山东中新世山旺组葛属(豆科)小叶的形态及其古生态学意义

Abstract

葛属Pueraria是豆科蝶形花亚科、菜豆族大豆亚族中最大的、具有三小叶复叶的属，其大多数种为攀援性藤本植物，分布于东亚、南亚、东南亚和大洋洲。化石记录表明，葛属早在中新世就已出现在东亚、巴尔干半岛和高加索地区的亚热带和温带植物群中，目前已知有3个化石种，即中国山旺中新世的荚果化石山旺葛藤P. shanwangensis、山旺和日本数个中、上新世产地的叶、小叶化石鲁葛藤P. miothunbergiana以及克罗地亚和格鲁吉亚阿布哈兹中新世的小叶化石大葛藤P. maxima。本文基于对中新世山旺组最近采集的葛属小叶印痕化石的观察和对现生种葛P. montana小叶的形态比较，研究了该属化石种的小叶形态和发育变异。结果表明，现生种葛与化石种鲁葛藤和大葛藤的小叶都具有不甚发育的间二级脉，这个特征在先前报道的鲁葛藤小叶化石中未见保存。另外，它们的主脉和二级脉远轴侧有时都会分别发出两条挨得很近、角度不同的二级脉和二级脉梳脉，这个特征过去在葛属中则被忽视了。总体上，葛属小叶化石的叶脉特征在中新世广阔的欧亚中纬度地区显示了高度的相似性。目前，仅在中国中新世山旺植物群和日本中新世高峰山组植物群中发现了与现生的葛小叶相似的、具有裂瓣的鲁葛藤小叶化石，但现生葛比化石葛的小叶更大些、似乎发育了更多具有裂瓣的小叶，这可能与中新世以来大气二氧化碳浓度的变化有关。现生葛长在荫蔽生境以及攀援于支持物(如藤架或其他木本植物)上的植株比生于开阔生境以及蔓生的植株发育了更多的、具有裂瓣的小叶，这可能有效促进了整株植物的叶片和冠层中的光照截取和通风散热。据此推测，中国和日本中、上新世的鲁葛藤居群可能比克罗地亚和阿布哈兹中新世的大葛藤居群的生态耐受性更加宽泛，东亚的居群既生于荫蔽的、更多依赖森林的生境中，也长在开阔的、较少依赖森林生境中，甚至蔓生。 Pueraria DC. is the largest papilionoid legume, trifoliolate genus of the subtribe Glycininae in the tribe Phaseoleae, the majority species of which are climbing lianas distributed in East Asia, South Asia, Southeast Asia, and Oceania. The known three fossil species of Pueraria described from the subtropical and temperate floras of the Balkan Peninsula, the Caucasus, and eastern Asia respectively are P. shanwangensis (fruit) from the Miocene Shanwang of China, P. miothunbergiana (leaf and leaflet) from the Miocene of Shanwang and numerous localities in the Mio-Pliocene of Japan, and P. maxima (leaflet) from the Miocene of Croatia and Georgian Abkhazia. On the basis of observations on the newly collected Pueraria leaflet impres-sions and comparisons with the leaflets of living P. montana, the morphology and developmental variation of Pueraria leaflet fossils are studied. The result shows that the leaflets of both living species P. montana and fossil species P. miothunbergiana and P. maxima bear poorly developed intersecondary veins, which were not observed in former reports on P. miothunbergiana. Also, two adjacent secondary veins or agrophic veins at different angles are sometimes diverged respectively from the primary vein (midvein) and the exmedial side of secondary veins in both extant and fossil Pueraria leaflets, which is a feature that has long been neglected. Overall, the venation of fossil Pueraria leaflets that are widely occurred across the Miocene of middle lati-tudes in Eurasia is highly similar, but the lobed leaflets similar to those of living P. montana are only discov-ered from the Miocene Shanwang flora of China and Takamine flora of Japan. Extant P. montana bears larger leaflets than fossil Pueraria and seems to have developed more lobed leaflets than fossil P. miothunbergiana does, which might have been related to the change of atmospheric CO2 concentrations since the Miocene onwards. Living individuals of P. montana growing in shady, closed habitats as well as climbing on supports (e.g., pergolas or other woody plants) develop more lobed leaflets than those inhabiting open habitats and trailing, which may efficiently enhance light interception and heat dissipation within leaves and canopies. It is inferred that populations of P. miothunbergiana lived in the Mio-Pliocene of China and Japan may have wider ecological tolerances than those of P. maxima occurred in the Miocene of Croatia and Abkhazia, so the eastern Asian populations may not only live in shady habitats more relied on forests, but also grow in open habitats less relied on forests or even sprawl.