The Geography of Phytochemical Races

5.2 Nonvascular Plants 235
Fig. 5.9 Map showing eastern (Le Trayas) and western (Sausset les Pins) ends of a transect in the
study of Cystoseira amentacea
5.2.14 Plocamium (Plocamiaceae)
Several genera of red algae (Rhodophyta) synthesize a variety of halogenated
terpenes that, in addition to exhibiting interesting biological activities, have been
found to be useful in chemotaxonomic studies. Several examples involving species
of Plocamium, Portieria, and Laurencia are discussed below. The very extensive
chemistry of the Rhodophyta has been reviewed by several authors, including
Fenical (1975) and Faulkner (1987, 1988, 1990, 1991), in a series of more or less
regular summaries. Owing to the ready availability of these reports, the examples
below represent only a sampling.
Before looking at the chemistry of these taxa, it is prudent to comment briefl y
on taxonomic problems associated with these organisms. Several factors contribute
to the diffi culties in species defi nition in the red algae, not the least of which is the
limited number of morphological features available to the systematist. Structural
distinctions can be blurred further because of the amount of convergence that has
occurred, much in excess of that commonly seen in vascular plants. Also lacking in
many instances has been the suffi cient sampling to enable limits of morphological
variation to be assessed. In fact, many taxa have not been revisited taxonomically
since the initial collections were made, with the attendant problem that species and
generic concepts may have changed dramatically in the meantime. Another diffi -
culty in assessing the taxonomic signifi cance of different chemical profi les in some
organisms was the tendency of some natural product chemists to accept the names
of organisms on faith and not concern themselves with taxonomic nuances such as
preparing voucher specimens. This is much less a problem nowadays than it was
in the past owing to increased awareness of these problems by practicing natural
product chemists, as well as increasing cooperation between chemists and marine
biologists in addressing the possible signifi cance of the chemical data. With these
considerations in mind, we can turn our attention to the fi rst example.
One of the more thoroughly studied rhodophyte genera is Plocamium (Plocamiaceae),
from which highly halogenated cyclic and acyclic compounds have been