The Transport System in Plants

 

Wilting

1. Non-woody plants depend on the water stored in the cells of the stem for support.

 

2. Wilting occurs in non-woody plants when water loss through the aerial parts of the plant exceeds water absorption by the roots.

 

3. Cells in the plants lose their turgidity (stiffness) and the plant droops.

 

4. Wilting can be important to the plants because the leaf surfaces are removed from the direct rays of the Sun and the stomata closed. Water loss from the plants will be reduced.

 

5. Normally, a plant which has wilted will remain in this condition until evening.

 

6. During the evening, water absorption exceeds water loss and the cells again become turgid.

 

Transpiration

1. Transpiration is the evaporation of water from the aerial parts of plants.

 

2. 90% of the water absorbed by the roots is lost by evaporation from the surfaces of cells in the leaves and subsequent diffusion of water vapour through the stomata. 9% is lost through the cuticle. The remaining 1% is used for photosynthesis.

 

Stomata

1. Stomata are pores in the epidermis of the leaves and stems.

 

2. A pair of bean-shaped guard cells bound each stoma.

 

3. In most dicotyledons, stoma occur only in the lower epidermis of the leaf.

 

4. In monocotyledons, stomata are found on both sides of the leaf.

 

5. Most stomata open during the day and close at night.

 

6. The main function of the stomata is to allow gases to diffuse in and out of the leaf. During photosynthesis, carbon dioxide diffuses from the atmosphere into the leaf and oxygen diffuses out from the leaf into the atmosphere.

 

7. When the stomata open, water vapour is released to the surroundings through the stomata by transpiration. However, the stomata will close when transpiration exceeds water absorption.

 

Factors Affecting The Rate Of Transpiration

1. Light 

Light stimulates the opening of stomata and consequently increases transpiration.

 

2. Temperature

Water molecules move faster in warm air, hence the rate of transpiration is increased.

 

3. Relative humidity

The rate of water loss depends on the difference in concentration of water molecules in the leaf and in the atmosphere. High relative humidity reduces water loss.

 

4. Wind speed

An increase in wind speed increases the rate of transpiration because the movement of air carries away the water vapour from the stomata.

 

5. Water availability

Short supply of water causes the plantto wilt and the stomata to close. This reduces the rate of transpiration.

 

6. Altitude

In the mountains, the atmospheric pressure decreases sufficiently to cause an increase in the rate of transpiration.

 

The Roles Of Transpiration

1. The pulling force developed by transpiration provides the pathway through which water and minerals are transported in the plant.

 

2. Evaporation from the leaf surface has a cooling effect which helps to prevent the heat to direct sunlight from damaging the delicate cells.

 

Vascular Tissues Of A Plant

1. The vascular tissues are concerned with transport and are functionally equivalent to the circulatory system of mammals.

 

2. The two types of vascular tissues are called xylem and phloem.

 

3. Xylem

    (a) The main function of the xylem is to transport water and mineral
         salts from the roots to the stem and leaves
.

    (b) Xylem forms wood in shrubs and trees. Therefore,
         xylem provides support to the plants.

    (c) Xylem consists mainly of vessels, which are elongated
         tubes with thick walls. The walls are strengthened with
         a substance called lignin. Xylem vessels are dead
         structure without protoplasmic contents and transverse
         cells.

 

4. Phloem

    (a) Phloem transports synthesised food substances from one part of 
         the plant to another
.

    (b) Phloem consists mainly of sieve tubes. Sieve
         tubes are living cells without nucleus. The
         walls of sieve tubes are composed largely
         of cellulose.
         The transverse walls of sieve tubes are
         perforated by pores and are called sieve
         plates
.

    (c) Each sieve tube cell has a companion cell
         beside it.