Transportation in plants:

Plants have a special capability that they make their own food so they are called autotrophs. Food is mainly prepared in leaves of plants. In making food they require essential water and minerals from the earth. Food prepared in leaves is to be supplied to different parts of plants to make it alive.

The first question that arises in your mind is how the supply of water, minerals and food is carried out in plants?

Definition of transportation in plants:

• The process involved in supply of such raw as well as synthesized substances from one part of plant to other part is called transportation in plants.
• Root, stem and leaves are the main constituents of plants.
• Roots absorb water and minerals from soil. Stem is supporting as well as transporting part of plant. Leaves mainly produces the food.

Types of conducting tissues in plants:

There are two conducting tissues present in plants which carries the process of transportation:

1. Xylem
2. Phloem

A. Xylem:

• Xylem begins with roots and is present in all parts of plants.
• It has main role of conducting water from roots to leaves.
• It also provides strength to stem of the plant.
• Xylem has tracheids and vessels.
• Xylem is present at the centre of plants body.

Mechanism of transportation in plants through xylem:

• Through osmosis water present in the soil enters in roots.
• Minerals present in the soil enters the root through a process called active transport.
• Now, both water and minerals present in the roots move upwards by osmosis.
• The cortex present in roots help in osmosis of water and minerals from one cell to other.
• From cortex, these water and minerals are moves to xylem.
• The water along with dissolved minerals solution is sap.
• The sap as it moves upward from root cells to xylem, this upward movement of the sap is called ascent of sap.
• The xylem vessels present in roots continues to the vessels (xylem) present in stem.

Transpiration:

• The loss of water from different parts (mainly leaves) of the plants in the form of water vapour is called transpiration.
• Transpiration takes place through stomata. Stomata are present on the underside of a leaf to reduce excess water loss, and guard cells surround them by open and close the pores.
• The concentration of water inside the mesophyll cells lowers after water loss through evaporation.
• To fulfill the water needs in mesophyll cells, water enters in it through osmosis from neighbouring xylem vessels.
• Evaporation of water from leaves creates a suction force which helps to draw more water from stem cells. Continuously it also forces root cells to draw more water from the soil.
• This phenomenon explains how water moves up in the plants, against gravity, without the use of any pump!
• Thus, if there is higher rate of transpiration there will be higher rate of absorption of water and minerals from soil.
• This process also helps plants to maintain the temperature of the body of plants.

B. Phloem:

• Phloem has main role to transport manufactured food from the leaves to different parts of the plant.
• One of the special characteristics of phloem tissue is the food present in the phloem can move in the upward and downward directions i.e. the flow is bidirectional.
• Food movement in the phloem occurs due to the pressure flow mechanism.
• Osmotic pressure differences help in the movement of food from the area of high concentration to areas of low concentration.
• Phloem has mainly sieve tube cells and companion cells.
• Phloem contains living cells called Sieve tubes. They contain cytoplasm but no nucleus.
• Sieve plates are form by the end walls of the cells.
• The small pores in sieve plates allow food to pass through the phloem.
• Each sieve tube continues to companion cell next to it.

Mechanism of transportation in plants through phloem:

• Food synthesized during photosynthesis from leaves is loaded into sieve tubes by utilizing energy in the form of ATP.
• The presence of food inside the phloem develops the concentration gradient for water. Thus, water enters the phloem by osmosis.
• Osmosis develops high pressure inside the phloem which transports the food from the phloem to plant parts where the concentration of food is less.
• This process of transportation of food from the area of higher concentration to lower concentration is Translocation.
• In spring, the sugar present in the root or stem tissues is transported to the buds.