1. Light and darkness appeared to have only an indirect effect, through their influence on growth. 2. High temperature with sufficient moisture tended to decrease hair production by increasing the elongation of the internal cells. 3. The slower the growth in air the better the hair development. 4. Retardation of growth by glass tubes, by wounding, or by resistance of the substratum favored hair production. 5. Roots of seedling corn in water first curled and produced hair, possibly because of the retardation of growth by the diminution of oxygen or its presence in the dissolved state. Later the roots grew straight and smooth, either on account of accommodation to the oxygen supply or because the gas was supplied by the aerial parts. 6. Saturated air with high temperature tended to suppress hair development (cf. 2). 7. Saturated soil tended to suppress hair in corn and wheat, but other factors must be considered when Elodea develops hair in the substratum. 8. Osmotic solutions gave very irregular results on...