Plants contain a variety of specialized structures, each serving a unique purpose. Among these are cystoliths, microscopic formations found in the cells of many plant species. These “cavity stones,” as their name suggests, are outgrowths of the plant cell wall.
What Are Cystoliths?
Cystoliths are mineralized bodies that develop as outgrowths from the epidermal cell wall. They are primarily composed of calcium carbonate, embedded within a matrix of cellulose and other polysaccharides. Some cystoliths also incorporate silicon.
These structures are housed within specialized, enlarged epidermal cells called lithocysts. Their shape can vary, but they are often described as spindle-shaped, club-shaped, or grape-like. They are microscopic, typically ranging from 65 to 300 micrometers in length.
Where Do We Find Cystoliths?
Cystoliths are not universally present across all plant species but are characteristic of certain plant families, primarily dicotyledonous angiosperms. They are commonly observed in the Urticaceae, known as the nettle family, though in some species, they form only during later flowering and seed-setting stages. The Moraceae, or mulberry family, including plants like figs and hemp, also frequently exhibit these structures.
The Acanthaceae family is another group where cystoliths are abundant and serve as a useful identification character. Within these plants, cystoliths are typically found in the vegetative parts, with a high concentration in the leaves and stems. In leaves, they can occur on both the upper (adaxial) and lower (abaxial) surfaces, and sometimes within specialized hairs called trichomes. They may be scattered across the leaf blade or arranged along veins.
Why Do Plants Produce Cystoliths?
Plants produce cystoliths for several functions, contributing to their survival and physiological regulation. One primary role is calcium regulation, as these structures serve as reservoirs for excess calcium. By sequestering calcium carbonate, plants can prevent calcium toxicity, which can occur when calcium uptake exceeds metabolic needs. This process helps in maintaining proper cellular calcium levels, which are important for various plant processes.
Cystoliths also provide a defense mechanism against herbivores. Their hardness and abrasive nature, due to the calcium carbonate composition, can deter grazing animals and insects from consuming the plant tissue. Studies have shown that herbivores, such as silkworms, prefer feeding on plant cultivars with a lower density of cystoliths, supporting their role in deterring predation.
Beyond defense, cystoliths may contribute to structural support for plant tissues. While often microscopic, their collective presence can add rigidity to leaves and stems. Additionally, there is evidence that cystoliths play a role in light scattering within leaves. Positioned in the epidermis, they can scatter incoming light more evenly throughout the photosynthetic tissue, the mesophyll. This redistribution of light can enhance photosynthetic efficiency, especially by directing light to deeper, less light-saturated parts of the leaf.