Vascular bundle
It is a group of specialized plant tissues responsible for transporting water, minerals, and other materials throughout the plant. It is made up of two main components: xylem and phloem.
Xylem transports water and minerals from roots to rest of plant, made of dead lignified cells.
Phloem carries products of photosynthesis, such as sugars, from leaves to other parts of plant, made of living cells.
Transverse sections of stems and roots of herbaceous dicotyledonous plants:
Xylem and phloem are arranged in a circular pattern around the vascular cylinder.
Xylem is located towards the center of the stem, surrounded by phloem.
Transverse sections of leaves of herbaceous dicotyledonous plants:
Xylem and phloem are arranged in discrete vascular bundles, located along the midrib and major veins of the leaf.
The bundles run parallel to each other, with the xylem towards the upper surface of the leaf and phloem towards the lower surface.
Xylem Vessel
Long, cylindrical cells with thickened walls that are arranged end-to-end to form tubes.
The thickened walls and lignified structure of the vessel elements allow for the transport of water and minerals against gravity.
The absence of protoplasm in the cells and large diameter of the tubes also allows for efficient water transport.
There are tiny openings in the walls between adjacent xylem cells (vessel elements) called pits.
The pits serve as conduits for water flow and allow for the movement of water from one vessel element to the next, with minimal resistance.
Lignin, a complex durable polymer that is synthesized by cells in the xylem are deposited in the cell walls.
Lignin strengthens the walls of the xylem cells and provides structural support to the plant.
The presence of lignin makes the xylem cells resistant to crushing and allows them to maintain their shape, even when water is present.
Hydrogen bonding between lignin and water causes cohesion.
Phloem Sieve Tube Elements
They are long, narrow cells with perforated walls that are arranged end-to-end to form tubes.
The perforated walls allow for easy transport of dissolved sugars and other photosynthetic products.
The cytoplasm and nucleus are still present in the sieve tube elements, which allows them to continue metabolic activities.
Companion Cells:
Small, specialized cells that are associated with sieve tube elements.
Provide metabolic support to the sieve tube elements, which lack a nucleus and other organelles necessary for metabolic processes.
The companion cells also regulate the transport of materials in the phloem.
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