PHYSIOLOGICALLY ACTIVE GASES 141 



plant organs respond to very low concentrations of ethylene, the concentra- 

 tion may vary many per cent without a noticeable difference in response. 

 The authors also spoke of the toxicity of ethylene. Later work has showTi 

 that ethylene is not a highly lethal chemical, that is, tissue-killing, but 



Figure 51. Response of the etiolated epicotyl of garden pea to ethylene. Seedlings 

 3 to 4 cm high exposed to ethylene for 3 days room temperature. A {left to right) : control, 

 1 ppm ethylene, 1.33 ppm ethylene, 4 ppm ethylene. B: (left) control; (right) 1.33 ppm 

 ethylene. 



rather an anesthetic and a growth modifier. The failure of the buds to grow 

 was due partly to anesthetic action, and the closing of the flowers was an 

 irreversible gro^\i:.h response called hyponasty. 



As ethylene is hard to detect chemically in the low concentrations that 

 produce plant responses, Knight and Crocker ^^ attempted to find plant 

 responses that would serve as reliable and delicate tests for traces of ethyl- 

 ene. They tried various sorts of peas as test plants and found certain varie- 

 ties of sweet peas, Gladys Umvin, for example, even more sensitive than 

 garden peas. One-tenth ppm of ethylene inhibited the elongation of the 



