LATERAL GEOTROPISM OF TWINING PLANTS 85 



The curvature here must be ascribed to the activity of the 

 cambium of the lower side of the branch, which becomes the con- 

 vex surface of the organ. 



131. Diageotropism of Flowers of Narcissus. Cut off some 

 stalks of Narcissus poeticus the flower buds of which have just 

 opened and place some in a horizontal position, and others in a 

 vertical position. Or simply lay a pot containing the plants on 

 the side. Either preparation should be placed to receive all of 

 the light reaching it, from one direction only. Note the move- 

 ment of the peduncles. These curve to place the calyx tubes 

 in a horizontal position. What direction do they take with re- 

 spect to the light? Note torsions as in last experiment (See 

 carpotropism). 



The geotropic stimulus induces the curvature toward the hori- 

 zontal position, but it will be found that the direction in which 

 this curvature is made will be determined by the rays of light, 

 the tubes pointing toward the source of the rays. Observe the 

 behavior of a specimen placed in a dark chamber. 



132. Lateral Geotropism of Twining Plants. A large number 

 of species of plants have acquired the habit of lifting their leaves 

 into the sunlight by twining their slender stems around other 

 plants, or any convenient support. In order to accomplish this 

 it has been necessary for them to develop a special form of re- 

 action to gravity, which may be termed lateral geotropism. By 

 this form of irritability the growing tip of the inclined stem of a 

 twiner is stimulated to curve horizontally. The curving portion 

 is directly connected with the firmer internodes below, and the 

 flexion is necessarily accompanied by a rotation of the axis of the 

 apical portion of the stem thus exposing a new region to the 

 stimulus of gravity and setting up a new curvature, which causes 

 the stem to be stimulated to curve in a slightly different plane, 

 and this repeated change of position and rotation induces a con- 

 tinuously altered geotropic response that sweeps the tip around 

 in an irregular circular manner. This action may be imitated if 

 a section of heavy rubber tubing a few cm. in length is held in the 



