Spores of E. hyemale L., collected under sterile conditions were germinated aseptically in Bold''s Basal Medium plus supplementary micronutrient solution. Various Si concentrations, from 2.5 mg/l-1000 mg/l, were used to supplement the control medium. Gametophytes developed on all but the 1000 mg Si/l medium. Fertilization occurred after the cultures were flooded with sterile distilled water, followed by the development of the sporophytic generation. A concentration of 10 mg Si/l yielded the greatest number of sporophytes. Sporophyte dry weights correlated with Si concentration up to 5 mg/l. Higher concentrations gave no additional increase in dry weights. Best overall growth and development of sporophytes occurred in media containing 10-20 mg Si/l. A concentration of 20 mg Si/l gave the highest percentage of sporophytes with strobili. Strobili shedding viable spores were produced only by sporophytes growing on medium containing 20 mg Si/l, indicating that Si is a required nutrient for the completion of the life cycle. With the light microscope, anatomical differences were not discerned in sections of internodes of sporophytes grown in the various media. Scanning electron micrographs revealed increasingly larger papillae of Si on the surface of the internodal epidermis with increasing Si concentration. No spores germinated and no cultures grew on media containing Ti.