126 RESPIRATION 



that sugar, more especially dextrose, was essentially the res- 

 pirable material, but no statement of general application can 

 be made. The action of foods in this connection varies with 

 different plants and their physiological condition, and is in- 

 fluenced by the operation of various natural internal factors, 

 such as acidity, which are not directly controllable in ex- 

 perimental work, together with external factors such as tem- 

 perature ; thus in Phycomyces the normal combustion of 

 fats changes to the consumption of carbohydrates at high 

 temperature.* 



But notwithstanding this, general physiological experience 

 shows that carbohydrates are highly important in aerobic 

 respiration. Palladin,f for instance, found that ioo grams 

 of etiolated bean leaves, with depleted carbohydrate, gave off 

 102-8, 95 -Q and 70-2 mg. of carbon dioxide respectively for 

 three successive hours ; but after these leaves had been grown 

 in darkness on a solution of cane sugar for two days, by which 

 means their carbohydrate content was increased, the rate of 

 evolution of carbon dioxide was 152-6, 147-5, 146-8 and 144-5 

 mgs. respectively for four successive hours. 



A more prolonged culture on cane sugar does not result in 

 a corresponding increase in the rate of carbon dioxide pro- 

 duction although the carbohydrate reserve is greater ; thus 

 after a treatment over a period of forty hours, the leaves 

 evolved no more carbon dioxide than they did after four 

 hours. Such observations indicate that a certain content of 

 carbohydrate is necessary, but an increase above this value, 

 which is a variable figure, is merely increasing the respirable 

 capital of the organ. 



Muller-Thurgau and Schneider-Orelli % found that the 

 respiration of sweet potatoes is very high compared with 

 normal tubers ; in the autumn, when little if any sugar is 

 present, the respiration is low, but as the tubers increase in 

 age and sugar accumulates there is concurrently a more intense 

 respiration. According to Knudston § the respiration of the 



* See De Boer : " Rec. Trav. bot. neerland.," 1928, 25, 117. 



f Palladin : " Rev. gen. Bot.," 1893, 5, 449. 



% Muller-Thurgau and Schneider-Orelli : loc. cit. 



§ Knudston : " Cornell Agric. Exp. Sta. Mem.," 1916, 9, I. 



