how respond to environmental stimuli can be physical chemical or biological

Any external agent that produces a reaction in the body of an organism in favour of it or away from it is known as stimulus. The reaction produced by the organism against stimulus is called response. 
Stimulus can be:
  1. Physical: Any physical agent that generates response is called physical stimulus.  For example, when we touch hot object by hand, we remove our hand away from it. The hot object is a physical stimulus while as removal of hand represents a response. 
  2. Biological:  Any biological agent that generates response is called biological stimulus. For example, sexual attraction between living organisms leading to reproduction or Immune response generated by our body to the pathogen. 
  3. Chemical: Any chemical agent that generates response is called chemical stimulus. For example,  the growth of pollen tube towards the ovule in the ovary (through the stigma and style) is due to the chemical present in stigma or response shown by taste buds to any food.


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