Intelligence is a term fraught with dificulties in definition.
In part, the problems arise because of the human slant
placed on the use and meaning of the word. However,
although as a species we are clearly more intelligent than
other animals, it is unlikely that intelligence as a biological
property originated only with Homo sapiens. There should
therefore be aspects of intelligent behaviour in lower
organisms from which our superlative capabilities are but
the latest evolutionary expression.

Stenhouse (1974) examined the evolution of intelligence
in animals and described intelligence as `Adaptively
variable behaviour within the lifetime of the individual’.
The more intelligent the organism, the greater the degree of
individual adaptively variable behaviour. Because this
de®nition was used to describe intelligence in organisms
other than humans, it is a de®nition useful for investigating
the question in plants. Do plants exhibit intelligent
behaviour? The use of the term `vegetable’ to describe
unthinking or brain-dead human beings perhaps indicates
the general attitude.

However, in animal terms, behaviour is equated with
movement, and since plants exhibit little if any form of
movement, plant intelligence on that basis does not exist.
Although some higher plants exhibit rapid movements (e.g.
Mimosa pudica), these are exceptions rather than common-
place. Mimosa captures our attention because it operates on
a time scale similar to our own, and it is the difference in
time scales that frequently makes plants seem unmoving.
The use of time-lapse facilities has indeed indicated that
plants operate on very much slower time scales than our
own, but once observed in this way, movement is quite
clear.

In addition, the majority of multicellular plants, including
macroalgae, are sessile, the result of a decision several
billion years ago to gather energy and reducing potential via
photosynthesis. Since light is freely available, movement
has never been particularly critical to plant survival. Such
movement as has been observed is usually limited to less
complex plants such as blue-green algae. Rejection of that
(photosynthetic) decision by the primordial animal eukar-
yotic cell ensured that movement became critical to ®nd
food and mates. Once animals started to prey upon each
other, the development of highly differentiated sensory
systems and specialized nerve cells to convey information
rapidly between sensory tissues and organs of movement
was an inevitable consequence. The predatoryprey relation-
ship has acted as a positive feedback loop to accelerate
complex development and equally complex organ differen-
tiation in animal evolution (Trewavas, 1986b). Movement
is, however, the expression of intelligence; it is not
intelligence itself. Stenhouse (1974) regarded the early
expressions of intelligence in animals as resulting from
delays in the transfer of information between the sensory
system and the motor tissues acting upon the signals. The
delay enabled assessment of the information and modifica-
tion of information in the light of prior experience, and it
was that assessment that formed the basis of intelligence.
The key difference between plants and animals in the
Stenhouse (1974) definition is in the word `behaviour’.
Silvertown and Gordon (1989) have defined plant behaviour
as the response to internal and external signals. In plant
terms these are familiar growth and development phenom-
ena, such as de-etiolation,  flower induction, wind sway
response, regeneration, induced bud break/germination,
tropic bending, etc. Thus, a simple definition of plant
intelligence can be coined as adaptively variable growth
and development during the lifetime of the individual. To
add significance to this definition, time lapse shows that
virtually all plant movements are indeed the result of growth
and development.