146 



GROWTH OF PLANTS 



produces coils in some of the petioles. In narcissus only the young leaves 

 respond, and the inequality of growth on the two faces of the leaves is 

 sufficient to produce coils at the tips of the leaves. The epinastic response 

 of tomato leaves has already been shown in Figs. 52 and 53, and it will be 

 shown for other plants in later figures. 



Figure 55. A, Marigold plants. Left: control. Right: in 1 part of ethylene to 1 billion 

 parts of air for 20 hours. B, Potato plants. Left: control. Right: in 1 part of ethylene to 

 300 million parts of air for 24 hours. 



Since leaf epinasty is a growth response, it is most easily induced in 

 thrifty plants growing under optimum conditions; but there is a great dif- 

 ference in the minimum concentration of ethylene or other effective gases 

 needed in the air to induce leaf epinasty in various kinds of plants. The 

 tomato is moderately sensitive. The leaves of thrifty plants in good growth 

 condition will respond to 1 part of ethylene to 10 to 20 million of air. In 

 old leaves under long exposure epinasty will occur in 1 to 25 million or less 

 of ethylene (Fig. 59). As is shown in Fig. 55, leaves of the young potato 

 plant respond to 1 part of ethylene to 300 million of air, and the African 

 marigold leaves to the extreme dilution of 1 part of ethylene to 1 billion of 



