Silica bodies of Selaginella erythropus: Detection, morphology and development

Abstract

Selaginellaceae (Lycopodiophyta), an early divergent group of vascular plants, are a species-rich and taxonomically challenging family. Here we study silica bodies on the leaves of a prominently studied Selaginella, S. erythropus, with various methods of microscopy and energy disperse X-ray analysis (EDX) to characterize their properties in mature and emerging leaves.To detect silica bodies, a light microscope or a fluorescence microscope can be used to reveal colorless silica body protrusions in the epidermal cell walls. Silica bodies show as light spots with scanning electron microscopy (SEM). However, the recent tabletop SEM featured with low accelerating voltage and backscattered electron detectors provides a better contrast than standard SEM. EDX shows that these protrusions on leaf surfaces are silica bodies and that trichome apices and some stomata also have silica deposits. Atomic force microscopy provides three-dimensional images and quantitative measurements, but with an upper limit on silica body size. Under transmission electron microscopy silica bodies are characterized as compact electron-dense particles, gradually thinning and mingling with the cell wall.These analyses recognized two major forms of silica bodies, conical (small and large subforms) and sinuate-carinate. Distinct forms are found on different types of epidermal cells. Patterns on a leaf surface differ between the four types of leaf surface, dorsal and ventral surfaces of dorsal and ventral leaves, and may have taxonomic value.The silicification process on a young leaf begins from the central part. Large conical silica bodies appear first, along leaf margins and trichomes. Then, small conical silica bodies develop. Sinuate-carinate forms arise from the merger of large conical forms.A full understanding of the functional significance of silica bodies in Selaginella has yet to be developed, but we hope this report can help researchers detect silica bodies, and inspire more studies.