Control Systems in the Living World - Building the Most Complex Structure on Earth - page 48

Figure 1.28 Basic control design of a system manipulating many aspects of plant behavior
and incorporating negative feedback ( Trewavas, 2006 ).
The heart of the control system in animals is the controller, the part of the control
system that monitors the state of the system; it detects deviations from the norm,
makes decisions, and sends instructions to restore the normal state. The controller
enables self-correction and maintenance of the normal state. As it will be shown in
the next chapter, in the animal's integrated control system, this supreme role belongs
to the brain (the neural net in lower metazoans).
An integrated control system would imply the physical omnipresence of the ele-
ments of the control system in plant tissues and organs or, theoretically at least, an
overarching mechanism of communication between the controller and the rest of the
plant body, as well as an ability to transmit information on the state of the system to the
controller. While one can theoretically argue the necessity of an integrated control sys-
tem for the coordination of the development and maintenance of the plant structure and
function, there are no substantial clues as of yet to track the whereabouts of the con-
troller of the control system in plants. Interesting in this regard is a theoretical model of
the control system in plants developed by Trewavas (2006) ( Figure 1.28 ).
As shown in this diagram, the control system in plants is a closed-loop system.
While the logic of the functioning of this control system is solid, the devil is in the
details. Central to this system are the comparator and the reference signal. At the
plant organism level, “the reference signal is currently unknown” ( Trewavas, 2006 ).
Neither is the comparator. The reference signal or the set point has to be part of the
control system, rather than an input from the environment. The set point is an evolu-
tionarily determined parameter of the species that unfolds in the process of develop-
ment, rather than an environmentally provided instruction. In discussing the control
system of animals, I came up with the idea that the determination of physiologically
precise norms is a function of living systems, which is related to the emergent com-
putational capabilities of the neuron, neural net, and CNS ( Cabej, 2005, pp. 63-66 ).