Introduction

All the living organisms show the irritability or sensitiveness. It is the property to give the response to stimulus. The stimulus can be external or internal. The living organisms adapt themselves to the external and internal factors with proper adjustment. This adjustment of the vital activities of life is called co-ordination.

The working of one system is co-ordinated with those of other systems. During eating our body performs several kinds of co-ordinated activities. The eyes help in translocation of food, the nose differentiates the smell of food and hands serve as the organs of ingestion of food. Thus various organs perform co-ordinated activities.

Living organism repond and react to their environment. The response to environmental factors (stimuli), such as light, heat, cold and pressure, varies in different organisms.

Plants bend towards light. Amoebae move towards food, and tend to aggregate in moderately warm water. Amoeba and other protozoa avoid mechanical obstacles. Unlike animals, most plants do not have any special structures for perception of external stimuli.

Multicellular animals, except sponges, have specialised cells called neurons, for responding to stimuli. A neuron or nerve cell is the structural and functional unit of the nervous system. Nervous system, which includes the brain, spinal cord and nerves, receives information from the surroundings, processess and interprets it, and responds accordingly.

The nervous system also passes information from one internal system to another. For example, smell or taste of food causes salivation. Nervous system, therefore, controls and coordinates various functions in the body. The other organ system for control and coordination of various life processes is the endocrine or hormonal system.Plants are devoid of any nervous system or muscular system to coordinate and control their responses.

Growth in plants is generally of two types it can either be growth dependent or growth independent. Germination of seeds is a growth dependent movement whereas sensitivity of a touch me not plant is growth independent movement.

Plants lack nervous system and sense organs as animals have them e.g. eye, ear, etc. Plants react to the environmental changes by using hormones called Phytohormones