A PALYNOLOGICAL STUDY OF CROTONOIDEAE (EUPHORBIACEAE)

Pollen of 69 species representing 34 genera from 12 of the 13 tribes of Crotonoideae sensu Webster was examined in light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The pollen of the vast majority of the subfamily is inaperturate, and all Crotonoideae exines have a similar architecture: triangular supratectal elements attached to a network of muri having short or irregular columellae. These three components, triangular supratectal elements, muri, and modified columellae constitute the Croton structure. The triangular elements, designated subunits, can be psilate, striate, furrowed and ridged, or pitted, and with echinate, rounded, or long-attenuate apices; the subunits can be closely spaced or arrayed in more open configurations. A Croton sculpture refers to the presence of triangular sculpture elements, which may or may not have muri and modified columellae. Inaperturate grains have thin footlayers and thin endexines, whereas colpate pollen bas both footlayer and endexine well developed; in porate grains, the footlayer and endexine appear intermediate between inaperturate and colpate. Tribes Micrandreae (Hevea, Micrandra, Micrandropsis examined) and Adenoclineae (Adenocline, Ditta, Glycydendron, Endospermum, Klaineanthus, Tetrorchidium) are 3-colpate and probably primitive, having retained not only apertures but (prominent) footlayers and endexines as well; Manihoteae (Manihot, Cnidoscolus examined) and Gelonieae (Suregada) are porate. The thicker aperturate exines, especially in the colpate grains, may be linked with retention of apertures. The remaining 22 genera examined, including the two largest, Croton and Jatropha, have inaperturate pollen. A thin footlayer/endexine also occurs in Oldfieldioideae, and a tectum with triangular elements is found in some Acalyphoideae. In both morphology and exine structure, the pollen of Crotonoideae closely resembles the (porate) pollen of Thymelaeaceae.