Pollen morphology of Chinese 19 species representing 6 genera in the tribe

Zizipheae Brongn., Rhamnaceae, was examined under light microscope and scanning electron microscope. The pollen grains oblate or suboblate, rarely subspheroidal.   Polar axis 13.1-26.1μm long, equatorial axis 16.5-29.6μm long, 3-colporate, colpus generally narrow, ora lalongate, with two ends connected with the thinned part of exine, forming a H-shape.  There are 4 thickenings where colpi and ora pass across, or sometimes the thickenings indistinct, forming a ring.  Stratification couspicuous or inconspicuous. Ornamentation of exine obscure, rarely obscurely reticulate under light microscope, but

conspicuously fine-reticulate, finely reticulate-foveolate, foveolate, striate-reticulate and shortly striate under scanning electron microscope.

     A key to the genera based on pollen grains is provided and the general morphology

of 6 genera: Paliurus, Ziziphus, Chaydaia, Berchemia, Berchemiella and Rhamnella are

described.

     Based on 4 thickenings and H-shape, pollen grains of the tribe Zizipheae may be divided into two groups: 1. the four thickenings rather larger and conspicuous, or stretched along the colpus in some species, H-shape distinct, e.g. in Berchemiella, Chaydaia and Rhamnella, and 2. the four thickenings smaller, sometimes forming a ring, H-shape more or less conspicuous, e.g. in Berchemia, Paliurus and Ziziphus.

     With respect to the concept of the genera Berchemiella and Berchemia, there were two different ideas among a number of taxonomists. H. Koidzumi, J. Ohwi and S. Kitamura place the former in Berchemia as a section, but on the contrary, T. Nakai, T. Yamazaki and K. Suessenguth consider Berchemiella and  Berchemia as two separate genera.  The data from pollen morphology support the latter treatment.

     Berchemia is closely related to Berchemiella, Chaydaia and Rhamnella in the habit

and the floral morphology, but differs from the others by its disc well-developed, eularged and becoming cup-shaped in fruit. They are also different from each other in the characters of pollen grains.

      T. Yamazaki places the genus Chaydaia in Rhamnella as a section, but according to the pollen morphology to treat Chaydaia and Rhamnella as two independent genera woild be reasonable.