Mentha (mint) is a genus in the Lamiaceae with a worldwide distribution. It has a complex classification due to frequent hybridisation at the interspecific level, variation in basic chromosome number and the occurrence of polyploidy (diploid to nonaploid). Although there have been many studies of Mentha leaf micromorphology, usually only a few taxa were described. The aim of this study was to characterise the micromorphology of Mentha leaves. Nineteen Mentha taxa, covering all four sections of the genus, were grown under controlled conditions and adaxial and abaxial leaf surfaces were examined using stereo and scanning electron microscopes. This study included examination of the previously uninvestigated Australian species, M. australis and M. diemenica. The study revealed that average lamina length varied from 3 mm (M. requienii) to 34 mm (M. × niliaca) and leaves were sessile (M. spicata) to where petiole length was 50% of total leaf length (M. requienii). Peltate and capitate glandular trichomes were found on the adaxial and abaxial leaf surfaces of almost all taxa. Most taxa were hypostomatous. A few taxa had amphistomatous leaves which was interesting given that Mentha is a mesophytic genus naturally found in moist environments beside streambanks and lake shores. Average guard cell length varied from 14 µm (M. suaveolens) to 27 µm (M. × piperita f. citrata ‘Basil’) with larger guard cell length correlated with larger DNA content and chromosome number. Two species in section Pulegium (M. requienii and M. pulegium) had small laminas, relatively long petioles and high adaxial stomatal density which distinguished them from taxa in the other three sections. Larger DNA content in plants can be associated with larger cell size. Most studies of Mentha leaf micromorphology make no mention of ploidy. The present study indicates this should be considered when comparing relative cell size between species.

中文翻译：

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19种薄荷类群的叶片微形态学

薄荷（薄荷）是唇形科的一个属，分布于世界各地。由于种间水平的频繁杂交、基本染色体数目的变化和多倍体（二倍体到非倍体）的发生，它具有复杂的分类。虽然对薄荷叶的微形态学进行了很多研究，但通常只描述了几个分类群。本研究的目的是表征薄荷叶的微形态。在受控条件下生长了 19 个薄荷类群，覆盖了该属的所有四个部分，并使用立体和扫描电子显微镜检查了正面和背面的叶面。这项研究包括对以前未调查的澳大利亚物种 M. australis 和 M. diemenica 的检查。研究表明，平均叶片长度从 3 毫米（M. requienii）到 34 毫米（M. requienii）不等。× niliaca) 和叶子是无柄的 (M. spicata)，其中叶柄长度为总叶长 (M. requienii) 的 50%。在几乎所有分类群的正面和背面叶面上都发现了盾状和头状腺毛。大多数分类群是下口的。一些分类群具有两栖性叶子，这很有趣，因为薄荷是一种天然存在于河岸和湖岸旁潮湿环境中的中生植物。平均保卫细胞长度从 14 µm (M. suaveolens) 到 27 µm (M. × piperita f. citrata 'Basil') 不等，保卫细胞长度越大，DNA 含量和染色体数量越大。Pulegium 部分中的两个物种（M. requienii 和M. pulegium）具有较小的叶片，相对较长的叶柄和较高的正面气孔密度，这将它们与其他三个部分中的分类群区分开来。植物中较大的 DNA 含量可能与较大的细胞大小有关。大多数薄荷叶微形态研究没有提到倍性。本研究表明，在比较物种之间的相对细胞大小时，应考虑这一点。