144 GROWTH OF PLANTS 



leaves have responded in gas and the plants are put in gas-free ah, the 

 young leaves recover their former position completely, and the older leaves 

 partially, by growth on the lower side, or hyponastic growth. Leaves of 

 plants rotated on a horizontal clinostat show much less epinastic response 

 to ethylene than do leaves of plants in a stationary, upright position; and 

 leaves of inverted plants show little or no epinastic response to ethylene. 

 With the plant upright in air, the position of the leaf is determined mainly 

 by its geotropic equilibrium position, but in part also by autonomic epi- 



FiGURE 53. Tomato plants showing change in length of upper faces of petioles when 

 sealed in Wardian cases for 24 hours: left, in air; right, in 1 part of Yonkers gas to 10,000 

 of air. 



nasty; ^ in some leaves light plays a part. With a little ethylene in the air, 

 either the geotropic equilibrium position is changed or the geotropic 

 effect is so weakened that autonomic epinasty plays the main role in leaf 

 position. In some leaves no doubt geotropism is still a factor in determin- 

 ing the ethylene position of the leaf, while in others autonomic epinasty is 

 the main determinant. We shall see later that petioles of some leaves in 

 the presence of ethylene continue to grow on the upper side until they 

 actually form coils. In such cases it would seem that autonomic epinasty 

 and not geotropism is the determining factor. 



Out of 202 species and varieties of plants tested,^ 89 gave leaf epinasty 

 in the presence of ethylene and 113 did not. Fig. 54 shows the type of 

 response in four different plants. In Fuchsia the petioles decline somewhat, 

 but the blade shows the greatest epinastic curvature. In buckwheat and 

 sunflower the gro^^'th on the upper side of the petiole is so extreme that it 



