The Geography of Phytochemical Races

96 2 Examples Within Continents
Fig. 2.60 Map of sites for the
Ambrosia ambrosioides study
also present in two populations from Baja California along with hydrodamsinic acid
otherwise not seen in this taxon. These data are summarized in Table 2.19.
Ambrosia camphorata (Greene) Payne is a shrubby native of the Sonoran Desert
that has been variously described as consisting of two varieties, var. camphorata
and var. leptophylla Gray, or two species (Rydberg, 1922). A reinvestigation of the
complex led Payne (1964) to conclude that the problem could not be resolved on
the basis of the available information. In an effort to overcome this state of affairs,
Seaman and Mabry (1979a, b) examined the sesquiterpene lactone chemistry of individuals
collected from 14 sites in Baja California and one from the Mexican mainland
near Puerto Libertad (see Fig. 2.61). Chromosome number was established for
individuals from nine of the Baja Californian sites; four were found to be diploids
(n = 18) and fi ve tetraploids (n = 36). Two of the diploids (Seaman and Mabry populations
No. 17 and 217) occurred near the northern extent of the collections and two
from much farther south (No. 174 and 175). Four of the tetraploids were also found
in this area, with the fi fth from near the tip of the peninsula. Also noteworthy is the
apparent heterogeneity of fl oral structure. Two types of head structure have been
observed, one with small heads with few spines that characterize plants throughout
the length of Baja Californica, except for an area that lies roughly between 24°30′N