A PALYNOLOGICAL STUDY OF CROTONOIDEAE (EUPHORBIACEAE)

Pollen of 69 species representing 34 genera from 12 of the 13 tribes o f Crotonoideae sensu Webster was examined in light microscopy (LM), sc anning electron microscopy (SEM), and transmission electron microscopy (TEM). The pollen of the vast majority of the subfamily is inapertura te, and all Crotonoideae exines have a similar architecture: triangula r supratectal elements attached to a network of muri having short or i rregular columellae. These three components, triangular supratectal el ements, 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 mor e open configurations. A Croton sculpture refers to the presence of tr iangular sculpture elements, which may or may not have muri and modifi ed columellae. Inaperturate grains have thin footlayers and thin endex ines, whereas colpate pollen bas both footlayer and endexine well deve loped; in porate grains, the footlayer and endexine appear intermediat e between inaperturate and colpate. Tribes Micrandreae (Hevea, Micrand ra, Micrandropsis examined) and Adenoclineae (Adenocline, Ditta, Glycy dendron, Endospermum, Klaineanthus, Tetrorchidium) are 3-colpate and p robably primitive, having retained not only apertures but (prominent) footlayers and endexines as well; Manihoteae (Manihot, Cnidoscolus exa mined) and Gelonieae (Suregada) are porate. The thicker aperturate exi nes, especially in the colpate grains, may be linked with retention of apertures. The remaining 22 genera examined, including the two larges t, Croton and Jatropha, have inaperturate pollen. A thin footlayer/end exine also occurs in Oldfieldioideae, and a tectum with triangular ele ments is found in some Acalyphoideae. In both morphology and exine str ucture, the pollen of Crotonoideae closely resembles the (porate) poll en of Thymelaeaceae.