References

40 P.W. Barlow
that a tropism is superimposed whenever a new direction of growth is
induced in response to a suitable stimulus (Darwin 1880). Much later, the
claim was made that the unidirectional movement of root gravitropism
actually suppressed the approximately circular movement of the nutation
(Ney and Pilet 1981). The two types of root movement – tropism and nutation
– might be likened to those animal movements which are responses
to the properties of the somatic and autonomous nervous systems, respectively.
The animal movements due to the latter system are of innate origin:
they are a property of the system itself and are not induced by any external
stimulus. Plant nutations are also innate. They are a consequence of an
innate program of rotating cell divisions in the meristem which is a function
of its cellular structure, whereas, as mentioned, plant tropisms are the
consequences of sensory perception and are quite distinct from nutation.
Moreover, tropisms are always directional with respect to the vector of the
initiating simulus.
There is one site along the root that gathers information for root movements.
It is located at the boundary between the root meristem and the
subjacent zone of rapid cell elongation, and has become known as the
‘transition zone’ (Baluška et al. 1996). The modulation of cellular growth
within the transition zone may be expressed either one-, two-, or threedimensionally.
In the first case, the transition zone can accumulate cells
in linear files and store them in an unexpanded state until some further
stimulus triggers their release and subsequent entry into the zone of rapid
cell elongation. In the second case of a two-dimensional response, opposite
flanks of the root’s transition zone show differential growth. Elongation
growth is either accelerated or delayed in response to the asymmetric signalsreceivedfromtherootcap,someofthesesignalsbeingintheform
of differential amounts of auxin. The resulting asymmetric, or directional,
growth response is registered as a tropism. When the third dimension,
which corresponds to the tangential plane, is brought into play, the differential
growth of root nutation is the result.
Just as in animals where the brain directly affects the motion of the
organism, the cells of the transition zone steer the tropic growth of the
plant root in a direction determined by the degree of asymmetry of the
incoming signal. In this way, root tips not only avoid potentially hostile
regions in the soil but also reach into environments more favourable for
the sustenance of the plant.
It seems that, although the transition zone receives asymmetric signals
which have their origin in environmental asymmetries, the response of this
zone is more akin to that which initiates, in animals, a muscular response
under the direction of the coordinating properties of a brain. The transition
zone may therefore correspond to a boundary between the body of the root
and the efferent portion of the root-brain. Other areas of the root-brain may