298 PLANT PHYSIOLOGY 



is quickly transferred into a cooler place, the respiration increases 

 for the first few hours. 



The respiratory quotient is at a minimum at about 12° C. 

 according to Purievich (1893) and then increases with both higher 

 and lower temperatures. The following figures derived from studies 

 on the geranium {Pelargonium zonale) indicate the kind of results 

 obtained : 



Temperature C. Respiratory quotient 



4-5 0.75 



12-14 0.54 



34-35 0.95 



At 12° there was about twice as much oxygen taken in as carbon 

 dioxide given off, according to these figures, while at both the 

 higher and lower temperatures the ratio more nearly approaches 

 unity. 



Food. — Respiration cannot take place without the presence of 

 respirable matter, the nature of which has been previously dis- 

 cussed. Palladin (1893) found that 100 grams of etiolated bean 

 leaves, containing only a small amount of carbohydrate, pro- 

 duced carbon dioxide at the rate of 90 mg. per hour, but the 

 same leaves when transferred to a sugar solution increased their 

 carbon dioxide output to 148 mg. per hour. No matter how 

 much carbohydrate they contained, the respiration never went 

 beyond this amount, indicating that when the carbohydrate 

 supply ceased to be a limiting factor the respiration could not be 

 increased. 



The interdependence of the various factors that affect a given 

 end product may well be illustrated here. We have seen that a 

 decrease in temperature decreases respiration and also that an 

 increase in available food increases respiration. When potato tubers 

 are placed in the cold, as pointed out by Hopkins (1924), this causes 

 the starch to be transformed into sugar, which is more readily 

 burned (respired) than the starch, and respiration increases. Thus 

 we see that the decrease in respiration caused by the drop in 

 temperature is more than counterbalanced by the increase pro- 

 duced by the addition of available (respirable) food materials. 



The nature of the food respired is also of importance. The 

 carbohydrates and fats are the chief materials respired. Most 

 of the work on respiration has been done with carbohydrates, and 

 concerning the fats very little precise information is available. 



