The Role of Root Apices in Shoot Growth Regulation: Support for Neurobiology at the Whole Plant Level? - Communication in Plants: Neuronal Aspects of Plant Life - page 68

evolutionary progression from lower to higher plants. This progression in-
dicates that lower forms of plant life without differentiated roots or vascular
tissues have survived successfully to this day (Raven et al. 1999; Poethig
2001). For example, multicellular brown algae such as the giant kelps found
in the Pacific Ocean show plasmodesmatal connections between cells as
in higher plants and often consist of a leaflike blade connected to a stem-
like stipe. Moreover, some kelps have elongated cells in the center of the
stipe which resemble the phloem transport tissue of vascular plants. Some
kelps may also develop a basal holdfast structure situated at the base of the
stipe. This anchors the algae to rocks and superficially resembles a root.
However, other kelps have no holdfast and form free-floating masses. Thus,
large marine algae appear to be able to function successfully with or without
differentiated roots and vascular tissues.
The first multicellular terrestrial plants may have been similar to the
small multicellular green algae Fritschiella , which has some common fea-
tures with present-day higher plants. Fritschiella has plasmodesmata and
forms multicellular rhizoids at the soil surface from which erect branches
sprout (N.B. although rhizoids occupy the same position as roots they have
no specialized apices and no differentiated transport cells). Fritschiella
can therefore function successfully without specialized conductive tissues
to facilitate nutrient and signal transmission and without complex root
apices.
Sporophytes and gametophytes of bryophyte mosses are thought to rep-
resent a subsequent stage in the evolution of higher plants and reveal more
similarities. Thus, the cells of bryophytes are interconnected by plasmodes-
mata and leaflike structures may be born on a stemlike structure which can
be attached to rhizoids at the base. Moreover, the stems of some bryophytes
may include central strands of nonlignified water-conducting tissues and
of nutrient-conducting tissues. Although some bryophyte species utilize
primitive conductive tissues, it is again noteworthy that others function
successfully as multicellular plant organisms despite the absence of con-
ductive tissues and of root apices.
In summary, various multicellular plant organisms can function with-
out roots, root apices or vascular tissues (Table 5.1). In contrast, modern
gymnosperm and angiosperm plants share a body architecture which in-
cludes roots, root apices and vascular tissues. The relatively large size and
world-wide distribution achieved by gymnosperm and angiosperm plants
suggest that they represent a winning formula for ecological success. Nev-
ertheless, many lower forms of plant life have retained an ability to survive
and propagate very successfully, in a world-wide variety of different envi-
ronments,despitetheabsenceoftheseattributes.Thedegreetowhichthe
relatively greater success of the higher plants is attributable to the facili-
tation of plant neurobiological activity by root tip “brains” and vascular