142 



GROWTH OF PLANTS 



epicotyl of this pea measurably; 0.2 ppm caused slight declination, together 

 with much greater inhibition of growth; and 0.4 ppm caused still greater 

 reduction in elongation, a greater declination from the vertical, and a swell- 

 ing of the declined portion. This constituted the "triple response" of Knight 

 and Crocker. They proposed the use of either the declination or triple 

 response of the sweet pea seedling as a means of detecting the presence of 

 ethylene. 



Later, several workers 4, s, it, 35 y^q^yq sho\vn that ethylene causes a 

 modification in the relative rate of growth on the upper and lower sides 

 of the petioles of leaves of many kinds of plants so that the leaves curve 

 downward, giving the epinastic response. Fig. 52 shows this response in 

 the tomato plant. 



Figure 52. Tomato plants after exposure for 28 hours in various houses of a com- 

 mercial range suspected of gas injury. No. 1, a house in which Acacias showed almost 

 complete bud and leaf fall; No. 8, a house in which newly forced roses were showing 

 leaf fall; No. 7, a house in which roses were showing similar injury to No. 8; No. 6, a 

 house in which roses showed no injury. 



Besides ethylene, acetylene, and propylene, carbon monoxide (CO) and 

 perhaps butylene will induce the declination or the triple response in the 

 sweet pea seedling and the epinasty of leaves, but these three gases must be 

 used in much higher concentrations than ethylene to be effective. Table 18 

 shows the minimum effective concentrations of the several unsaturated 

 C-gases for inducing declination of the sweet pea seedling and epinasty in 

 the tomato leaf.^ The gases of the olefin series (ethylene, CH2=CH2, 

 propylene, CHaCH^CHz, and butylene, CHaCH^CH-CHs*) fall off 

 rapidly with increase in length of the chain. Acetylene, CH^CH, with a 



* Formula given is one of three isomers. 



