化石及现生真蕨类植物的根系:形态演化及古环境意义*

根是植物的重要营养器官,对维持植物生理活动及生态系统物质循环和能量流动极为重要。真蕨类植物的根系以可预测的方式起源于枝,被称为枝生根,通常也被称为不定根。相比于对种子植物根系的广泛研究,对真蕨类植物根系的认识较为薄弱。通过广泛收集各类文献中有关真蕨类根系的信息,以及观察大量化石、现生植物标本,对若干化石及现生真蕨类植物根系的形态学特征进行了综述。在繁盛于中、晚泥盆世的早期真蕨类植物中,密集的枝生根着生于茎轴、或推测的根状茎、或主干膨大的基部上;根系的分枝罕见,若存在,则仅表现为等二歧式分枝;而在真蕨纲冠群植物中,枝生根密集或稀疏,根系呈单轴式分枝,侧根发育,可达2级甚至更多。真蕨类根系统的形态演化可采用“顶枝学说”进行解释: 真蕨类植物的根系具有一定的演化保守性,均为枝生根;早期真蕨类的原始根系可视为不分枝的或偶见二歧式分枝的顶枝束;经历顶枝束的越顶生长,侧根系统在真蕨纲冠群中演化出现。真蕨类根系的构型、形态、解剖结构、生物量分配等信息可在化石记录中得以揭示,这些信息可为探讨植物根系的演化及研究地史时期的植物—土壤系统提供重要依据。最后,简述了这方面研究的1个案例,即北京西山侏罗纪地层中的真蕨类根系化石及根系—土壤系统。

Roots of fossil and extant ferns: Morphological evolution and paleoenvironmental implications

Roots are important vegetative organs of plants,and play essential roles in plant physiology as well as in the nutrient cycle and energy flow of an ecosystem. Fern roots are known to arise from the shoots in a predictable pattern,and thus are called shoot-borne roots,or more commenly adventitious roots. In contrast to the extensive studies of seed plant roots,relatively little information is available on fern roots. Based on a compilation of the available data and our own observations on both fossil and extant fern specimens,this study presents a review of root morphology of several fossil and extant ferns. In the early ferns that flourished during the Middle-Late Devonian,dense shoot-borne roots are attached on stems,or inferred rhizomes,or on expanded bases of trunks;such roots seldom branch,but when they do they display an isotomous branching pattern. In the crown groups of ferns,however,shoot-borne roots may be either densely or sparsely arranged,and show a monopodial branching pattern with two or more orders of lateral roots. The morphological evolution of fern roots may be interpreted by“the telome theory”: shoot-borne roots seem to be evolutionary conservative;primitive roots in early ferns can be considered as telome trusses that occasionally branch isotomously;and lateral rooting systems in crown groups of ferns evolved through overtopping of telome trusses. Information on fern root architecture,morphology,anatomical structure,and allocation ratios is documented in the fossil record,and is of significance in understanding the evolution of plant rooting systems and plant-soil interactions over geological time. As a case study,fossils of fern roots and root-soil systems from the Jurassic of Western Hills,Beijing,are briefly introduced.