Abstract: Pollen morphology of 25 Chinese species, representing 6 five genera in the tribe Rhamneae of Rhamnaceae, was examined under LM and SEM. Pollen grains are subspheroidal or suboblate, obtuse-triangular in polar view, 3-colporate, with the polar axis (P)10.4-28.7μm long and equatorial axis (E) 11.3-30.5μm long, P/E = 0.81-1.11. Colpi generally narrow and long, ora lalongate, with two ends connected with the thinned part of exine, forming a H-shape. Four thickenings were observed where colpi and ora cross, but sometimes the thickenings were indistinct or formed a ring. The stratification indistinct. The ornamentation of exine obscure or indistinctly reticulate under LM, but rugulose, short-striate, rugulose-foveolate or reticulate under SEM. The exine ultrastructure of Rhamnus ussuriensis was examined under TEM. It consists of imperforate tectum, granular layer, foot layer and endexine. The tectum is rather thick with uneven surface, while the granular layer is rather thin. A key to the genera based on pollen characteristics is provided and the general pollenmorphology of five genera, i.e. Sageretia, Rhamnus, Hovenia, Colubrina and Alphitonia, is described respectively. Based on pollen size, the four thickenings and the ornamentation, the five genera under study may be distinguished from each other. For example, pollen grains are smaller in Sageretia; the four thickenings are larger and distinct in Hovenia and Alphitonia. However, the ornamentation is short-striate under SEM in Hovenia and Alphitonia, while reticulate in the Colubrina and Rhamnus ( Subgen. Rhamnus) .Two different concepts of Rhamnus exist among taxonomists. Heppeler and Suessenguth divided Rhamnus into two subgenera (Subgen. Frangula and Subgen. Rhamnus), whereas Grubov separated Subgen. Frangula from Rhamnus as an independent genus. According to the pollen morphology, the separationis reasonable.

Key words: Pollen morphology, Rhamnaceae, Rhamneae