158 GROWTH OF PLANTS 



has had rather extensive use and is in great favor with the anesthetists who 

 do use it. 



Coloring and ripening of fruits. Denny 9. i". " was the first to note the 

 effectiveness of ethylene in coloring citrus fruits. When commercially 

 mature as determined by their size, color, or chemical composition, citrus 

 fruits are often still green in color. For years the citrus growers had been 

 burning kerosene stoves in their storehouses to hasten the proper coloring 

 of the fruits. At first they thought the stoves did this by raising the tem- 

 perature, but it was discovered that the stoves gave off some gas that 

 increased the rate of chlorophyll decomposition. Denny found that low 

 concentrations of ethylene in storage houses would hasten the coloring. 

 Any concentration from 1 : 5000 to 1 : 5 million of the air had some effect, 

 with 1 : 5000 to 1 : 1 million representing the most effective range of concen- 

 tration. Very high concentrations, such as 80 per cent, slowed the process. 

 The process was fastest at about 82° F (28° C) . Besides accelerating the 

 coloring of lemons, ethylene increased the rate of respiration 100 to 250 per 

 cent, depending upon the concentration of the gas and other conditions. 

 Acetylene and carbon monoxide were effective, but must be used in much 

 higher concentrations; and butadiene showed some effect. 



In practical application of this discovery the rooms were gassed two to 

 four times a day and aired out at least once a day. Ethylene came to be 

 very generally used instead of the kerosene stoves. The significance of this 

 discovery was well put by the late Dr. Henry G. Knight, former Chief of 

 the U. S. Bureau of Agricultural Chemistry and Engineering, on the occa- 

 sion of his receipt of a medal from the American Institute of Chemists for 

 his achievements in agricultural chemistry. Science ^^ reports a portion of 

 his speech as follows: "As a single, dramatic example of the returns obtain- 

 able on small outlay in research, Dr. Knight cited the case of the ethylene 

 gas treatment of oranges, to bring a bright color to the skins of some types 

 of fruit that persist in staying green after the oranges themselves are ripe. 

 'The treatment bleaches out the predominant green color and leaves the 

 orange a beautiful natural yellow. The chemical investigations leading to 

 the development of this treatment, which is now in rather general use, cost 

 the taxpayers of the country about $4,000 and is estimated to be worth 

 about $4,000,000 a year to the producers of citrus fruits in Florida alone 

 and about the same amount to producers in California. And yet some 

 people say that research doesn't pay. . . .'" 



Ethylene induces the decomposition of chlorophyll in many different 

 plants and plant organs. This is not a direct effect of ethylene upon the 

 chlorophyll but an indirect effect through the protoplasm. In high con- 

 centrations, such as 80 per cent, ethylene inhibits the decomposition, as 

 Denny showed for the lemon, because it produces partial rigor in the 

 protoplasm and lowers the speed of many of its activities. Fig. 60 shows 

 that illuminating gas (the ethylene portion) induces the decomposition of 

 chlorophyll in rose leaves.''* 



