STEINMANN, VICTOR W. Rancho Santa Ana Botanic Garden, 1500 North College Avenue, Claremont, CA 91711. - Comparative anatomy of the New World succulent Euphorbia(Euphorbiaceae).
The genus Euphorbia(Euphorbiaceae) contains nearly 2000
species, of which as many as a third are succulent. The majority of
the succulent Euphorbiaoccurs in Africa and lower Asia. Within
the New World there only about 15 succulent species. Based on a broad
molecular phylogenetic analysis of Euphorbiaand related genera,
it appears that these comparatively few species represent at least
five independent derivations of this habit, none of which are closely
related to the Old World taxa. Succulence in the New World species is
achieved by the proliferation of water-storing parenchyma in the pith
and/or cortex. Periderm development is delayed, the epidermis remains
intact, and the stem stays green and photosynthetic for many seasons.
In addition, there is a trend towards microphylly, and most species
possess minute, caducous leaves, and the stem is the chief
photosynthetic organ. Despite this convergence to reduced-leafed,
green-stemmed succulents, there are many differences at the cellular
level. Chlorenchyma organization varies from neatly layered rows of
narrow palisade cells to unlayered zones of wide spheroidal cells.
Tanniniferous cells are present or lacking. The vasculature also
differs, and leaf traces that descend many centimeters through the
cortex occur in one species. The vascular cambium is round in most
taxa but is variously angled in others. In addition, there is
diversity in secondary xylem. Some species posses highly lignified
wood lacking axial parenchyma, while the wood of other species is less
lignified with well-developed axial parenchyma, this sometimes
occurring in bands. Although both the molecular phylogenetics and the
many anatomical differences suggest the repeated evolution of stem
succulence within Euphorbia,it is still uncertain what promotes
this trend.
Key words: anatomy, Euphorbia, Euphorbiaceae, succulence