Neurons can be divided into five categories. namely-
1. Apolar: A non-polar neuron has no protracted segment. Apolar neurons are located in the outer layer of the cerebral hemisphere and in the intermediate nuclear layer of the retina.
2. Unipolar: A unipolar neuron has an elongated segment. It is a sensor. The peripheral nervous system of vertebrates consists of unipolar neurons.
3. Bipolar: Bipolar neurons have two elongated parts. One is the axon and the other is the dendrite. These later transform into unipolar or multipolar neurons. In the human embryo, there are bipolar neurons in the retina, nose and cochlea.
4. Multipolar: Multipolar neurons have many elongated parts. The mammalian brain and spinal cord contain multipolar neurons.
5. Pseudopolar: Pseudopolar neuron has two pronged parts first. Later merge into one. It arises from bipolar neurons. Spinal ganglia and carotic ganglia contain pseudopolar neurons.

The structural, functional and basic unit of nervous system is called neuron. It arises from the neuroplast of the ectoderm. It is accompanied by a special banana called neurogia.

Response to any kind of change inside and outside the organism is called stimulus. In organisms, receptors receive the stimulus, neurons transmit it, and effectors express the response.
The coordination between different cells, organs, organs and systems of the body by the nervous system is called neural coordination. Electrical impulses are transmitted from one cell to another through neurons. Electrical stimulation is then converted into chemical stimulation by neurotransmitters and causes action.

1. Cuticle present on stem.
2. The chondroma is multicellular.
3. The epidermis is composed of collenchyma cells.
4. The cortex can be divided into extracortical and intracortical regions.
5. Pericycles are multilayered.
6. Its vascular bundles are connected, collateral and free.
7. Its metaxylem is towards the periphery and protoxylem towards the center i.e. Endarch.
8. Vascular bundles are located circularly or circularly.
9. Xylem is elongated.
10. Contains marrow and medullary rays.

A thin cross-section of the stem of a dicotyledonous plant shows the following.
1. Epidermis: Epidermis is single layered and composed of parenchyma cells. Cells are densely packed and cysts are absent. It has multicellular chondroma. Cuticles are present outside the skin.
2. Hypodermis: Below the epidermis lies the hypodermis. It is 5-7 layered and composed of collenchyma cells. It has no budget. Some cells contain chloroplasts.
3. Cortex: It is 2/3 layers below the hypodermis. It is composed of parenchyma cells containing chloroplasts. Cells are large and round with intercellular spaces. It stores food.
4. Endodermis: The inner layer of the cortex is called endodermis. It is composed of single-layered and tubular parenchyma cells. The cells are densely packed and without gaps. It is called starch sheath because there are many white grains deposited in the endodermis.
5. Stele: The portion extending from the bottom to the center of the endothelium is known as the styli. Different regions of Stele are-
(i) Pericycle: Pericycle is 3-4 layered and composed of parenchyma or sclerenchyma cells. At the head of the vascular bundle is a cap-like ring made up of sclerenchyma cells. It is called Bundle cap. Pericycle originates from the connective tissue of the head.
ii) Basal tissue: Basal tissue is composed of multi-layered parenchyma cells below the pericycle. The cells are large and the walls are very thin. Vascular bundles are located within this layer.
(iii) Vascular bundles: Vascular bundles are numerous and arranged in two rows in a ring shape. It is connected, parallel and free. Its components are:
Xylem: Metaxylem towards the periphery and protoxylem towards the center i.e. Endarc. Xylem is located between exocambium and endocambium. It transports water. The xylem breaks down to form lysigenous cavities.
Phloem: Phloem is located on both sides of xylem. The phloem outside the xylem is called exophloem and the phloem inside is called endophloem. It transports food.
Cambium: Cambium is located on both sides of xylem. The cambium outside the xylem is called exocambium and the cambium inside is called endocambium. It creates new cells.

