Department of Botany, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan.
J Plant Res. 2015 Jan;128(1):161-75. doi: 10.1007/s10265-014-0672-9. Epub 2014 Nov 16.
Helwingia, a shrub or small tree of four species distributed in East Asia, has been assigned to various families, mainly Cornaceae. However, molecular analyses show that the genus belongs to its own family Helwingiaceae which is sister to Phyllonomaceae (Phyllonoma only) in the order Aquifoliales. On the basis of H. japonica, we investigated the poorly understood floral and embryological characters of Helwingia, and compared their features with those of other Aquifoliales, particularly those of Phyllonomaceae. Results showed that perianth leaves of Helwingia represent sepals, because in plesiomorphic pentamerous flowers, they agreed in position with sepals (not with petals) in pentamerous flowers of Phyllonoma. Overall comparisons based on available information show that, while sharing with Phyllonoma the epiphyllous inflorescence, the inferior ovary, and an epigynous disc nectary as syapomorphies, Helwingia is characterized by loss of petals, obhaplostemony, large recurved stigmas, poorly developed disc nectary, tenuinucellate ovules with a mature female gametophyte filled with densely stained cytoplasm, and a thin mature seed coat. Morphological evidence, like molecular evidence, confirms that Helwingia is sufficiently distinct to be placed in its own family. Morphological and field observations suggest wind and insect pollination in H. japonica, which is the first example of ambophily in Aquifoliales.
五列木属（Helwingia），一种分布于东亚的四个物种的灌木或小乔木，曾被分配到多个科中，主要为山茱萸科。然而，分子分析表明，该属属于其自身的科五列木科，与冬青科（仅 Phyllonoma）在冬青目（Aquifoliales）中互为姐妹群。基于五列木属（Helwingia）的模式种五列木（Helwingia japonica），我们调查了五列木属植物的花部和胚胎学特征，这些特征在水生或陆生植物中较为罕见，且难以研究。我们比较了这些特征与其他冬青目植物的特征，特别是冬青科（Phyllonomaceae）的特征。结果表明，五列木属的花被片代表萼片，因为在原始的五基数花中，它们与 Phyllonoma 的五基数花中的萼片（而不是花瓣）在位置上一致。基于现有信息的综合比较表明，虽然五列木属与 Phyllonoma 具有共同的附生花序、下位子房和上位花盘蜜腺等近裔特征，但它具有花瓣缺失、不完全离生雄蕊、大的反曲柱头、发育不良的花盘蜜腺、具薄珠被的十细胞珠心胚珠以及薄的成熟种皮等特征。形态学证据，如同分子证据一样，证实了五列木属的独特性足以将其置于自身的科中。形态学和野外观察表明，五列木属（Helwingia）的传粉方式为风媒和虫媒传粉，这是冬青目植物中首次发现的兼性传粉现象。
