| Rhododendron subgen. Tsutsusi, which comprises ca. 110 species, was divided into three sections by Sleumer. They are mainly distributed in E Asia. Based on literature, extensive field work and herbarium examination, the morphological characters, which were used for taxonomy by the previous taxonomists, were evaluated. Eighty-four pollen samples (containing 68 taxa) and 50 seed samples (containing 42 taxa) were used for SEM observation. As a result, a worldwide taxonomic revision of Subgen. Tsutsusi is made to clarify species delimitation, and modify infra-subgeneric system. Main results in our study are summarized as follows:1. Morphological charactersFor population collection, a special attention was paid to four groups, namely the R. simsii group, R. taipaoense group, R. mariae group and R. fuchisiifolium group. Using statistical analysis, the morphological characters which were used to distinguish species were evaluated one by one. Habit and shape and size of corolla, can be used to distinguish sections or series. The indumentum on shoots, corolla and style, stamen number, shape of calyx lobes, etc. can be used in species delimitation. The characters of flower number and leaf size can be limitedly used in a few cases, while corolla color, leaf shape, stamens equal or unequal in length, and style longer or shorter than stamens should be better avoided.2. PalynologyEight-four pollen samples were obtained from 68 taxa, of which, 13 species and one variety belong to Sect. Brachycalyx, 46 species and two varieties to Sect. Tsutsusi, and six species to Subgen. Pentanthrea. The pollen grains in all the samples were revealed spheroidal or tetrahedral, and 3-colporate, ranging from 37.67μm to 61.06μm in diam., and compactly granulated or unconspicuously granulated. From the observations on different samples of R. simsii (containing 5 samples) and R. mariesii (4 samples), pollen grains were consistent in shape, size and surface pattern. There was a significant difference in pollen size between Sect. Brachycalyx and Sect. Tsutsusi. Pollen of R. tashiroi, which is the only member of Sect. Tsusiopsis, indicated its affinity with Sect. Brachycalyx, although Sect. Tsusiopsis was sometimes treated as synonymy of Sect. Tsutsusi. Size and surface pattern of pollen grains can not be used to delimitate the species of Sect. Tsutsusi, but these two characters are often consistent with external morphology. According to palynology and morphology, R. huadingense, a species which was once placed in Sect. Brachycalyx, should be placed in Subgen. Pentanthera.3. Seed micromorphologySeeds of 40 species and two varieties (totally 50 samples) were observed under SEM. Size and shape of seeds were fairly stable gathered among different samples of the same species. Based on shape, size index, flat rate, shape of two ends and surface micromorphology, five types of seeds should be recognized. Seed micromorphology can not only easily delimitate the two sections within Subgen. Tsutsusi, but also divide the species of Sect. Tsutsusi into the three types. The species, belonging to the R. seniavinii-type, were characterized by corolla campanulate-infundibular and <1.5 cm in length, and the seeds tiny, narrowly ovoid or oblong, with the cells of surface often concaved and a transverse raphe. The seeds in the R. rivulare-type were long-ovoid, with tassels and tails at the two ends, surface cells concaved, and the corolla of these species was tubular-infundibular, 2-3 cm long. The seeds of the R. simsii-type were slightly larger than those in R. seniavinii-type and R. rivulare-type, narrowly ovoid or oblong, with surface cells often slightly concaved, sometimes with a transverse raphe, and their corolla was infundibular, >3 cm long. The seeds of the R. farrerae-type were more similar to those in R. simsii-type, but bigger in size index and larger in flat rate, the corolla of these species was rotate-infundibular, and they were often deciduous.4. Taxonomic revisionA worldwide taxonomic revision of Subgen. Tsutsusi is made in this work, with 59 species, two subspecies and five varieties recognized. They are keyed, described and illustrated in most cases. About the infra-subgeneric system, two sections, namely Sect. Brachycalyx and Sect. Tsutsusi are accepted, and Sect. Tsusiopsis was reduced to a synonym of Sect. Brachycalyx. Sect. Tsutsusi was classified into three series, namely Ser. Tsutsusi, Ser. Mariana and Ser. Seniaviniane, and the latter two are new. R. xiangganense and R.×yunyianum are described as new, and three combinations, R. mariae subsp. flumineum, R. chrysocalyx subsp. meridionale, and R. rivulare var. kwangtungense are made. One hundred and fifty-nine names are reduced to synonyms, and geographical distribution of each taxon is given in Appendix.5. Geographical distributionUsing the result of the taxonomic revision, the geographical distribution of Subgen. Tsutsusi was analyzed. According to Takhtajan's (1978) regionalization of the world flora, this subgenus is distributed in E Asiatic Region, with a few species extending to the Indochina Region. The distribution of Sect. Tsutsusi is obviously broader than that of Sect. Brachycalyx, ranging from NE India-Hengduan Mountain Region eastwards to central Hokkaido. Luzon Island (the Philippines) and Chichijima Island (Japan) are two disjunctive plots. The distribution of Sect. Brachycalyx is from SE Yunnan to central Honshu, but not extending to S Asia, and Changping of Beijing (China) is a disjunctive plot. In Sect. Tsutsusi, the primitive series, Ser. Tsutsusi, has two evolutionary trends. One evolved the deciduous group (Sect. Brachycalyx), and the other evolved Ser. Mariana, which have tubular-infundibular corolla. Ser. Seniaviniana was evolved from Ser. Mariana. Based upon the analysis of the primitive group of Rhododendron, environments of S China, and fossils, the mountainous region of S China is considered as the most possible origin center of Subgen. Tsutsusi. The analysis of distribution patterns of species indicates that the mountainous region of S China and central to south region of Japan are the two centers of differentiation. The four possible dispersal routes are proposed as well, and the eastward route may be the mainest one and the northward route is weakest.
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