160 GROWTH OF PLANTS 



used in too high a concentration. Denny ^^ points out that Harvey got 

 increased respiration in one of his determinations, namely, the one with 

 shortest exposure, probably before the anesthetic effect became evident. 



A number of investigators ^^- ^" have determined the effect of ethylene 

 upon the metabolism of celery and a number of fruits, citrus and pomaceous, 

 generally using 1000 ppm of ethylene in the ah. Ethylene caused an in- 

 crease in reducing and simple sugars, at the expense of polysaccharides if 

 the latter were present. If not, the simple sugars were decreased because 

 ethylene increased respiration. In all the fruits ethylene hastened coloring 

 and ripening. Ethylene decreased the tannin and catalase *~ and increased 

 the peroxidase content. Ethylene, 1 ppm in air,^^ increased the respiration 

 of potatoes in storage from the first and increased the soluble sugar content 

 in later storage. There was one exception to tliis; in tubers rich in sugars 

 caused by a period of storage at low temperature, ethylene did not increase 

 respiration. If such tubers were de-sugared by a storage period at high 

 temperature, ethylene again stimulated respiration. 



Hansen ^^- ^^ has confirmed the findings mentioned above for the effects 

 of ethylene upon fruits and added another .very important metabolic 

 change. Ethylene in 1000 ppm in air hastens the hydrolysis of insoluble 

 protopectin of cell walls of the fruits into soluble pectin. This accounts for 

 the more rapid softening of fruits when ripened with ethylene. Hansen also 

 found that ethylene hastened the formation of pectin from protopectin in 

 citrus rinds and English walnut shucks. This no doubt is a factor in shuck- 

 ing walnuts with ethylene. Hansen also found that little ethylene was pro- 

 duced by green fruits and that ethylene production increased as the fruit 

 ripened. Ethylene did not induce ripenmg and softening of fruits after they 

 had been m cold storage at 31° F ( -0.5° C) for three weeks. 



Lynch ^^ suggests that ethylene acts as a respiratory coenzyme in the 

 ripening fruits, and Nord ^^ suggests that ethylene increases the enzymatic 

 formation of plant hormones. Englis and Dykins -° find that ethylene does 

 not modify the rate at which salicin, a glucoside, is hydrolyzed by the 

 enzyme emulsm. They conclude that ethylene, in modifying the metabo- 

 lism and hastening the ripening of fruits, does not act directly upon enzymes 

 but indirectly upon the living protoplasm of the fruit. 



Root and root-hair initiation. The discovery that the unsaturated C-gases 

 induce rooting was accidental. For some years Zimmerman and Hitchcock 

 had been studying the rooting of cuttings, and during this time had been 

 seeking unthout success chemicals that would induce rooting. On the side 

 they were working with the author on the effect of illuminating gas and 

 its constituents on greenhouse plants, a problem that would seem to have 

 no relation to their main researches. In general the exposures of the plants 

 to the gases were for a duration of three to four days. 



On one occasion a tomato plant was exposed to 1 per cent CO in a bell 

 jar, and because of a holiday making a long week-end was kept in the gas 

 several days more than usual. The gassed plant showed many roots along 



