Biology Reference
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glucose, glucocorticoids, mineralocorticoids, electrolytes in body fluids, water con-
tent, blood pressure, etc., are determined in brain structures. We are still far from
being able to answer the question of where the set points are that determine the phys-
iological levels of hormones and other parameters of plant fluids in plants.
Being a prerequisite of the maintenance and reproduction of any ordered structure,
the control system implies mechanisms of monitoring and communicating with the rest
of plant organism. The mechanisms of communication have been the Achilles' heel of
our understanding of the nature of the control system in plants. However, undeniable
progress has been made in this regard, especially during the last decade or so. Biologists
on both sides of the Atlantic have provided substantial evidence on the existence of ele-
ments of a neuroid system and attempted to put it on a solid theoretical foundation.
Let us remember that neuroid phenomena evolved in unicellular ancestors of
both multicellular plants and animals and that the nervous system plays the role of
the controller in animals. In view of the common unicellular origin of animals and
plants, these two facts may have some bearing on the issue of the control system in
plants. Transmission of electrical signals between plant cells has been observed for a
long time, and a number of environmental agents can induce action potentials in cell
plants. It is interesting to point out that Arabidopsis possesses 30 percent of the nerv-
ous system-related genes of flatworms (platyhelminthes).
It is argued that “[n]eural aspects of biological systems are obvious already in
bacteria and unicellular biological units such as sexual gametes and diverse uni-
cellular eukaryotic organisms” ( Baluška and Mancuso, 2009 ). Indeed, elements
of electrical impulse conduction are observed in protozoans. Based on their com-
plex morphology and physiology, the American ethologist James L. Gould called
Paramecium “a promising nerve cell analogue” that has “all the complexity of real
nerve cells and more” ( Gould, 1982 ).
In plants, the situation with respect to impulse conduction is closer to the one we
observe in sponges, which have no neurons or a nervous system but show coordi-
nation of activity and endogenous contraction rhythm for water exchange. Sponges
have a nonneural control system ( Cabej, 2012, pp. 421 ). They show an electri-
cal conductance of the type observed in epithelial cells of lower metazoans, which
is conserved in particular epithelia of higher animals. Sponges also possess many
chemical messengers involved in their systemic contraction behavior, and even a
number of synaptic genes.
Plants display electrical excitability (i.e., the ability to respond to various stimuli
by electrical signaling); they can perceive light of various wavelengths, sense gravity,
and even “smell” various volatile substances and determine their behavior accord-
ingly. This gives rise to the well-known phenomena of phototropism, gravitropism,
and “olfactotropism.”
In 1991, an electric potential on the surface of the root apex of the Lepidium sati-
vum L. , was reported for the first time. The authors concluded that “unevoked fluc-
tuations of the potential … are due to signals of an unknown nature” ( Hejnowicz
et al., 1991 ). Now the evidence of the existence of synchronized electrical activity
in the transition zone of the root apex is believed to reflect the integration of internal
and external stimuli for adapting plant physiology and metabolism to the changes
occurring in the environment ( Masi et al., 2009 ).
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