LONG DISTANCE TRANSPORT OF WATER
$\displaystyle \small \bullet$ Diffusion is a slow process. It accounts for only short distance movement.
$\displaystyle \small \bullet$ Long distance transport systems are necessary to move substances faster across long distances in plants.
$\displaystyle \small \bullet$ Movement of substances in bulk (en masse) from one point to another due to pressure differences between two points is called Mass (bulk) flow.
$\displaystyle \small \bullet$ Bulk flow can be achieved either through a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient.

Absorption of Water by Plants
$\displaystyle \small \bullet$ Absorption of water and minerals occurs by diffusion through millions of root hairs present at the root tips.  
$\displaystyle \small \bullet$ Root hairs increase the surface area for absorption.
$\displaystyle \small \bullet$ The absorbed water is moved deeper into root layers by 2 pathways:
Apoplast pathway and Symplast pathway  

Apoplast Pathway 
 $\displaystyle \small \circ$ Movement occurs through the intercellular spaces or walls of the cell, without entering the cytoplasm.
 $\displaystyle \small \circ$ The apoplast does not provide any barrier to water movement and water movement is through mass flow.  
 $\displaystyle \small \circ$ Apoplast movement is fast.
 $\displaystyle \small \circ$ Movement of water occurs via apoplast except at the casparian strip.
Symplast Pathway
 $\displaystyle \small \circ$ Water enters the cell through cell membrane and travels intercellularly through plasmodesmata.
 $\displaystyle \small \circ$ Water movement is slow via symplast.
 $\displaystyle \small \circ$ Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed. So water can move without resistance.
 $\displaystyle \small \circ$ The water movement through the root layers is ultimately symplastic in the endodermis. This is the only way water and solutes can enter the vascular cylinder.
 $\displaystyle \small \circ$ Some plants have additional structures associated with them that help in water and mineral absorption called mycorrhiza.
 $\displaystyle \small \circ$ Mycorrhiza is a symbiotic association of a fungus with a root system.
 $\displaystyle \small \circ$ The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
 $\displaystyle \small \circ$ Eg: Pinus seeds cannot germinate and establish without mycorrhizae

WATER MOVEMENT UP A PLANT
Root pressure
$\displaystyle \small \bullet$ Root pressure theory was forwarded by Priestley in 1916.
$\displaystyle \small \bullet$ Root pressure is positive pressure that develops in the xylem sap of the root of plants.
$\displaystyle \small \bullet$ It can be responsible for pushing up water to small heights in plants.
$\displaystyle \small \bullet$ Root pressure re-establishes the continuous chains of water molecules in the xylem which often break under the tensions created by transpiration.
$\displaystyle \small \bullet$ Loss of water in liquid phase by herbaceous plants from the tips of leaf blades is known as guttation.

Transpiration pull
$\displaystyle \small \bullet$ Water is mainly ‘pulled’ through the plant and that the driving force for this process is transpiration. This is known as cohesion-tension-transpiration pull model of water transport.  
$\displaystyle \small \bullet$ Cohesion Tension theory was put forwarded by Dixon and Joly in 1894.