Plants are less active so their cells do not need all the materials so quickly. Moreover, the plant parts (roots, stem and leaves) can get oxygen and carbon dioxide for respiration and photosynthesis from the air directly by diffusion.
So, only water and minerals must be carried from the soil to the plant body.
Functions of Transport in Plants:
- The supply of water and minerals to the plant which they cannot obtain from the air through diffusion.
- The supply of food prepared in the leaves to various parts of the plant like stems, roots etc.
Conducting Tissues :
The tissues present in plants which help in the transport of materials from one part to the other are known as conductive tissues.
The plants have two types of conducting tissues called xylem and phloem.
- Xylem tissues carry water and minerals.
- Phloem tissues carry the food prepared by plants.Phloem also carries the hormones made by the plants in the root and shoot tips.
- Plants require water for making food by the process of photosynthesis.
- They also require mineral salts to be absorbed from the soil by the roots of the plant and transported to various parts like stem leaves and flowers.
Conducting tissue : Xylem - Water and minerals transport
- The water and minerals move from the roots through two kinds of elements of the xylem tissue : Xylem vessels and xylem tracheids.
Process of transport of water and minerals :
The water containing minerals called xylem sap is carried by the xylem vessels . The roots of a plant have hair called root hairs.
- These root hairs absorb water and minerals from the soil since these are directly in contact with the film of water in between the soil particles.
- Water gets into the root hair by the process of diffusion .
- These minerals and water pass from cell to cell by osmosis through the epidermis, root cortex, endodermis and reach the root xylem.
- The pressure at the top of the xylem vessels is lowered whereas the pressure at the bottom of the xylem vessels remains high.
- Due to this pressure gradient, the water flows up the xylem vessels into the leaves.
- Evaporation of water from leaves of a plant is called transpiration.
- The pressure at the top of the xylem vessels in a plant is reduced due to transpiration.
The continuous evaporation of water or transpiration from the cells of a leaf creates a suction which pulls up water through the xylem vessels.
Tracheids are long, thin, spindle shaped cells with pits in their thick cell walls.
- Tracheids are dead cells with lignified walls but they do not have open ends , so they do not form vessels. Lignin lends rigidity to cell walls.
- Water flows from one tracheid to another through pits.
- All plants have tracheids but they are the only water conducting tissue in non-flowering plants.
- Leaves make food by the process of photosynthesis. This food is in the form of simple sugar (glucose)
- This food needs to be transported to all parts through a tube called phloem.
- Transport of food from the leaves to all other parts of the plant is called translocation . Phloem translocates food made in the leaves.
- Movement of food materials and hormones through phloem depends on the action of living cells called Sieve tubes .
Sieve tubes are long , living cells , without nucleus , that join end-to-end to form the Phloem.
- Sieve tube cells do not have lignin in their walls.
- The end walls are not completely blocked. They have sieve plates on either end which have small holes to let food through .
Process of transport of food:
- The movement of food in phloem takes place by utilizing energy.
- The energy from ATP is utilized to load the food made in the leaves onto the phloem tubes.
- Water enters the sieve tubes through osmosis due to which the pressure rises in the phloem tissue.
- This pressure gradient in the phloem moves the food to all parts of the plant having less pressure in their tissues
- This allows the phloem to transport food according to the needs of the plant.