PHYSIOLOGICALLY ACTIVE GASES 



143 



Table 18. Comparative Effectiveness of Gases in Producing Declination in Sweet Pea 

 Seedlings and Epinasty in Tomato Petioles. 



* 3 daj's' exposure used. 



t 2 days' exposure used. 



triple bond and chemically more active than ethylene, is much less effective 

 in inducing either response; more than 1200 times the concentration is 

 needed to induce the pea response and 500 times the concentration to 

 induce the tomato response. Other unsaturated short chain compounds ^ 

 such as allyl alcohol, CH2=CH— CH.OH, acrolem, CH2=CH— CHO, 

 and isoprene, CH2=CH — C(CH3)=CH2, did not induce these responses. 

 Allyl alcohol killed the tomato in 125 ppm of air and acrolein in 30 ppm 

 of air. Denny ^^ has found three out of 77 other volatile chemicals tested 

 that produce leaf epinasty: ethyl bromide, ethyl iodide, and propyl chloride. 

 Later we shall see that most so-called plant hormones induce this reaction. 

 The latter in general have low volatility and neither the three halides 

 Denny mentions nor the hormones are likely to be present at all, or at 

 least in a way that will interfere with the tests we describe below for ethyl- 

 ene and other effective unsaturated C-gases. 



Responses Induced by Gases Containing Unsaturated Carbon 



The four effective gases mentioned above induce the following responses 

 in plants: (1) epinasty of leaves; (2) proliferation of tissues; (3) abscission 

 of leaves, flowers, and fruits; (4) anesthesia and inhibition of gro'«i:h; 

 (5) coloring and other ripening changes in fruits; (6) other metabolic 

 changes in living plant tissues; (7) root and root-hair initiation; and 

 (8) other phj^siological effects. 



Epinasty of leaves. The effective gases cause epinasty in leaves by in- 

 ducing a rapid elongation in the upper side of the leaf or petiole. This is 

 sho\^■n in Fig. 53.^ In this case only the older basal leaves of the tomato 

 plants in which the base of the petioles had ceased to elongate were used. 

 The gas starts gro^vth again in the tissue on the upper side of the base of 

 the mature petiole. In young leaves in which the petiole is still growing 

 even at the base, the gas causes gro^\i,h on the upper side of the petiole 

 in excess of that on the lower side, and thus turns the leaf downward, the 

 curvature extending over a great part of the length of the petiole. After 



