Silica Ghosts of Ancient Plants
As plants absorb water through their roots, dissolved silica (monosilicic acid) travels up the xylem and precipitates as solid amorphous silica within cells, cell walls, and intercellular spaces. These precipitates — phytoliths — take on the shape of the cells they fill: dumbbell-shaped bilobates from grass epidermis, spiny spheres from palm leaves, blocky polyhedrals from wood. When the plant decays, phytoliths persist in the soil for millions of years.
Morphotype Classification
Phytolith identification relies on shape. Grasses produce distinctive short-cell morphotypes: bilobates and crosses (Panicoideae, warm-climate C4 grasses), saddles (Chloridoideae, drought-adapted), and rondels (Pooideae, cool-climate C3 grasses). Forest trees produce globular granulate, blocky, and elongate forms. Palms produce large globular echinate (spiny spherical) phytoliths. The relative abundances of these morphotypes reveal the vegetation composition at the time of deposition.
The D/P Ratio and Forest-Grassland Balance
The D/P ratio — forest indicator phytoliths divided by the sum of forest and grass phytoliths — is the standard metric for reconstructing tree cover. Applied to sediment records spanning the last 20 million years, D/P reveals the late Miocene grassland expansion in dramatic detail: values shift from above 0.7 (closed forest) to below 0.3 (open grassland) between 8 and 5 million years ago across Africa, South America, and South Asia.
Advantages Over Pollen
Phytoliths complement pollen analysis with distinct advantages. They reflect local vegetation (deposited in situ rather than wind-transported), survive in oxidizing soils where pollen is destroyed, and are abundant in drylands and tropical sediments. Their main limitation is lower taxonomic resolution — most morphotypes are diagnostic only to family level, whereas pollen can often be identified to genus. Combined phytolith-pollen studies yield the most complete vegetation reconstructions.