Vascular tissues and Vascular Bundles
The xylem and phloem are altogether known as vascular bundles or tissues. The xylem and phloem act as the blood vessels found in the animals and humans. Except these are plants and the vascular tissues do not have any blood cell in it or plasma or platelets.
Xylem is a dead vessel which provides minerals and water to the green parts of the plant especially leaf. The xylem has lignin deposits in it which strengthens it. The lignin present in it is of 3 types. It is either present as rings, spiral or is covered completely with lignin with pits in it. The pits are for entry and exit of the materials. The innards of a xylem is hollow.
Phloem is long vessel which provides food molecules to the growing regions of the plant where energy is utilized. The phloem has a hollow tube but it has sieve tubes and sieve plates inside it. The phloem has a degenerate protoplasm in it but has granular cytoplasm which helps to sieve the coming food materials. There is a long cell present beside the phloem called the companion cell which helps it to transport the material and keep it alive.
Structure of stem
The xylem and phloem are structured together with a solution cambium between them. This forms the vascular bundles which then forms a ring around the structure called the pith. The layer of the stem has epidermal cells with cuticle on top. The structure found between the epidermal layer and the vascular bundles is called the cortex.
Structure of root
Root has epidermal cells forming its layer. The epidermal layer has tubular outgrowths which are called root hair cells. This layer is called piliferous layer. The vascular bundles are arranged the same way around the root as in the stem. No cuticle is present on the epidermal cells here as transpiration is impossible here.
How water is absorbed?
Water and minerals like salt, calcium etc. are present in the soil. These enter the roots through osmosis according to the diffusion gradient. When the concentration of water is high in the soil and low in the root diffusion occurs. However, active transport is the most common tool for sucking in the minerals from the soil. Do you remember what is active transport? If not click here.
How is water pulled up in the vessels against gravity?
Water and minerals travel from the root to the stem and then on to the leaf by xylem vessels. These travel by means of osmosis and active transport. However in the stem the process of pulling up the water is difficult. It is pulled up by transpiration. When water is evaporated from the plants the water vapor in the gaps of the mesophyll cells leave the leaf through the stomata. This creates a suction action which pulls up the water.
Excess transpiration results in loss of too much water making the cells of a plant flaccid. This is why the plant wilts.