Plants respond too
Irritability is not confined to animals. Plants respond to stimuli too.
One stimulus, opposite responses
Some insects fly towards the dark; some fly towards the light. The stimulus is the same light — but the response differs with the organism. So the response is decided not by the stimulus alone but by whoever receives it.
Mimosa — sensitive to touch
The leaves of the mimosa plant fold when touched. Those leaves are touch-sensitive.
Thora, tamarind, kathurumurunga — sensitive to light
The leaves of these plants fold up as night comes on. Those leaves are light-sensitive.
A plant has no nerves
This is the interesting part: a plant has neither a nervous system nor muscles. Yet it responds. So nerves are not essential for a response — irritability is a characteristic of living things, not a characteristic of a nervous system.
There is another way plants respond — through the direction in which they grow. That comes up in section 8.6.