GROWTH. 183 



in all these cases, however, has been previously elaborated 

 elsewhere. 



321. Light is found to exercise a retarding influence 

 upon growth. A plant, for instance, in a window will 

 bend towards the light, because the cells on the side 

 nearest the window grow more slowly than those which 

 are shaded, thus causing curvature of the stem and 

 petioles. 



322. Gravitation also affects growth, as we know that 

 the stem and root, or axis of the plant, are usually in the 

 line of the radius of the earth at the place of growth. If 

 a seedling plantlet be laid with the stem and root 

 horizontal, the stem will curve upward and the root 

 downward in the endeavour to restore the vertical direction. 



323. The twining movement of the stems of many 

 plants is due to inequality of growth at successive points 

 in the sides of the stems. Leaves unfold from the bud 

 because the growth on the upper side at the time of 

 unfolding is more rapid than on the under side. These 

 movements are called nutations, and are not due to the 

 external action of light, but entirely to internal causes. 

 The movements of tendrils, however, are affected by 

 contact with the object which they grasp. 



