References

66 P.M. Neumann
Finally, I examined the plant neurobiology concept in the light of several
experiments from our laboratory. These involved the effects of partial root
excision, or of treatments which inhibit root activity, on root-to-shoot signaling
and shoot growth responses to environmental stresses. The findings
are presented in the following three sections.
5.2
The Comparative Need for Rapid Neurobiological Activity
in Animals and Plants
Multicellular animals and plants appear to have evolved from a common
ancestral cell type and they share many basic molecules together with lifeassociated
activities such as respiration, cell division and long-distance
transport systems. However, there are, of course, intrinsic differences between
animals and plants. For example, brains and networks of specialized
nervecellshaveclearlyevolvedinanimalsbutnotinhigherplants.The
animal brain can be viewed as a device which rapidly processes sensory
input signals, makes decisions and sends motor output. The human brain
consists of around one hundred billion neurons intimately connected via
the spinal chord to a nervous network which can rapidly transmit signals to
and from every point in the body. The brain of higher animals is essential to
life since decapitation or brain inactivation immediately leads to death of
the organism. In contrast, excision of roots or root apices of plants need not
have immediate adverse effects on the remaining parts (consider cut flowers
andseeSect.5.3).Centralizedbrainsare,however,notessentialfortheexistenceofsometypesofanimals.Forexample,membersofthemarinephylum
Cnidaria (e.g., hydra, jellyfishes, corals, sea anemones and Portuguese
man-of-war) have survived and thrived for about a half billion years without
brains. Body control in these organisms is based on a diffuse nerve net
which provides the communication between sensory cells and the muscle
cells involved in essential movements. Cutting nerves in lower or higher
animalsisexpectedtoresultintheparalysisofassociatedmuscularfunction
and is likely to be life-threatening. In contrast, phloem girdling (i.e.,
interruption of potentially nervelike tissues) in branches of fruit trees, as
practiced by farmers in Israel, does not appear to have much adverse effect
and can result in the production of larger and sweeter fruits. Thus, plants
and animals can differ in their responses to disruption of signaling events.
A more important and major difference between animals and plants is
that the animals need and use contractile muscles to facilitate their heterotrophic
life style, while plants do not. A heterotrophic life style means
that animals depend on obtaining preformed organic foodstuffs from their
environment. Regulated patterns of muscular activity and directional mo-