PHYSIOLOGICALLY ACTIVE GASES 



163 



ethylene, propylene, and acetylene, like CO, were found to induce rooting. 

 These gases were effective in lower concentrations than CO, as has been 

 found for other plant responses, but the minimum root-inducing concen- 

 tration was not determined for any of them. In some plants roots were 

 induced along considerable stretches of the stem, as shown m Fig. 61 for 



Figure 63. African marigold to show the effect of acetylene on orientation of roots 

 to gravity and on formation of root hairs. Left: normar growth of roots six days after 

 a three-day exposure to the gas. Right: the same roots after a 48-hour exposure to 

 0.25 per cent acetylene. Root hairs were induced and the roots changed their direction 

 of growth. 



