In plants, special long distance transport is necessary as sites of production or absorption and sites for storage too far and at much faster rate. This is generally done by mass or bulk flow system.

As a result of pressure differences between two points causes movement of substances in bulk or enbulk.

Bulk flow depends upon +ve hydrostatic pressure gradient or –ve hydrostatic pressure gradient +ve in garden hose and –ve in suction through straw. 

Translocation:- Through conducting or vascular tissues of plant, bulk movement of substances takes place. i.e. translocation.

Specialized Vascular Tissues:-
Xylem	Phloem
Translocation of mainly water, mineral, some organic N2 and hormones (From root to aerial parts of plant)	Translocation of variety of organic and inorganic solutes (leaves to other parts of plant)

Water absorption of plants:-

• Water absorbs from soil by root hairs which are present at the tips of root.
• These root hairs increase the area for absorption and absorbs water and minerals by the process of diffusion.
• Then further movement of water in roots takes place by two different methods.

1. Symplast pathway:- It consists of interconnecting protoplasts.

• Plasmodesmata: Neighboring cells are connected through cytoplasmic strands that extend through plasmodesmata.
• Water moves through plasmodesmata (intracellular movement)
• Movement of water is slower as water has to enter into the cell through down a potential gradient.
• Aided by cytoplasmic streaming (e.g. in cells of hydrilla leaves)

2. Apoplast Pathway:- It consist of adjacent cell walls (i.e. continuous throughout the plant except at casparian strips of endodermis in roots).

• Movement of water is through intercellular spaces and walls of cells.
• Water movement is through mass flow and dependent on the gradient.
• Adhesive and cohesive properties of water is responsible for mass flow as water evaporates, tension develop in continuous stream of water in apoplast.

• The cortical cells are loosely packed so no resistance to water movement, so most of the water movement is through apoplast in roots.

 

• Casparian strip presents in the endodermis (the inner boundary of cortex) impervious to water.
• Penetration of water molecules are unable, so they are directed to wall regions that are not suberised into the cells.
• Then, water movements is symplastic and again cross a membrane to reach the cells of xylem.
•  Water and other solutes enters in vascular cylinder in this way only.
• The water is full to move between cells as well as through them, once inside the xylem.
• In young roots, water enters directly into xylem vessels or tracheids. Which are part of apoplast.

• Some associated structure also help in absorption like in mycorrhiza is symbiotic association of fungus with root system.
• The fungal filaments penetrate root also. The hyphal absorb mineral ions and water from soil because of their large surface area.
• The root provide sugar and N-containing compounds to fungus. In return, fungus provides minerals and water to roots.
• In plants like Pinus, seeds can’t germinate without presence of mycorrhizae.

Movement of water up a Plant:

Root Pressure:-

• Water inside root follows and increases pressure inside xylem as various ions from soil are activity transported into vascular tissues of roots. This pressure is called root pressure.
• This pressure is responsible for pushing water up to short distance.

Guttation:- It is the process of water in liquid phase

•  Root pressure helps in pushing water modestly.
• Don’t involve in water movement in tall trees.
• It plays a great role in re-establishing the continuous chains of continuous chains of water molecules in xylem which may break due to transpiration.

Transpiration Pull:-

• In plants, Movement of water upward through xylem can achieve fairly high rate upto 15 meters/hour.
• Water is pulled through the plant with the driving force for this process is transpiration from leaves.
• This is called cohesion-tension-transpiration pull model.

A very small amount of water less than 1% is used in photo synthesis and plant growth.