The morphology and systematic position of the Didiereaceae of Madagascar

Abstract

The Didiereaceae, endemic to the arid south-west of Madagascar is one of the most remarkable plant families in the world. Four genera and 11 species are known, of which 3 have been described by the author:  Decaryia (1 species), Alluaudiopsis (2 spp.), Alluaudia (6 spp.) and Didierea (2 spp.). The species show different growth forms, but their shoot-stem-systems are basically similar; they are all spiny, semi-succulent deciduous shrubs or trees, with a stem (shoot) system differentiated into long-shoots and short-shoots (brachyblasts). The long shoots bear normal leaflets, which soon fall off. In their axils, brachyblasts develop proleptically, which produce two types of leaves (1) thorns which develop proleptically in different numbers (1 in Alluaudia, 2 in Alluaudiopsis marnieriana and several Didierea)-, (2) leaves which are green, deciduous assimilation organs (2 per brachyblast in Alluaudia, several in Didierea).

The stem organization is comparable to that of the Cactaceae of the New World. The brachyblasts of the Didiereaceae are homologous to the areoles of the Cactaceae, most obviously to those of the primitive members, the Pereskioideae. The affinities between the two families can be demonstrated by means of grafting. It is possible to graft Didiereas onto Pereskioideae and vice versa.

The flower structure of Didiereaceae is very uniform. Flowers are unisexual and dioecious and show affinities to those of the Portulacaceae. The development of the gynoecium, the ovule and the female gametophyte as well as the presence of betalains in the flowers indicate that the Didiereaceae do not belong to the Sapindales as was sometimes assumed, but to the Centrospermae. They should be placed between Portulacaceae and Cactaceae. The ancestors of the Didiereaceae are not known.