Laetsch, W. M., and Winslow R. Briggs. (Stanford U., Stanford, Calif.) Kinetin modification of sporeling ontogeny in Marsilea vestita. Amer. Jour. Bot. 48(5): 369–377. Illus. 1961.—Sporelings of the fern, Marsilea vestita Hook. and Grev., were cultured aseptically with a wide concentration range of kinetin. The elongation of the primary and all succeeding roots was significantly inhibited by concentrations as low as 0.3 γ/l., and was almost completely inhibited at concentrations above 30 γ/l. Root inhibition by kinetin was a result of decreased cell division and was not influenced by the amount of available carbon in the medium. The primary leaves also were inhibited by kinetin, but only by higher concentrations. The relative numbers of organs present were altered by intermediate concentrations, with a rise in the number of roots per plant, and a corresponding decrease in the number of leaves. Sporelings maintained in these kinetin concentrations for long periods developed abnormal leaves and roots. The shoot apex lost its organization in higher concentrations and proliferated into callus. The histology of the callus is discussed. The kinetin effects were independent of light conditions. Various other growth‐active compounds were studied in combinations with each other and with kinetin for their effects upon sporelings. Interactions between these compounds were not apparent, and they neither substituted for nor modified the effects of kinetin. The use of kinetin as a tool in morphogenetic study is discussed.