<p>1. Solid biosilica (phytoliths) deposited in plant tissues is thought to function as mechanical support, as a cost-effective alternative to lignin, and in herbivore defence, by limiting nutrient access/extraction and abrading herbivore mouthparts. It has been assumed that active phytolith accumulation evolved for these purposes, but this hypothesis remains untested. For example, an influential idea holds that grasses became so silica-rich through antagonistic coevolution with mammalian grazers during the Cenozoic. 2. We examine whether phytoliths fulfil criteria established for adaptations, focusing on three aspects. First we evaluate the recent debate concerning whether plant silica wear herbivore mouthparts/teeth. Second, we test whether the evolutionary pattern of phytolith accumulation is consistent with adaptive hypotheses by mapping silica content onto time-calibrated land plant and grass phylogenies. Third, we compare with paleontological evidence for the timing of the &lsquo;demand&rsquo; for the hypothesised function (support, herbivore defence). 3. Our survey demonstrates that phytoliths meet several adaptive criteria, but key aspects require further study. For example, phytoliths wear teeth but are likely less important than dietary grit, suggesting that silica deterrence is ineffective against large mammalian grazers. 4. Mapping analysis indicates that active silica accumulation evolved numerous times, rather than being ancestral in land plants. However, a clear temporal link between these events and hypothesised functional &lsquo;demands&rsquo; is still missing. For example, we find no convincing evidence for Cenozoic grass-grazer coevolution. 5. Synthesis: Phytoliths help support and defend plants today, but the adaptive origin of this trait requires further testing. Such tests should integrate the phylogenetic distributions of phytoliths with ecology and biomechanics, and use fossil evidence to evaluate the correlation between functional &lsquo;demand&rsquo; and plant evolution.</p>