250 GROWTH OF PLANTS 



drin the vapors counteracted each other so far as the respiration is con- 

 cerned. Cutting the tubers ^^ into seed pieces increases the respiration 

 rate enormously but does not force the dormant buds. Perhaps the best 

 evidence that this early chemically-induced flush in respiration is not 

 causally related to bud forcing is the fact that it precedes bud growth and 

 has receded to normal considerably before bud gro^vth begins. No doubt 

 after bud growth begins there is a second great rise in respiration. 



It has long been knouTi that in darkness various succulents, Bryophyllum, 

 cacti, etc., do not oxidize the sugars completely to carbon dioxide and water 

 but partly to the organic acid stage; hence citric, malic, and oxalic acids 

 accumulate in such plants during darkness. Under illumination such 

 plants complete the oxidation of these acids. Guthrie '^ has shown that 

 treating Bryophyllum leaves in darkness with ethylene chlorhydrin vapors 

 induces them to oxidize citric and probably malic acid, in this way render- 

 ing the tissues less acid. This is similar to the effect of ethylene chlorhydrin 

 on potato tubers. 



Enzymes. Since starch is the main food storage in the potato tubers, 

 investigators seem justified in asking whether bud-forcing chemicals are 

 effective by increasing the activity of the amylase already present in the 

 tuber ("direct effect"), or by increasing the amount of amylase produced 

 by treated tubers ("indirect effect"). Possible correlations between either 

 of these effects on amylase and the bud-forcing action of chemicals were 

 sought. In the main, the good bud-forcing chemicals ^' did not increase 

 the activity of either plant or animal amylases in vitro. Potassium thiocya- 

 nate ^^ did increase the activity of animal amylase in low pH, had no effect 

 in intermediate pH, and inhibited that action in high pH. Also hydrocyanic 

 acid," slightly effective as a bud forcer, increased the amylase activity of 

 undialyzed potato juice. Neither of these throws any light on bud-forcing 

 action. 



Ethylene chlorhydrin treatment " of tubers led to a later great increase 

 in the amylase activity of the tubers. Treatment of tubers with sodium 

 thiocyanate, another good bud forcer, led to some increase in the amylase 

 activity of the tubers if the tubers were not too dormant; but often 

 dormant tubers showed no increase in amylase activity when treated with 

 this chemical. Denny concludes that the bud-forcing action of chemicals 

 cannot be explained either on the direct or indirect effect on amylase 

 activity. 



Dormant tubers were treated mth ethylene chlorhydrin, sodium thiocya- 

 nate, and thiourea,^^ and the later effect of these treatments was determined 

 on the catalase and peroxidase activity and reducing power of the juice of 

 the tubers. The last was determined by the power of the juice to reduce 

 methylene blue, indophenol, iodine, in phosphotungstic reagents. The 

 increase in catalase and peroxidase activity began about 24 hours after the 

 treatment, a little earlier with ethylene chlorhydrin than with the other 

 two treatments; also the former chemical gave much greater increases than 



