1. Roots of Vicia faba, on the horizontal clinostat revolving at velocities of 1 revolution per hour and 1 revolution per 2 hours, grew faster than vertical roots. 2. When roots were placed in a horizontal position at rest the upper side grew faster than vertical roots during the first hour. The growth rate of the lower side was not significantly different from the growth of vertical roots during the first hour. After that period the growth of the lower side decreased markedly. 3. It is suggested that the growth-increase of roots on the horizontal clinostat is caused by the fact that each root cell is at the upper side for a sufficiently long time to react with accelerated growth, but that this period is too short to inhibit the growth of the root cells when these are at the lower side.