We all have brains and we all have nervous system which helps to feel the pain. But plants like us do not have nervous system to feel the pain. But the plants can feel if they receive any stimuli externally in the form of wound or in the form of environmental factors.

It has been seen that when a plant reacts to a stimulus then it must also be able to perceive that stimulus. This perceiving reaction is called tone or tonus of reacting protoplast. The plants which respond are due to the presence of this protoplast. Sometimes, if a plant comes in contact or submit its stimulus no response is caused. This response is dependent on the condition of tone which in turn is dependent upon the stimulus which the plant has submitted previously and also on the stage of its development.
After few days it is noted that when a plant has stimulated a number of times it stop giving further response. This is because the plant has said to be passed in another stage called rigour. The plants also does not stimulate because the plant organ have lost tone and they are no longer in a condition to respond to wound.

It has been noted that there is no relationship between stimulus and response. Stimulus is caused mainly to release a certain chain of reactions.
In some other cases the plants organ does not response until some the stimulus has been removed. So, the time interval separates the perception of the stimulus and response.