Embryology of Acanthothamnus, Brexia and Canotia (Celastrales): a comparison

TOBE, H. & RAVEN, P. H., 1993. Embryology of Acanthothamnus, Brexia and Canotia (Celastrales): a comparison. Acanthothamnus, Brexia and Canotia , which have been variously discussed with respect to their mutual relationships, are studied embryologically in an effort to clarify their proper systematic positions. Comparisons in embryological characters show that, although Acanthothamnus and Canotia are more similar to eacli other than is any of the genera to Brexia , they all share quite a number of features with one another and further with other Celastraceae. The shared features include a distinctive combination of features, which is common to Celastraceae and related families, and which comprises (1) tenuinucellate (or weakly crassinucellate) ovules, (2) a small nucellus with an early disintegrating tissue, (3) Nuclear type endosperm, (4) albuminous mature seeds with a straight embryo and (5) a fibrous exotegmen. Evidence from embryology supports the inclusion of the two closely related genera Acanthothamnus and Canotia in Celastraceae. Furthermore, combined with evidence from floral anatomy and development as well as from chemistry, it also supports the concept of a unigeneric family Brexiaceae in Celastrales. The affinities of Brexia with any of the families of Saxifragales sensu laio , which have been widely accepted, are not supported.