梧桐科梅蓝的研究兼论属的系统位置

A STUDY ON MELHANIA HAMILTONIANA IN RELATION TO THE SYSTEMATIC POSITION OF THE GENUS

梅蓝(Melhania hamiltoniana Wall.)是梅蓝属在中国的唯一代表,仅限于云南元江干热河谷一个极为有限的区域内;国外也间断出现于缅甸中部伊洛瓦底江河谷。梅蓝在形态特征和分布上是属内一个特殊的种类。梅蓝种子小,长1.5—2.5mm,子叶折扇状,2深裂;外种皮4—5层细胞厚,最外1—2层细胞显著伸长、变大,内种皮4—5层细胞厚;背腹叶,中脉维管束2束。叶柄维管束1束,圆形;气孔限于下表皮,气孔器为不等细胞型;木材管孔大小不均匀,由2或更多管孔组成复管孔,本薄壁组织极少;射线异型(Kribs IIB),具瓦形细胞;体细胞染色体数60,同小型染色体;花粉粒球形,64.7×62.7μm,2—1孔,外壁具刺。根据梅蓝及本属的其它资料,与梧桐科和锦葵科相关类群比较,除外部形态的个别特征以外,梅蓝属的外部形态、解剖、花粉形态及染色体基数等综合特征支持梅蓝属应该保留在梧桐科Dombeyeae族,不宜移至锦葵科。

Melhania hamiltoniana Wall. is distributed in extremely limited dry and hot areas of Yuanjiang River (Red River) valley in Yunnan and Irrawaddy River valley in Burma. Its distribution pattern is of interest for further floristic and original study. The systematic position of the genus Melhania Forsk. has been debatable. It is traditionally placed in the Sterculiaceae, but recently some systematists (e.g., Airy Shaw, etc.) considering that its familial placement may be put in the Malvaceae. The morphology, anatomy of leaf, petiole, wood and seed coat of M. hamiltoniana Wall, were investigated. The chromosome number of 2n = 60 for this species is reported for the first time. The pollen grains were investigated with light microscopy and scanning electron microscopy. As a result, the anatomical, cytological and palynological descriptions are presented. A number of characters, such as axillaryraceme with 3-4 (5) flowers, the xerophytic habitat, the occurrence of tile cells in the wood rays, imply that this species probably is a relatively isolated one in Melhania. Comparisons with related taxa in the Malvaceae and in the Sterculiaceae have been made in order to ascertain the affinity of the genus Melhania. With regard to gross morphology, its dithecal anthers, 2-partite cotyledons, 5 stamens with 5 staminodes and the fusion of stamens and staminodes at the base, recall the Dombeyeae of the Sterculiaceae, but the 3 large, persistent epicalyces recall the Malvaceae. The anatomical structure of petiole, wood and seed coat resembles that of certain Sterculiaceae but differs from that of the Malvaceae in a number of characteristics, such as the arrangement of petiolar vascular bundles, the pattern of wood pore groups, the size of pores, the ray types, and the thickness of seed coat, etc. Based on the analysis of the first reported chromosome number 2n = 60 and the formerly reported number n = 30, it is suggested that the basic chromosome number of Melhania be x = 10. This basic chromosome number is accordant with the main basic chromosome number of the Sterculiaceae but different from the Malvaceae with basic chromosome number x = 7, 13 or 11-20. Furthermore, pollen grains of Melhania, which are spheroidal, 3- (1)-porate, with spiny exine, are almost identical with those of the Dombeyeae of the Sterculiaceae but disagreed with those of the Malvaceae in aperture numbers. Conclusively the placement of the genus Melhania Forsk. in the Sterculiaceae is supported by the data from the grounds of gross morphology, anatomy, cytology and palynology.