POLLEN EXINE ULTRASTRUCTURE OF THE APETAL OUS CROTONOIDEAE

The crotonoid pollen type defines the subfamily Crotonoideae in the Eu phorbiaceae. The apetalous Crotonoideae are composed of 5 tribes (Micr andreae, Manihoteae, Elateriospermeae, Adenoclineae, Gelonieae), and 1 subtribe, Neoboutonineae (tribus Aleuritideae). In the pollen of thes e Crotonoideae, the exine is basically tectate. The tectum is of the t ilioid type. The more or less distinct reticulum is supratectal. The m uroid ridge supports ornamented excrescences, clavae or gemmulae, regu larly arranged around a circular area. Although morphologically analog ous, the columellae are not in fact all homologous in the exine. They are of two types. The very short ones, sometimes reduced to spheroidal grains rest on the foot-layer. The others are elongated and in two ro ws, supported by the tectum in the meshes. In the first case, they con cern a part of the real infratectum and in the second, the columellae are a tectal formation. In the Elateriospermeae and in the Neoboutonin eae, where the tectum is reduced to some clavae or verrucae in the mes hes, only the tectal columellae are present and constitute a new form of infratectum. The foot-layer is continued or reduced to masses and t he endexine is partly or totally lamellate. According to the nexine st ructure, the pollen grains of apetalous Crotonoideae are classified in two groups: Group I, Micrandreae, Glycydendron (Adenoclineae), Maniho teae, Elateriospermeae; Group II, Adenoclineae (incl. Cladogelonium; e xcl. Glycydendron), Gelonieae, Neoboutonineae (Aleuritideae). In the t wo groups, the pollen types with a maximum of primitive characters are subequatorial; group I is South American and Southeast Asiatic and gr oup II is pantropical and principally palaeotropical. The oldest fossi l pollen dates from the Late Cretaceous of the tropics and their polle n type is derived. The diversity of the pollen types of the apetalous Crotonoideae may correspond to regression associated with a new organi sation of the exinous structure that is well developed in the petalifl orous Crotonoideae and in the Euphorbioideae. The apetalous Crotonoide ae show some relationships with many palaeotropical Phyllanthoideae su ch as Martretia, Protomegabarea, Savia. In comparison with the other E uphorbiaceae, the crotonoid pollen type cited in many Angiosperms woul d only be used to describe the pollen ornamentation independently of t he structure, which varies considerably with the taxa. This ornamentat ion is found in many families without close relationships. In contrast , the study of the different exinous structures in the Crotonoideae de monstrates certain relationships with other Dicotyledones such as Malv ales (Tiliaceae, Bombacaceae), Dilleniales (Dipterocarpaceae),but less certain affinities with Buxaceae and Thymeleaceae. (C) 1997 Elsevier Science B.V.