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intercellular spaces, cell wall of endodermis) that occur continuously throughout the
plant, except at the casparian strips of endodermis in roots. In this pathway, the
movement of water molecules takes place through intercellular spaces and the walls
of the cells only. The water movement takes place along the gradient” from root
hairs to xylem through the walls of intervening cells without crossing any membrane
or cytoplasm.Thus, it does not provide any barrier to the movement of water, which
occur through mass flow , due to adhesion and cohesion of water molecules.
There is no involvement of osmosis in the apoplast pathway.
Symplast Pathway
This system includes the living part of the plant cells made up of interconnected
protoplasts of neighbouring cells and connected through cytoplasmic strands
extending through plasmodesmata.
The water that enters into the cell sap of root hair as a result of active absorption,
moves into the underlying cortex cells bounded by a continuous selectively
permeable membrane through plasmodesmata.
The movement is relatively slower as water enters the cells through the cell
membrane. The movement is again down the potential gradient. This pathway is
aided by cytoplasmic streaming, which helps in quicker movement across individual
cells. Thus, facilitating transport.
Cytoplasmic Streaming is an autonomous vital movement that occurs in eukaryotic
cells continuously, e.g., around the central vacuole. Also known as cyclosis. It can be
easily observed in the cells of Hydrilla leaf. The movement of chloroplast due to
streaming is easily seen under microscope.
It is generally believed that both apopiast and symplast pathways are operative in
plants but apopiast pathway offers less or no resistance. Thus, water continue to
move through apopiast in the roots.
As the apopiast pathway is bfbcked by the bond of a suberised matrix called the
casparian strip in the inner boundary of the cortex, the endodermix is imperious to
water.
Therefore, the movement of water by and the endodermis occurs by the symplast
pathway, i.e., water from the cell walls enter the cell cytoplasm and moves from one
cell cytoplasm to another cell cytoplasm crossing the cell membrane through
plasmodesmata and finally reaches the xylem elements.
Mycorrhizal Absorption
Instead of using root hairs for absorption many plants use mycorrhiza for water
absorption. Mycorrhiza is a symbiotic association between a fungus and a young
root system of a plant. Fungal hyphae have a very large surface area and extend into
soil for sufficient distance, where a root cannot reach.
They absorb both mineral ions and water from the soil and provide these substances
to the roots. The roots in turn provides sugar and N-containing compound to the
mycorrhizae. The mycorrhizal association is obligate in many cases, e.g., seeds of
Pinus and many orchid cannot ’ germinate properly without the development of
mycorrhizal association.
Water Movement up a Plant (Ascent of sap)
Translocation of water or ascent of sap is usually upwards from the roots toward the
top of the plant via stem, i.e., to the leaves and growing points or apical meristems
and other aerial plant parts. It occurs through the tracheary elements of xylem. Many
theories have been put forward to explain the upward movement of water.