The Geography of Phytochemical Races

2.3 Europe 25
to put the specifi c study in perspective. The Ramalina siliquosa (Huds.) A. L. Sm.
species complex consists of moderately large, fruticose, sexually reproductive
organisms widely distributed on acid substrates along the coasts of western and
northern Europe. The chemistry of this species complex has been described in considerable
detail (Culberson and Culberson, 1967; Culberson, 1967, 1969, 1970).
In all, six chemical races have been described, fi ve differing in the nature of their
β-orcinol derivative, with the sixth characterized by the absence of medullary substances.
Structures of the compounds identifi ed in this study are protocetraric acid
[60], hypoprotocetraric acid [61], stictic acid [62], norstictic acid [63], and salazanic
acid [64] (see Fig. 2.12 for structures 60–64).
The salazanic acid race is the most northerly, occupying suitable habitats in arctic
Norway, Iceland, and throughout the Baltic region. The norstictic acid race and
medullary negative races occur in southwestern Norway, western Great Britain, and
in the Brittany Peninsula. The hypoprotocetraric acid race is common in western
Great Britain and France, and extends south to Portugal. Finally, the protocetraric
and stictic acid races are not only most common in Portugal, but also occur northwards
to southern Norway. All six races occur “sympatrically” in western coastal
Wales. In the strict sense, however, use of the term sympatric is incorrect. Careful
analysis of individuals along several transects on a rocky headland (Culberson and
Culberson, 1967; see for photograph) revealed a signifi cant level of specialization
of chemical race in (apparent) response to the degree of harshness of the particular
aspect of the site. Thus, the southern (Portuguese) race, characterized by hypoprotocetraric
acid, occurs on the northern-most, sheltered side of the promontory. The
stictic acid race occurs on the most exposed faces in this locale, the seaward faces to
the west and south sides. Plants characterized by norstictic acid occupy intermediate
sites. These studies revealed that an otherwise apparently homogeneous area can
harbor chemically different races separated by no more than a few centimeters. How
the different chemicals elaborated by these races aid in the survival of individuals in
their respective habitats, if in fact they do, remains an unanswered question, at least
so far as this writer is aware.
2.3.2 Cyanogenesis in Clover (Trifolium, Fabaceae)
The study of cyanogenesis in European clover involves both latitudinal and elevational
transects and represents one of the most frequently cited examples of pattern
variation in the production of a secondary metabolite by a fl owering plant. We
can introduce this example with an interesting tale, related by Briggs and Walters
(1997), involving the discovery of cyanogenic glycosides in species of Lotus that
were toxic to transport animals used by British forces in the Sudan campaign at the
end of the nineteenth century. It was subsequently demonstrated that some plants
of Lotus corniculatus L., one of the species responsible for the poisonings, gave a
positive reaction for hydrocyanic acid (HCN), whereas others did not (Armstrong
et al., 1912). Subsequently, Dawson (1941) reported an HCN polymorphism within