Background and aim <p>Silica accumulation varies considerably among plant species and depends on the uptake mechanism they possess. Some species use a passive mechanism, in which silicon is transported with the transpiration stream, while others employ an active mechanism involving specific transporters. The lycophyte genus <i>Selaginella</i>, which originated 383 million years ago, can accumulate up to 10% of its dry weight as silica, although whether these plants absorb silicon solely through the transpiration stream or also possess an active uptake mechanism remains unclear. </p> Methods <p>This experimental study evaluated the effects of two factors, transpiration and silicon concentration in the irrigation solution, on silica accumulation and phytolith formation in the leaves of <i>S</i>. <i>horizontalis</i> (C. Presl) Spring.</p> Results <p>The results indicate that the accumulated silica was higher in plants with high transpiration and in those receiving irrigation solutions with higher silicon concentrations. The observed accumulation of silica per unit of leaf mass exceeded the levels expected by passive transport alone in all treatments, reaching up to five times more than predicted. The phytoliths observed on the leaf surface were similar in size, shape, and distribution among treatments, but differed in silica content.</p> &#xa0;Conclusions <p>These results suggest the coexistence of a passive mechanism dependent on transpiration and an active mechanism independent of environmental factors in an early diverging plant group.</p>