Neurobiological View of Plants and Their Body Plan - Communication in Plants: Neuronal Aspects of Plant Life - page 25

Fig. 2.2. Vascular bundles throughout the plant body. Thin strands of vascular tissue form
networks in leaves, join into bundles in shoots, and transform into a large central cylinder
of roots which is encircled by pericycle and endodermis
2.6
Vascular Strands as Plant Neurons
Vascular strands represent not only plant 'nerves' but also supply the plants
with an endoskeleton. Along the plant body axis, there is a gradient of in-
creasing volume and complexity of vascular/stelar tissues (Fig. 2.2). Leaves
contain single thin strands which join together to form the vascular bun-
dles. The latter extend along the stem up to the root to form the vascular
cylinder (Sachs 2000). In roots, a large portion (up to 50% of the root di-
ameter) of the organ is the vascular tissue, and its strands are supported
by numerous 'nursery' cells forming the vascular cylinder (Sachs 2000).
Moreover, stelar tissues in roots are completely enclosed by meristematic
pericycle and protective endodermis. The latter tissue is ontogenetically
related to the quiescent centre, while both endodermis and pericycle, like
all vascular cells, are very active in transcellular auxin transport. More-
over, pericycle cells initiate lateral root formation in a process very closely
resembling early zygotic events during embryo formation.
Phloem can be viewed as a supracellular axon-like 'channel' which con-
nects the shoot and root apices. Phloem is specialized for transmission of
action-potential-driven electric signals (Mancuso 1999). 'Axon-like' means
that it is specialized for the rapid transfer of RNA molecules (Lucas et
al. 2001) but it does not accomplish ribosome assembly and messenger