Describe the movement of water from the soil into the root xylem.
What will be an ideal response?
Answer:
Plants absorb capillary water from the soil through root hairs along the concentration gradient by the process of imbibition and endoosmosis. Root epidermis or epiblema extends to form thin walled unicellular root hairs. When water enters plants, it follows both apoplast and symplast pathways to reach the vascular tissue xylem through the cortex. Water passage from root hairs to xylem through cortex can follow the following pathways :
- Apoplast pathway :
- Water moves from root hair to xylem through the walls of intervening cells.
- Water movement in this pathway do not cross any membrane or cytoplasm.
- The passage is interrupted by the presence of impermeable lignosuberin casparian strips in the walls of endodermis.
- Apolast pathway offers the least resistance to movement of water.
- Non-vacuolar Symplast Pathway :
- Water movement occurs from cell to cell through their protoplasm.
- Vacuoles are not included in this pathway as water does not enter cell vacuoles.
- Plasmodesmata connects the cytoplasm of the adjacent cells.
- Vacuolar Symplast Pathway :
- Water movement occurs from cell to cell through their protoplasm.
- Vacuole is involved in this pathway.
- The pathway is cell wall, plasma-lemma, cytoplasm, tonoplast and central vacuole.
- Vacuolar pathway offers a lot of resistance to the passage of water
- It is followed only when individual cells are to take up water.
Moving through the above mentioned pathways in a partly apoplastic and partly symplastic manner, water reaches the endodermis which separate the vascular tissue from thecortex. The endodermis is present exclusively in the roots. It functions as a checkpoint for materials entering the root’s vascular system. A waxy, impermeable substance called suberin is present on the walls of the endodermal cells. This waxy region is called the casparian strip. Being impermeable to water, the casparian strip forces water to cross the cell membranes (symplast) of endodermal cells instead of slipping between the cells (apoplast). The water now reaches pericycle inner to which lies the vascular tissue (xylem and phloem). Xylem is responsible for conduction of water in plants. Water first reaches protoxylem and then metaxylem. Xylem parenchyma also facilitates the conduction of water. Upward movement of water from root xylem is known as ascent of sap.
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