(a) Nerve cell or neuron is a functional unit of the nervous system.
Function. The information acquired at the end of the dendritic tip of a neuron sets off a chemical reaction which creates an electrical impulse. This impulse travels from the dendrite to the cell body along the axon at its end. At the end of axon, the electrical impulse sets off the release of some chemicals, which cross the synapse and start a similar electrical impulse in a dendrite of the next neuron. In this way nerve impulses travel in the body, from one neuron to another till it reaches the brain of the target organ. Thus, a nervous tissue is made up of an organised network of nerve cells or neurons which are specialised in conducting information via electrical impulses from one part of the body to another.
(b) Phototropism is the response of the plant parts to the external stimulus of light. The stem of the plant grows in the direction of light while root grows away from the direction of light. This growth is controlled by the auxin hormone of the plant. The concentration of auxin stimulates the cells to grow longer on the side of the shoot (stem and branches) which is away from the light. Thus, the plant appears to bend towards light.