1. Cuticle present on stem.
2. Condrom absent.
3. Epidermis is composed of squamous cells.
4. The cortex cannot be divided into extracortex and intracortex.
5. Vascular bundles are connected, contiguous and closed.
6. Its metaxylem is towards the periphery and protoxylem towards the center i.e. endarch.
7. Vascular bundles are scattered.
8. Xylem is Y or V shaped.
9. There is no marrow or medulla.

A thin cross-section of the stem of a monocot plant shows the following.
1. Epidermis: The epidermis is single-layered and composed of parenchyma cells. It lacks chondroma and sacrum. Cuticle present.
2. Hypodermis: Below the epidermis lies the hypodermis. It is multi-layered and composed of squamous cells. It has no budget.
3. Ground tissue: Ground tissue extends from the bottom of the hypoderm to the center. It is multilayered and composed of parenchyma cells. Cells are round and have intercellular spaces.
4. Vascular bundles: Vascular bundles are numerous and scattered. It is connected, adjacent and closed. It does not contain cambium. Vascular bundles are surrounded by a sheath of sclerenchyma cells. It is called bundle sheath. Its components are:
(i) Xylem : Metaxylem towards the periphery and protoxylem towards the center i.e. endarch. Xylem looks like the letter Y or V. The xylem breaks down to form lysigenous cavities.
(ii) Phloem: Phloem is located between the two arms of Y-shaped xylem tissue. It lacks phloem parenchyma.

1. Cuticle is absent at root.
2. Its rhizome is unicellular.
3. It has no hypodermis.
4. The cortex is not organized into different layers.
5. The endothelium is composed of single cells and the cells contain Caspian ribbons.
6. The pericycle is composed of single cells.
7. Vascular bundles are anterior.
8. Protoxylem towards the periphery and metaxylem towards the center i.e. exarch.
9. Its xylem or phloem bundles number 2-6.
10. Marrow is very small.

A thin cross-section of the root of a dicotyledonous plant shows two zones. Extracellular region and Intracellular region.
1. Extra-steel region
The region outside the steel is called extra-steel region. Its various parts are-
(i) Epiblema or Root: The outermost layer of the root is called epiblema or root. The cortex is single-layered. Its wall is thin. It has unicellular rhizomes. It absorbs water and mineral salts.
(ii) Cortex : Cortex is multilayered and composed of parenchyma cells. Cells are round or ovoid with intercellular spaces. It is parenchyma like cells and the wall is very thin.
(iii) Endodermis: It is composed of single layered and tubular cells. The cells are densely packed and without gaps. Casparian ribbons are present in the walls of the cells. It has few passage cells. This layer stores food, protects the stele region, and allows water and solutes to enter through the passage cells.
2. Intrastellar region
The region from pericycle to medulla is called stellie or intrastellar zone. Its various parts are-
(i) Pericycle or pericycle: Below the endothelium there is a layer of pericycle. It is composed of parenchyma cells and the cell wall is very thin.
(ii) Vascular bundles: Xylem or phloem bundles are four in number. They are arranged radially and circularly. Protoxylem towards the periphery and metaxylem towards the center i.e. exarch. It transports water and food.
(iii) Medulla: The part surrounded by vascular bundles situated in the center of the root is called Medulla. It is composed of parenchyma cells. The cells are spherical, thin-walled, and capsuled.
(iv) Medullary Rays: The portion from medulla to pericycle through vascular bundles is called medullary rays. It is composed of narrow and elongated parenchyma cells. It stores food.

1. Cuticle is absent at root.
2. Its rhizome is unicellular.
3. It does not have a hypodermis.
4. In this the cortex is not organized into different layers.
5. Pericycle is monolayer.
6. Its vascular bundles are anterior and alternately arranged.
7. Protoxylem towards the periphery and metaxylem towards the center i.e. exarch.
8. Its xylem or phloem bundles are more than 6 in number.
9. The marrow is quite large and conspicuous.