References

3 Charles Darwin and the ‘Root Brain’ 43
a response. In roots, it seems that this response is effected by motor-type
cells within the inner cortex portion of the transition zone, and that these
are especially important for ‘driving’ the movements of tropism and root
swelling (Baluška et al. 1993).
The action of animal nerve synapses is facilitated by electrical impulses,
and it is these which result in rapid muscle responses. In plants, electrical
impulses are also recorded in response to a variety of stimuli, including
reorientation within a gravity field (Monshausen et al. 1996). However, it
is unclear at present how these impulses relate to the much slower growth
responsesgovernedbyauxinmovementacrosstheplantsynapses.Itmaybe
that such electrical signals play a different role – for example, in facilitating
increasingly rapid responses to repeated stimuli. If so, this would suggest
arolefortheseimpulsesinsimplelearningandmemoryprocessesinplants
(Thellier et al. 2000).
3.4
The Anterior Root-Brain
That auxin transport is largely directed away from the shoot apices and the
young leaves, which are the sources of auxin, and is instead directed towards
the roots stimulates new thinking about plant morphology. In particular,
it is necessary to engage with Darwin’s idea of the root being an anterior
structure (Darwin 1880, p. 572). There are two common perceptions that
might be regarded as reasons for assigning as ‘anterior’ the head-end of
an organism (as in an animal organism). The first is that the property of
anteriority is generally associated with an organism’s forward movement.
The second concerns the presence of a brain. These two propositions also
apply to plants. First, the anteriority of the root tips accords with them
being the location where rapid forward growth occurs. Generally, at any
given temperature, the rates of cell division and cell elongation in root tips
are faster than those in the shoot tip by an order of magnitude. Secondly,
theroottip,asarguedbefore,isapparentlythesiteoftheroot-brain.Athird
criterion of anteriority could be related to the direction of afferent nervous
impulses. In animals, afferent nervous activity directs impulses away from
what are often peripheral sites of sensory perception, leading them towards
the central nervous system and thence to the brain (Tortora and Grabowski
1996). Efferent nervous activity then transmits impulses away from the
central nervous system to regions of response, such as the muscles. The
major direction of flow of the plant neurotransmitter, auxin, is towards
the root tip, where it is then redirected basipetally out of the root cap and
back along the tip towards the muscular transition zone (Blancaflor and
Masson 2003). The acropetal, tipwards flow of auxin is analogous to the