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afferentnervousimpulsewithinthesomaticnervoussystem.Theredirected
basipetal flow of auxin from the brain composed of root tip and its cap-
enclosed sensory tissues is analogous to the efferent nervous impulse. In
addition, the movement of auxin, either acropetally or basipetally, occurs
in much the same way that animal neurotransmitter substances are moved
from neuron to neuron by means of chemical synapses (Lodish et al. 2000).
It seems that one consequence of the polarised flow of auxin towards the
anterior root tip is that it leads to the regulation of auxin level by auxin
catabolism in the quiescent centre. A second consequence is that a polarised
flow can be redirected so that when an appropriate stimulus is perceived
differential growth can be initiated. In the plant embryo, also, the flow
of auxin is polarised towards the root tip (Friml et al. 2003), this being
established after an early phase during which auxin transport is subject
to maternal influence. In the embryo, therefore, the root tip can also be
regarded as the anterior, or front end, of the plant. It is also worth noting
that the root tip is often the first organ to emerge from a germinating seed -
an anterior property akin to head-first emergence from hatching eggs or
during mammalian parturition.
Theseobservationsaboutanteriorityupsettheusualperceptionofthe
plant where, because of its visibility and upward growth, a false pre-
eminence is implied to the shoot. Some authors even explicitly regard
the shoot as apical (anterior) and the root as basal (posterior) (Friml et al.
2003). But, for the reasons mentioned, the converse may be a more valid
conception of the plant, thereby vindicating Darwin's remark about the
anterior root.
3.5
Closing a Gap in Living Systems Theory
The presence of a channel in the form of a cellular system which transports
auxin either towards (in the xylem parenchyma) or away from (in the outer
cortex and epidermis) the 'brain' of the root tip and its cap, together with
the redefinition of the root tip as an anterior element of the plant, results
in a congruence between plants and animals with respect not only to their
morphology and anatomy but also to the way in which sensory information
is processed. It follows that the theoretical analysis of living systems, as it
applies to plants (Barlow 1999), can now be completed.
Living systems theory was propounded by James Grier Miller, first alone
(Miller 1978), then later in collaboration with his wife, Jessie Louise Miller
(Miller and Miller 1981, 1995). In this theory, the organism and the so-
cial environment within which it lives and which it has helped to cre-
ate are analysed into 20 subsystems. These subsystems in turn fall into
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