References

38 P.W. Barlow
ThestimulipresentedtoroottipsbyCharlesDarwinandhisson,andto
whichtherootsresponded,includedgravity,light,moisture,andtouch.The
Darwins also noticed that two or more simultaneously applied stimuli could
be distinguished by a root tip, and that its response was such as to suggest
that it could discriminate between the stimuli and judge which was the more
important response for the survival of the whole plant. A discrimination
between touch and gravity stimuli has been confirmed in recent times by
Massa and Gilroy (2003), and between gravity and a moisture gradient by
Takahashi et al. (2003).
Whether or not Charles Darwin considered the root-tip ‘brain’ as a seriouspostulate,orsimplyasafancifulnotion,isnotentirelyclear.Thestrong
advocacy of a root-brain in the mentioned letter to Hooker suggests that he
did take this proposition seriously. However, despite the two mentions of
a ‘brain’ and the acknowledgment that a stimulus perceived in one site, such
as the root tip, results in an “influence from the excited part” moving to
another part where a response takes place, the Darwins disclaimed the possibility
of a nervous system: “Yet plants do not of course possess nerves or
a central nervous system” (Darwin 1880, p. 572). It may be that the authors
refrained frommaking such a radical assertion because they did not know of
any supporting anatomical evidence. In any case, some years were to elapse
before convincing images of the neurons of the brain and the central nervous
system of mammals were published by Santiago Ramón y Cajal (1909).
Why, then, did Charles Darwin use the term brain? In the absence of
any corroboration from anatomical evidence, the possibility of there being
a brain was presumably suggested by the evident perception of ambient
environmental stimuli by the root tip, the discrimination between stimuli,
and the subsequent growth response of the root. A further question is what
Darwin had in mind when he wrote that the brain was “seated within the
anterior end of the body”? It is not entirely clear whether Darwin was here
referring to the brain in relation to the body of a lower animal, to animals
in general, or to the body of a plant. And if to a plant, is the ‘body’ that of the
root or of the whole plant? Taking one extreme of all these possibilities, it
seems that Darwin viewed the plant as possessing an anterior, or front end.
For him, the anterior end was represented by the root tips. Further, in each
of these tips there was a brain sensing the environment around the tip and
then bringing about a response to those stimuli which the tip was capable
of perceiving. The plant brain thereby guided the forward progress of the
plant. It mattered not that the shoots failed to follow the movements of
the roots: in fact, the shoots mostly remained anchored where they were –
except if the plants happened to be geophytes, in which case the shoots
would be pulled along by their roots (Galil 1980). The job of the shoots
is to engage in sexual or vegetative reproduction (or both) and to scatter
progeny.