(a) Vascular bundles: They are originated from the derivatives of procambium, which itself is derived from the apical meristem. Each vascular bundle is made up of Xylem and Phloem. The cambium may be present or absent. Depending upon this, vascular bundles are of two types.
1. Open Vascular bundle: In these bundles cambium is present between Xylem and phloem. For instance, dicotyledons.
2. Closed vascular bundle: In these bundles cambium is absent in the vascular bundle. For instance, monocotyledons.
(b) Nervous tissue: The cells of nervous tissue are highly specialised for being stimulated and then, transmitting the stimulus very rapidly from one place to another, within the body. The brain, spinal cord and nerve, all are composed of the nervous tissue. The cells of this tissue are called nerve cells or neurons. Each neuron has three parts- the cyton or cell body, dendrites and the axon.
1. Cyton: It is the cell body of a nerve cell, that has a large central nucleus and cytoplasm, from which long thin hair-like parts arise.
2. Dendrite: It is the short branched fibre of neuron, which receives nerve impulses.
3. Axon: It is the single long conducting fibre extending from a neuron, that transmits impulses away from the cell body.
(c) Bacteriophage: Bacteriophages can be found in all types of bacteria. Structurally, bacteriophages are almost similar to other types of viruses. They are also composed of protein and a nucleic acid but differ from other viruses in having bacteria as their host cells. It has a tadpole-like shape, with a hexagonal head and tail. The length of the head is 650 A in diameter. It has a protein coat, in which a double-stranded DNA is tightly packed. The cylindrical tail is made entirely of proteins. It has a core tube. The basal end of the tail has a hexagonal spiked end plate. At each corner of the hexagonal plate, there is a thin tail fibre. The core tube of the tail is surrounded by a contractile sheath. It is attached to the neck by a collar.
(d) Sclerenchyma:
(i) The cells of sclerenchymatous tissue are dead.
(ii) They are long and narrow in appearance.
(iii) Wals are thickened due to lignin, a chemical substance, which acts as cement and hardens them.
(iv) Due to the presence of such strong walls, there is no internal space inside the cell. This type of tissue is present in stems around vascular bundles in the veins of leaves and in the hard covering of seeds and nuts.
Functions:
(i) It is known to be the chief mechanical tissue, which makes plant hard and stiff, e.g. husk of coconut is made up of sclerenchymatous tissue.
(ii) It provides strength and enables them to bear stress.
(iii) It forms a protective covering around seeds and nuts. It gives rigidity, flexibility and elasticity to the plant body.
