References

3 Charles Darwin and the ‘Root Brain’ 39
3.2
The Advancing Root Front and Brain System
By entertaining the notion of an anterior root a rather different perspective
of the plant is reached from that which is usually considered. Here, the
roots are all important. Their tips form a multiheaded advancing front. The
complete set of tips endows the plant with a collective brain, diffused over
a large area, gathering, as the root system grows and develops, information
important for plant nutrition and survival. Roots also seem to confer a sense
of ‘self ’ upon the plant (Falik et al. 2003).
Theadvancingfrontofroottips,eachtipwithitsbrainandsensory
surface, can be extensive. For example, after 2 weeks of growth a sorghum
(Sorghum bicolor) plant may already have developed 2.6×10 3 root tips
(Iijima and Kono 1991). If these tips are assumed to be contained within
a conical soil volume, the average root density would be 24 tips per cubic
centimetre of soil. This value, however, is extremely modest compared with
the value of 1. 1 × 10 3 root tips per cubic centimetre which was recorded in
the floor of a mixed hardwood forest by Lyford (1974). Tree roots can have
uptosevenormoreordersofbranching,tipsofeachorderhavingtheirown
characteristic dimensions, anatomy, and tropic responses. High-order root
branches are, however, often ephemeral and associated with mycorrhizae,
a feature which might remove the previously mentioned ‘sense of self ’
that normally regulates root development by rendering the root brainless,
hence permitting these high root densities.
3.3
The Location of the Plant Root-Brain
3.3.1
Clues from the Transition Zone
Can the location of the ‘brain’ which Darwin considered to reside in the
root tip be more clearly defined? To do so would first require agreement
upon the definition of a simple brain, such as it being a group of cells that
not only receives neuronal-type signals from sensory cells or organs but
which also processes those signals, thereby bringing about a response. The
response might be recognised as a directional root movement, or tropism.
At this point it is important to distinguish root tropism from root nutation,
another type of movement. In fact, a major theme of the Darwins’
book The power of movement of plants wasthatnutationisaninherent
autonomous feature of the root tip, and that it is upon this mode of growth