Size always matters, shape matters only for the big: Potential optical effects of silica bodies in Selaginella

Abstract

Silica bodies are commonly found in Selaginella, but their function is unclear. Lens-like appearance and location in many species above giant chloroplasts of dorsal epidermal cells suggest optical functions. Silica body morphology in three Selaginella species was studied by microscopy. Optical effects were assessed by wave-optic simulations. Large convex, approximately hemispherical (papillose) and small approximately conical (concave-convex) silica bodies were found in different species. Both types lead to a concentrated spot of light high in the dorsal epidermal cell. Large convex bodies concentrate light 10-25 times in a shape-dependent manner by refraction, and small silica bodies concentrate light in a shape-insensitive, but wavelength-dependent, manner by diffraction (red light: approx. 2.3 times; blue light: approx. 1.5 times). Due to chloroplast movement, this concentrated light is above the chloroplast under high light, but within it under low light. Beyond the spot of concentration, light is dispersed into the chloroplast. Thin Selaginella leaves mean these effects may enhance light capture and minimize photodamage, but other effects such as inhibition of herbivory, mechanical support, and immune responses need to be considered. Silica bodies undoubtedly have optical effects, but their significance to the functioning of the plant requires direct studies of ecophysiological performance.