PHYSIOLOGICALLY ACTIVE GASES 



151 



plant recovered A\'ithout injury when the gas was removed. In proper con- 

 centrations of ethylene, Mimosa pudica was unable to respond to the con- 

 tact or heat stimulus; it recovered after removal from the gas, but showed 

 some later injury. Neljubow ^^ and Knight and Crocker ^^ showed still 



Figure 57. A, Crassula arboresceris showing leaf fall. Left: control. Right: sealed in a 

 10-liter bell jar with 3 puffs of cigarette smoke for 5 days. B, Salvia showing petal fall. 

 Left: control. Right: treated 24 hours with 1:10,000 illuminating gas. 



earlier that the rate of elongation of etiolated pea seedlings was reduced 

 by very low concentrations of ethylene in the air, and that as the con- 

 centration of ethylene increased, the rate of elongation fell until in relatively 

 low concentrations the elongation ceased, that is, growth rigor was pro- 

 duced. Anesthetized pea seedlings showed no killing of tissue and resumed 



