse into the atmosphere through the stomata.

Figure 4.8: Movement of water through leaves

FACTORS AFFECTING THE RATE OF TRANSPIRATION

The rate of transpiration in plants is affected by Internal or structural factors (plant

features) as well as External (environmental) factors.

Internal or structural Factors (plant features)

(i)

The size of the leaves. Plants with large leaves lose more water than those with

sealer leaves. This is because, a large leaf has more stomata than a small leaf.

An extensive root system. The plants with extensive roos absorb more water and

can therefore lose more water than those with fewer roots.

(ii)

(iii) Leaf Cuticle. A thick cuticle resists water loss by transpiration while a thin cuticle

makes water loss by transpiration easier.

(iv) Number of Stomata. The more the stomata a leaf has, the higher the rate of

transpiration.

(v)

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.

(vi) Size of air spaces. Larger air spaces allow for a faster rate of transpiration because

the leaves can hold more water vapor. Smaller air spaces slow down the rate of

transpiration.

(vii) Sunken Stomata. Sunken stomata occur in pits. They are not exposed to moving air

so they slow down transpiration rate.

(viii) Epidermal hairs. Epidermal hairs trap water on the surface of the leaves, thus

decreasing the rate of water loss.

External (Environmental) Factors

(i)

Temperature: Transpiration rates increases as the temperature rises. Higher

temperatures cause the stomata to open hence increasing rate of water loss to the

atmosphere. Lower temperatures cause the stomata to close hence preventing or

slowing down the transpiration process.

(ii)

Relative Humidity: Humidity is the amount of water vapour in the air or

atmosphere. The higher the humidity of the surrounding air, the lower the

transpiration and vice versa. It is easier for water to evaporate into dry air than into

air saturated with moisture.

(iii) Wind and Air Movements: Increased movement of the air around a plant results

in a higher transpiration rate and vice versa.

(iv) Availability of soil moisture: When soil moisture is low in the soil, plants begin to

senesce (age prematurely) resulting in leaf loss and reduced transpiration.

(v)

Light Intensity: Brighter sunlight increases the rate of photosynthesis in the guard

cells, causing them to become turgid and open the stomata allowing more water

loss to the atmosphere. Also, higher light intensity increases the internal

temperature of the plant hence increases the rate of transpiration and vice versa.

(vi) Atmospheric pressure: When atmospheric pressure is low, for example at high

altitudes, plants lose water more easily.

Significance/Advantages of Transpiration in Plants

(i) It helps to maintain transpiration pull which is important for maintaining a

constant stream of water and mineral salts between the roots and the leaves.

(ii) It enables the loss of excess water from the plant.

(iii) It helps in cooling a plant.

(iv) It enables absorption and conduction of water and mineral salts from the soil to

different parts of the plant.

(v) It helps to balance water levels within the plant through continuous elimination of

water from the plant body.

(vi) It maintains osmosis and keeps the cells rigid.

Disadvantages of Transpiration

(i) It leads to wilting and death of the plant due to excessive transpiration (loss of water).

(ii) It also interferes with the process of photosynthesis, excretion and respiration, all of

which need water.

(iii) It reduces the water content of the soil which may make the land a desert.

(iv) It can reduce growth rate of the plants and so reduce harvesting quantities.

GUTTATION

Guttation is the process through which plants lose water in form of liquid droplets. It does

not take place through the stomata, but through special pores or openings called

hydathodes, which are found on the leaf margin or surface. Guttation occurs mostly at

night or in plants growing in wet areas.

Table 4.2. Differences between guttation and transpiration.

S/N

(i)

Guttation

Transpiration

It occurs early in the morning and at

night.

It occurs during the day.

(ii)

It takes place through hydathodes.

It takes place at low temperatures.

Water is lost in liquid form through

the hydathodes

It takes place through the stomata.

It takes place at high temperatures.

Water is lost in form of vapour via the

stomata.

(iii)

(iv)

(v)

Root pressure plays a vital role in this

process.

Root pressure is not involved in this

process.

(vi)

It takes place on the margin of the leaf.

It takes place mostly in the lower surface

of the leaf.

(vii) It is hindered by dry conditions.

It is favoured by dry conditions.

Excessive transpiration results in wilting

of the plant.

(viii) It never results in wilting of the plant.

(ix)

It

cannot

be

regulated

since

It can be regulated since stomata can

open and close.

hydathodes do not open or close.

EXERCISE

1. What do you understand by the following terms as used in Biology?

(i) Transportation of materials

(ii) Guttation

2. Explain what would happen when a plant cell is immersed in hypertonic solution?

3. Which of the following describes the function of root hairs?

A. Provide anchorage for the root

B. Protect the delicate cells of the enlarging root

C. Add to the root length repeated cell division

D. Provide large surface area for absorption of water

4. Consider a green leaf which has been smeared with Vaseline on the lower surface.

Which of the following groups of physiological processes will be most affected?

A. Transpiration, photosynthesis and respiration

B. Irritability, photosynthesis and digestion

C. Osmosis, assimilation and excretion

D. Secretion, photosynthesis and assimilation

5. The main product of photosynthesis is transported away from the leaves by the

A. Veins

B. Phloem

D. Cortex

C. Xylem

6. The force which is generated as a result of transpiration is called

A. Osmotic pressure

B. Root pressure

C. Transpiration Pull

D. Transpiration stream

7. Some plants such as baobab shed off leaves during dry season in order to;

A. Avoid heavy photosynthesis

B. Combat water loss

C. Expose their body parts for good aeration

D. Be more exposed to sunlight