From an examination of data as to the relative sizes of growing stems and roots in cotton, pea, carrot, turnip, and in other plants and of leaf lamina and petiole in Tropaeolum majus, it is concluded that the relative sizes conform to the equation, x=cy k, where x and y are the sizes of the organs, c is a constant expressing their relative initial sizes and k is a quantity expressing the ratio of their logarithmic growth rates and the spatial relationships of their meristems. This quantity usually approaches unity for stems and roots. It is low in carrot and turnip, where there is volume growth of the root, and high in etiolated peas, where there is an abnormal type of apical growth in the shoot.