Phytoliths, often called “plant stones,” are microscopic structures created within plants from silica absorbed from the soil. They form inside and around the cells of living plants, creating a durable record of the plant’s structure. These tiny mineral deposits can persist long after the plant itself has decayed.
How Plants Create Phytoliths
The formation of phytoliths begins when a plant’s roots absorb water from the ground containing a form of silica called monosilicic acid. This soluble silica is then transported throughout the plant’s body via its vascular system, the same network that carries water and nutrients to the stems and leaves.
As water moves through the plant and is used for metabolic processes or evaporates from the leaves, the concentration of silica increases. This leads to the precipitation of solid, non-crystalline silica, also known as opal, in and between the plant’s cells. The silica deposition creates a cast of the cellular structure, providing structural support to the plant tissues.
This biomineralization process occurs in a wide variety of plants but is particularly prominent in grasses. The amount of silica a plant takes up can vary, sometimes accounting for up to 10% of the plant’s total dry weight, depending on the species and soil conditions. This solidified silica fortifies the plant against stressors, from insects to extreme temperatures.
A Microscopic Record of the Past
When plants die and decompose, their organic parts break down, but the durable phytoliths are left behind. Composed of inorganic silica, they resist decomposition and can remain preserved in soil, sediments, and on artifacts for millions of years. Because they are inorganic, they can survive in conditions where materials like pollen or seeds would be destroyed, making them a stable source of information.
Archaeologists rely on these microscopic remnants to reconstruct ancient environments and human behaviors. By analyzing phytoliths from archaeological sites, researchers can identify the types of plants that grew in an area, providing a picture of the past landscape. This evidence helps track the development of agriculture by identifying domesticated crops like maize or rice from residues on ancient pottery or stone tools.
Beyond archaeology, phytoliths are a tool in paleoecology. They are recovered from fossilized dental plaque and animal dung, revealing the diets of extinct animals. This helps scientists understand ancient food webs and animal adaptations. Analyzing phytoliths in sediment cores allows researchers to track major ecological shifts, such as the global expansion of grasslands.
Decoding Phytolith Shapes
The scientific value of phytoliths comes from their distinctive shapes. Because the silica solidifies within and around the plant’s cells, it takes on their shape, creating a microscopic replica. These shapes can be so specific that they allow scientists to identify the family, genus, and sometimes the species of the plant that produced them.
Different parts of a plant produce different shapes. For example, grasses are known for producing dumbbell-shaped phytoliths, while woody plants might produce more spherical or blocky types. The surfaces of leaves can create phytoliths with wavy or intricate patterns that are characteristic of certain plant groups.
To accurately identify ancient phytoliths, scientists build extensive modern reference collections. This involves collecting living plants, extracting their phytoliths, and documenting their shapes under a microscope. When researchers find phytoliths in a sample, they can compare them to this reference library to determine their origin. This comparative process is how the microscopic shapes are translated into a larger story about past ecosystems and human activities.