Biomineralization plays a fundamental role in the global silicon cycle. Grasses are known to mobilize significant quantities of Si in the form of silica biominerals and dominate the terrestrial realm today, but they have relatively recent origins and only rose to taxonomic and ecological prominence within the Cenozoic Era. This raises questions regarding when and how the biological silica cycle evolved. To address these questions, we examined silica abundances of extant members of early-diverging land plant clades, which show that silica biomineralization is widespread across terrestrial plant linages. Particularly high silica abundances are observed in lycophytes and early-diverging ferns. However, silica biomineralization is rare within later-evolving gymnosperms, implying a complex evolutionary history within the seed plants. Electron microscopy and X-ray spectroscopy show that the most common silica-mineralized tissues include the vascular system, epidermal cells, and stomata, which is consistent with the hypothesis that biomineralization in plants is frequently coupled to transpiration. Furthermore, sequence, phylogenetic, and structural analysis of nodulin 26-like intrinsic proteins from diverse plant genomes points to a plastic and ancient capacity for silica accumulation within terrestrial plants. The integration of these two comparative biology approaches demonstrates that silica biomineralization has been an important process for land plants over the course of their >400 My evolutionary history.phytolith | fern | lycophyte | silicon | aquaporin I n modern ecosystems, land plants play a major role in the silica cycle through the accumulation and synthesis of amorphous biominerals composed of SiO 2 , known as phytoliths or silica bodies. It is widely appreciated that actively accumulating plants such as grasses are important components of the terrestrial biological pump of silica (1-3). Plant silica also plays a key role in connecting the terrestrial and marine carbon cycles, because silica is an important nutrient for marine silica-biomineralizing primary producers (i.e., diatoms) (1, 2, 4-7). However, both grasses and diatoms evolved in the latter part of the Mesozoic Era (8-10) and rose to ecological dominance within the Cenozoic Era (6,9,(11)(12)(13)(14). Determining precisely when and how the terrestrial-marine silica teleconnections evolved remains an obstacle to reconstructing the history of the silica cycle.Direct analysis of silica bodies in the fossil record provides limited insight into this problem. When fossiliferous material is macerated, it is often challenging to identify whether residual silica bodies are the result of primary biomineralization or secondary diagenetic processes, and if a living plant origin is suspected, it is often difficult to assign taxonomic identity to the phytolith producer. In addition, with rare exceptions (e.g., ref.15), lagerstätten that preserve exceptional anatomical detail in fossils, and might therefore be expected to preserve silica bodies, tend to be over...