74 RESPIRATION 



when the plant is ground and allowed to undergo autolysis 

 or decay, under which circumstances carbon dioxide and 

 hydrogen are produced. From these facts it is concluded 

 that the gas is a respiratory product and has no connection 

 with carbon assimilation. 



The accumulation of the products of physiological com- 

 bustion will bring about a modification, if not a complete ces- 

 sation, of the process which will lead to the termination of 

 other activities. Thus rotation of protoplasm in the cells of 

 Elodea will come to an end in the presence of an undue 

 amount of carbon dioxide. The germination of seeds is re- 

 tarded or inhibited by high partial pressures of carbon dioxide 

 in the atmosphere : this inhibition may remain in force only 

 so long as the seeds are exposed to the enriched atmosphere, 

 germination taking place after removal to a normal atmosphere 

 as in the bean, cabbage, barley, pea, and onion ; or, the in- 

 hibition may continue indefinitely after removal to normal 

 surroundings, germination only taking place after complete 

 drying and re-wetting or by the removal of the testa as in 

 Brassica alba. The degree of increase in the partial pressure 

 of carbon dioxide required to effect inhibition of germination 

 varies for different plants, and the retardation of germination 

 depends on the time of exposure and the character of the seed. 

 Similarly the sprouting of a potato is inhibited by an increase 

 of 20 per cent in the carbon dioxide of the atmosphere. A 

 higher concentration causes marked injury and ultimately 

 death.* 



These facts are of considerable importance not only as re- 

 gards the economic aspect but also in their bearing on experi- 

 mental work : results obtained for subjects contained in closed 

 vessels, as is not infrequent in experiments on respiration, 

 in which the products of the oxidative processes accumulate, 

 may be an expression of the plant's activity not in a normal 

 but in a pathological condition and, therefore, may be value- 

 less. 



INTENSITY. 



In general terms, the more active the body the more intense 

 the respiration, provided that the conditioning factors such as 



* Kidd ; "-Proc. Roy. Soc.," Lond., B., 1914, 87, 408 ; ' New Phytol," 1919, 

 18, 248. 



