云南腾冲上新统Litsea cf.chunii的角质层微细构造特征

在云南省腾冲县上新统芒棒组发现了保存较好的被子植物压型化石高山木姜子(相似种)Litseacf.chunii Cheng.该化石叶片呈窄椭圆形,基部楔形,顶端短尖,叶全缘,中脉直,二级脉羽状,与中脉夹角为30°～50°,近叶边缘微弯曲,羽状脉。进一步分析了化石叶片的角质层微细构造,利用叶结构结合表皮特征的分析方法提高了对化石植物分类鉴定的准确性。化石叶片的角质层特征主要表现为:上表皮无气孔器,表皮细胞多为四或五边形,排列呈规则的网格状,垂周壁直或微弯,平周壁光滑,毛基较少;下表皮细胞排列同上表皮,具平列型气孔器,其长轴无定向性,气孔器由下陷的保卫细胞和两个上拱的副卫细胞组成,具有单细胞毛基,根部由5～6个辐射状的细胞组成。同现生高山木姜子Litseachunii相比,化石叶片具有更小的毛基密度,推测毛基的增加是由于气候变冷所致。由此通过化石和现生叶片角质层的对比分析推断滇西地区上新世气候比现在温暖湿润。

Cuticular microstructure of Litsea cf. chunii from the pliocene of Tengchong, Yunnan province

A fossil leaf identified as Litsea cf. chunii Cheng was discovered in the late pliocene sediments of the Mangbang formation in Tengchong county, Yunnan province, China, which was well preserved with gross morphological structure and microstructure of cuticles in situ. The fossil leaf is narrowly elliptic, base cuneate,with obtuse apex and entire leaf margin. Its primary vein is straight and moderately thick; secondary veins are pinnate, which arise from the primary vein at angles from 30°～50°, and they are slightly curved near the leaf margin. Additionally, the anatomical characteristics of the present specimen are investigated as follows:the upper epidermis is free from stomata, composed of mostly tetragonal, pentagonal or slightly elongated cells, which are arranged in regular areola; anticlinal cell walls are straight and moderately thick, periclinal walls are smooth and slightly concave; trichome bases are rarely found. The lower epidermis is also smooth,with paracytic stomata; stomatal complex has sunken guard cells embedded below the overarching subsidiary cells with two cuticular ledges occurring between them; unicellular trichome bases can be found over the veins or in the areolas, surrounded by 5～6 radiating cells. It is more credible to decide the taxonomic position of fossil plant using leaf architecture and cuticular characteristics together. Compared with the modern leaf Litsea chunii, the fossil species has fewer trichome bases than the modern one. Consequently it was concluded that the increase of trichome bases resulted from their being in a cooler climate. Thus, based on the comparison in cuticular characteristics between the fossil and the modern leaves, it can be inferred that the climate was warmer and more humid in the pliocene of western Yunnan than the present.