Plants are in a continuous state of sleep. A person who does not judge accurately in these things can easily fall into the error of ascribing a kind of consciousness also to plants that is similar to that of animals and men in their waking state. That, however, can happen only if he has an unclear idea of the nature of consciousness. It is then stated that if an external stimulus is applied to the plant it makes certain movements like the animal. One speaks of the “sensitivity” of some plants that, for example, contract their leaves if certain outer stimuli act upon them.
Yet it is not the characteristic of consciousness that a being reacts to certain stimuli, but that the being experiences something in its inner nature that adds something new to the mere reaction. Otherwise, one could also speak of consciousness when a piece of iron expands under the influence of heat. Consciousness is present only when, through the effect of heat, the being, for example, inwardly experiences pain.
Source: Rudolf Steiner – GA 13 – An Outline of Occult Science: II: THE ESSENTIAL NATURE OF MANKIND
Translated by Maud and Henry B. Monges and revised for this edition by Lisa D. Monges