PHYSIOLOGICALLY ACTIVE GASES 167 



work of Crocker and Ivnight (1908) in answering the practical question on 

 the effect of artificial illuminating gas upon carnations. Denny's dis- 

 covery (1923) of the effectiveness of ethylene in coloring lemons was the 

 second great stimulus to work in this field. 



Qualitatively the four gases act similarly on plants; so far as tested, each 

 will produce the responses that any other produces provided they are all 

 used in the proper concentrations. Quantitatively they are very different; 

 the minimum effective concentration of ethylene is very much lower than 

 that of acetylene and propylene, and the minimum effective concentration 

 of carbon monoxide is much higher than that of the last two. In general, 

 the effectiveness can be expressed as ethylene > acetylene and propylene > 

 carbon monoxide. 



Ethylene modifies the geotropic equilibrium position of plant organs, 

 causing such changes as declination or horizontal nutation of pea seedlings, 

 leaf epinasty, and change in the orientation of roots. Leaf epinasty of the 

 tomato seedling and potato shoot has been used rather extensively in prac- 

 tice in detecting the presence of ethylene in the air and through it the 

 presence of traces of a number of gas mixtures, such as illuminating gas 

 and exhaust gases from internal combustion engines. Leaf epinasty has 

 been especially useful in scientific research for detecting the emanation of 

 ethylene from respiring plant tissues. The declination or horizontal nuta- 

 tion of the pea is the least sensitive of the tests. With a very sensitive 

 variety of sweet pea, using declination as the indicator, one might detect 

 0.2 ppm of ethylene in the air. With leaf epinasty of the tomato one can 

 detect easily 0.1 ppm of ethylene in the air. If one observed only the mature 

 leaves of the tomato and gave a long exposure, 0.04 ppm of ethylene could 

 be detected. The leaf epinasty of the potato furnishes a considerably more 

 delicate test than that of the tomato. Finally, leaf epinasty of the African 

 marigold is the most sensitive test kno^^^l for ethylene. It will detect 

 0.001 ppm of ethylene in air. On the basis of the small amount of the 

 chemical needed to induce the response, this is one of the most delicate 

 responses of an organism to a chemical kno^^^l to date. 



Ethylene causes cells that are dormant and would otherA^se not grow, to 

 enlarge and divide, forming soft, rather massive tissues. This is especially 

 true for lenticular and cortical tissue. Traces of ethylene ^\'ill even cause 

 cells with thickened walls to dissolve the walls and to enlarge and multiply. 

 The proliferation of tissue is induced by concentrations of ethylene as low 

 as 0.01 ppm in the air. 



Ethylene induces the abscission of leaves, flowers, petals, and fruits by 

 inducing flat cells of the abscission layer to enlarge and become spherical, 

 thus furthering organ drop. Hydrolysis of the insoluble protopectins of the 

 middle lamella of cell walls is also a factor in abscission. Ethylene is used 

 for defoliating plants when desirable in horticultural practice. This is 

 probably the response that makes ethylene useful in shucking English 

 walnuts and pecans. 



