References

5 Essentiality of root-shoot communication 67
bility are needed to allow animals to hunt, ingest and digest essential organic
foodstuffs and to actively excrete resultant solid or liquid waste products.
In contrast, plants have an autotrophic life style, i.e., while remaining
stationary they can synthesize all the life-essential organic materials they
need from readily available supplies of carbon dioxide, light, water and mineral
nutrients. Moreover, waste products can be stored internally by plant
cell vacuoles. On the other hand, plants may produce up to 100,000 secondary
compounds needed, for example, as defense against herbivores and
diseases, or to help attract pollinators (Poethig 2001; Lev Yadun et al. 2002).
In animals, aural, olfactory, visual and tactile input signals are involved
in feeding and in additional essential activities such as prey capture, fighting,
fleeing or mating. These varied inputs need to be continuously processed
by nervous systems in order to produce the appropriate muscular
contractions within seconds. Speedy signal transmission and processing
is clearly essential to animal survival. Plants appear to generally operate
within a slower time scale than animals. For example, the important and
relatively fast stomatal changes induced by soil water deficit or darkness
take minutes rather than seconds to complete (Kramer and Boyer 1995).
Similarly, significant geotropic responses to changes in root position take at
least 10 min to establish (Fasano et al. 2001; Aloni et al. 2004). Finally, stem
growth inhibition by a severe soil water deficit in a cactus species took weeks
to establish (see later). Of course every rule has its exceptions and rapid
mechanosensory responses such as leaf folding in Mimosa or trap closure
in Venus’s-flytraps can take only seconds to complete. The action potentials
and chemical stimuli associated with rapid leaf folding in Mimosa can
certainly be compared with the electrical and chemical processes involved
in nerve signal transmission. However, the velocity of propagation of electrical
signals in plants is about 2 cm s −1 , whereas action potentials travel
along nerve cells at tens of meters per second (Salisbury and Ross 1985).
In summary, the immobility of plants and their generally slower responses
to environmental changes suggest that a plant capacity for rapid
long-distance signal transmission via nervelike cells and rapid signal processing
in brainlike centers of activity is less essential than it is in animals.
5.3
Plants That Manage Without Roots, Root Apices
and Vascular Tissues
The plant neurobiology concept would be supported if functional essentiality
of the root apex “brain” and associated vascular system “nerves” for
the continuity of plant life could be established. In this respect it is interesting
to trace morphological and anatomical changes which occurred in the