根据树冠形状、针叶形状、脱落性小枝在侧枝上的排列与着生方式、针叶在脱落性小枝上的排列与着生方式、生长期叶色、秋冬叶色、干色、生长特性和是否结球果等 9 类定性指标以及脱落性小枝长度和宽度、针叶长度、叶夹角角度和脱落性小枝上每厘米的叶片数等 5 个定量指标,对 27 个落羽杉属( Taxodium Rich.)树种(包括种类、栽培变种及杂种)外部形态特征的变异状况进行了观察和比较,在此基础上进行了聚类分析,并根据外部形态特征编制了供试 27 个树种的检索表。 研究结果表明,不同杂种的外部形态特征既综合了亲本的形态特征,也表现出一定程度的变异,其中,9 类定性指标中,秋冬叶色和生长特性等指标变异较大,针叶形状及脱落性小枝在侧枝上的排列与着生方式等指标变异较小。 27 个树种的 5 个定量指标差异均极显著( P<0. 01),脱落性小枝长度为 4. 68 ~ 11. 19cm、宽度为 0. 59 ~ 2. 38 cm;针叶长度为 0. 84 ~ 1. 84 cm;叶夹角的角度为 24. 1毅 ~ 52. 9毅;脱落性小枝上每厘米的叶片数为 6. 3 ~ 16. 0 片。 通过聚类分析可将 27 个树种分成 5 组:落羽杉[T. distichum (L.) Rich.]、墨西哥落羽杉(T.mucronatum Tenore)和中山杉 9 分别各自独立成组,池杉(T. ascendens Brongn.)、池杉栽培变种‘Nutans’、中山杉 91、中山杉 102 和中山杉 401 聚为一组;落羽杉栽培变种‘Pendens’和‘Fastigiata’、中山杉 1、中山杉 24、中山杉 27、中山杉 46、中山杉 86、中山杉 118、中山杉 136、中山杉 140、中山杉 146、中山杉 149、中山杉 302、中山杉 405、中山杉 406、中山杉 407、中山杉 501、中山杉 502 以及中山杉 503 聚为一组。 研究结果显示,基于外部形态变异的聚类分析结果能在一定程度上反映落羽杉属种类、栽培变种及杂种间的亲缘关系。

According to nine qualitative indexes including canopy shape, needle shape, arrangement and attachment modes of deciduous sprig on lateral branch, arrangement and attachment modes of needle on deciduous sprig, leaf color in growing stage, leaf color in fall and winter, trunk color, growth characteristics and having cone or not, as well as five quantitative indexes including length and width of deciduous sprig, needle length, leaf angle and leaf number per centimeter on deciduous sprig, the variation status of external morphological characters of twenty-seven tree species ( including species, cultivars and hybrids) of Taxodium Rich. were observed and compared. On the basis, the cluster analysis was carried on, and a key of the twenty-seven tree species of Taxodium was compiled based on the external morphological characters. The results show that different hybrids not only possess parent’ s external morphological characters, but also vary in some degrees. In nine qualitative indexes, variation of  some indexes (such as leaf color in fall and winter, growth characteristics, etc.) is greater, but that of needle shape, arrangement and attachment modes of deciduous sprig on lateral branch, etc. is smaller. The five quantitative indexes among twenty-seven tree species have significant differences (P<0. 01) with
4. 68-11. 19 cm of deciduous sprig length, 0. 59-2. 38 cm of deciduous sprig width, 0. 84-1. 84 cm of needle length, 24. 1°-52. 9° of leaf angle and 6. 3 -16. 0 of leaf number per centimeter on deciduous sprig. The twenty-seven tree species have been divided into five groups by cluster analysis: T. distichum (L.) Rich., T. mucronatum Tenore and T. ‘Zhongshansha 9’ are independent groups, respectively; T.ascendens Brongn., T. ascendens ‘ Nutans’, T. ‘ Zhongshansha 91’, T. ‘ Zhongshansha 102’ and T.‘Zhongshansha 401’ are clustered together; T. distichum ‘ Pendens’, T. distichum ‘ Fastigiata’, T.‘Zhongshansha 1 ’, T. ‘ Zhongshansha 24’, T. ‘ Zhongshansha 27 ’, T. ‘ Zhongshansha 46 ’, T.‘Zhongshansha 86’, T. ‘ Zhongshansha 118’, T. ‘ Zhongshansha 136’, T. ‘ Zhongshansha 140’, T.‘Zhongshansha 146’, T. ‘ Zhongshansha 149’, T. ‘ Zhongshansha 302’, T. ‘ Zhongshansha 405’, T.‘Zhongshansha 406’, T. ‘Zhongshansha 407’, T. ‘Zhongshansha 501’, T. ‘ Zhongshansha 502’ and T.‘Zhongshansha 503’ are clustered as another group. It is suggested that the result of cluster analysis based on external morphological variation can reflex the relationship among species, cultivars and hybrids of Taxodium to a certain extent.