514 



Journal of Agricultural Research 



Vol. V, No. 12 



vidual experiments. The first experiment, however, is in marked con- 

 trast to the others in this respect, for, although the sugar content of these 

 sweet potatoes rose from 0.94 to 1.23 per cent, there was no corresponding 

 rise in the respiration. The paralleUsm between the respiration and the 

 sugar content is less marked when the different experiments are com- 

 pared. Thus, the roots in the second experiment contained approxi- 

 mately the same percentage of reducing sugar as those in the third, yet 

 the respiration was much lower in the second. This fact, as has been 

 pointed out, may probably be ascribed to the treatment to which the 

 sweet potatoes had been subjected before the experiment. It is evident 

 on the whole that the respiratory activity of the sweet potatoes is as 

 greatly influenced by seasonal changes and environmental factors to 

 which they have been exposed as by the sugar content. It is clear, of 

 course, that with the exhaustion of the carbohydrates immediately 

 utilized in respiration, the rate of respiration will fall, as in the case of 

 seedlings grown continually in the dark, but it seems that an increase 

 of the available carbohydrate supply does not necessarily entail a con- 

 tinued increase in the respiratory activity. That there is sufficient sugar 

 present in sweet potatoes, as well as in plant organs generally, to support 

 a more active respiration than usually takes place, is shown by the 

 increased respiration as a result of wounding. Table II gives the carbon- 

 dioxid output per kilogram per hour of two lots of sweet potatoes for a 

 short period before and after they were split lengthwise. 



Table II. — Carbon-dioxid output in milligrams per kilogram per hour of two lots of 

 sweet potatoes for a short period before and after being split lengthwise 



The great increase in respiration after the sixth day, when the roots 

 were split, shows that there was sufficient sugar present to support a 

 more energetic respiration than that which took place in the whole roots, 

 but that other limiting factors than the sugar supply determined the 

 rate of respiration. 



In the consideration of the question of the relative availability of the 

 monosaccharids and the disaccharids as sources of material for respira- 



