KOUKOL, SCOTT R.* and CURTIS CLARK. Biological Sciences, California State Polytechnic University, Pomona, CA, 91768; Rancho Santa Ana Botanic Garden, Claremont, CA, 91711. - Comparative anatomy of the leaves and stems of Encelia (Asteraceae: Heliantheae).
Encelia is a genus of mainly drought-tolerant shrubs found in
arid to semi-arid areas of southwestern North America and western
South America. Several species have well-characterized
ecophysiological and morphological adaptations for dealing with
drought. All but one species are currently assigned to one of two
clades, based on morphological, phytochemical and preliminary genomic
data. The purpose of this study was to determine whether the internal
anatomy of leaves, stems, and peduncles reflects the adaptations and
external morphology of individual species, or instead the basic
phylogenetic patterns of the genus. The basic anatomy of stems and
leaves is consistent with other xerophytic Asteraceae. Resin ducts are
present in more species than indicated in earlier studies. The unique
leaf shape of Encelia ventorum is reflected in its internal
anatomy, but the similarities among the species are otherwise much
greater than habitat, external morphology, or phylogeny might imply.
This provides further evidence of the recent diversification of the
group.
Key words: adaptation, anatomy, Asteraceae, Encelia, phylogeny