The Geography of Phytochemical Races

2.7 North and Central America 113
visited fl ower heads indiscriminately in mixed populations. Thus, the involvement of
fl oral color in the establishment and/or maintenance of the geographic partitioning of
the two forms of Encelia farinosa remains a mystery.
Interested readers can fi nd information on geographic variation in aboveground
architecture and leaf characters of Encelia farinosa in response to temperature and
available water in Housman et al. (2002), and a discussion of chemical variation and
defense against predation in a report by Wisdom (1985).
2.7.18 Larrea tridentata (Zygophyllaceae)
Later in this review we examine the genus Larrea with reference to the origin of
the North America–South American disjunction of the genus. Here, we look at
variation within one of the components of that system, Larrea tridentata (Seese &
Moc. ex DC.) Coville, the creosote bush, which occurs widely in the deserts of the
southwestern United States, Baja California, and northwestern Mexico in the State
of Sonora. The chemistry and biology of “creosote bush” have been reviewed in
detail in a volume edited by Mabry et al. (1977). The name derives from the copious
amounts of resin that accumulate on aerial parts of the plant, material that is
thought to serve in several capacities, including waterproofi ng, as an ultraviolet
shield, and as a feeding deterrent. Downum et al. (1988) undertook a detailed study
of this species in order to gain a better idea of the composition of and seasonal
dynamics of the exudate. To this end, they collected material over the entire range
of the species, including many sites in Sonora and along two transects: (1) one that
ran the length of Baja California; and (2) one that ran across southern California
and Arizona (Fig. 2.70). Analyses were performed to determine the amount of
exudate produced and variation, if any, of the major component, nordihydroguaiaretic
acid [229] (NDGA) (see Fig. 2.71). Mean percentages of hexane-soluble and
methanol-soluble components varied, but the variation did not correlate with either
latitude or longitude. In contrast, the concentration of NDGA correlated reasonably
well (r 2 = 0.59) with latitude, with a low mean value of 6.4 mg g −1 (dry weight
basis) near the southern boundary of the species’ range (southern Baja California,
24°N) and a high of 60.2 mg g −1 for plants near the northern extent of the range
(32°N). Seven collections in a roughly east to west transect across southern Arizona
(111°W) and California (115° 35′W) showed a relation between NDGA concentration
and longitude, but the r 2 value was much lower (0.22).
2.7.19 Chrysosplenium glechomaefolium (Saxifragaceae)
Worldwide, Chrysosplenium consists of at least 55 species; most occur in the Northern
Hemisphere, but the genus is also represented in southern South America where
two species are known (Hara, 1957). Chrysosplenium glechomaefolium Nutt. occurs