252 RESPIRATION, FERMENTATION AND DIGESTION 



such as dried mosses, lichens and seeds in which it is practically 

 zero. Absolute cessation of respiration should occur in seeds 

 and bacteria exposed to the extreme low temperatures of liquid 

 hydrogen ( 252 C). 



319. Aerobes. A supply of free oxygen is necessary to the 

 continued respiration and existence of aerobes, although such 

 forms are capable of liberating sufficient energy for shorter periods 

 by means of anaerobic processes. The external manifestations 

 of aerobic respiration are the excretion of carbon dioxide (not al- 

 ways shown), heat, which may be diffused so rapidly as to be 

 incapable of measurement, and in rare instances, phosphorescence 



Since physiological combustion is not always complete, it is 

 evident that the proportion of carbon dioxide to the amount of 

 oxygen used, must vary greatly. The respiration of oily seeds 

 produces less of this substance than the amount of oxygen ab- 

 sorbed, but in seeds containing starch or sugar the amounts are 

 practically equal, while Penicillium is claimed to excrete 2.9 times 

 as much carbon dioxide as oxygen absorbed when fed on tartaric 

 acid, although this disproportion is doubted by some writers. 

 Respiration is most rapid in the more vigorous parts of the plant, 

 although not always in the regions showing the most rapid growth, 

 and the amount of carbon dioxide excreted may amount to six 

 per cent, of the bulk in a mould, and as much as 2.4 per cent, 

 of the bulk of the organism, daily in certain bacteria. A temper- 

 ature of 10 to 1 5 C. is probably the minimum for respiration, 

 and the optimum probably lies at the maximum, or at the point 

 of heat rigor. Light has but little influence on the process. 1 



Anaesthetics and narcotics may increase respiration, although 

 their final and continued influence would lower the activity of 

 the organism in several ways. 2 



1 Puriewitsch, K. Physiologische Untersuchungen iiber Pflanzenathmung. Jahrb. 

 \Viss. Bot. 35 : 573. 1900. 



See also Palladine, M. W. Influence des changements de temperature sur la res- 

 piration des plantes. Rev. Gen. d Bot. n : 241. 1899. 



2 Morkowine, M. N. Recherches sur Tinfluence des anesthesiques sur la respira- 

 tion des plantes. Rev. Gen. d. Bot. u : 341. 1899. 



