Figure 3.opju from Size always matters, shape matters only for the big: potential optical effects of silica bodies in <i>Selaginella</i>

Silica bodies are commonly found in <i>Selaginella</i>, 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 <i>Selaginella</i> 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 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 <i>Selaginella</i> 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.硅体（silica bodies）广泛分布于卷柏属（Selaginella）植物中，但其具体功能仍未明确。多数物种中，硅体呈现透镜状外观，且定位于背表皮细胞（dorsal epidermal cell）的巨型叶绿体（chloroplast）上方，这提示其可能具备光学功能。本研究通过显微成像技术，对3种卷柏属植物的硅体形态进行了分析，并借助波动光学模拟（wave-optic simulations）评估了其光学效应。研究发现，不同物种中存在两类硅体：一类为大型凸面、近似半球形（乳头状）的硅体，另一类为小型近似圆锥形（凹凸形）的硅体。两类结构均可在背表皮细胞内形成高度集中的光斑：大型凸面硅体通过折射（refraction）作用，以形态依赖性方式将光线聚集10~25倍；小型硅体则通过衍射（diffraction）作用，以形态非依赖性但波长依赖性的方式聚光（红光约2.3倍，蓝光约1.5倍）。由于叶绿体可发生运动，这种集中的光斑在强光下会位于叶绿体上方，而在弱光下则处于叶绿体内部。在光斑之外，光线会弥散至叶绿体中。卷柏属叶片较为薄嫩，上述光学效应或可增强光线捕获效率并最大限度减轻光损伤（photodamage），但硅体的其他功能，如抑制植食性、提供机械支撑以及参与免疫应答等，也需纳入考量。硅体无疑具备光学效应，但其对植物整体生理功能的实际意义，仍需通过直接的生态生理性能研究加以验证。