EFFECT OF CARBON DIOXIDE 109 



and the method of its physiological combustion, whether by 

 aerobic or anaerobic means. 



The ordinary green plant, and certain non-green plants, 

 in their respiration absorb oxygen and ultimately give off 

 carbon dioxide and water. This respiration is unceasing and 

 is continued in all living members whether active or passive 

 until death ensues. If oxygen be entirely withheld, growth, 

 movement, irritability and activity in general ultimately will 

 come to an end.* 



The accumulation of the products of physiological com- 

 bustion will bring about a modification, if not a complete 

 cessation 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 retarded 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, germi- 

 nation 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 cf 20 per cent, in the carbon dioxide of the atmosphere. 

 A higher concentration causes marked injury and ultimately 

 death, f 



* The irritability of plants is outside our present consideration : an 

 introduction to the problems regarding the minimum pressure of oxygen 

 necessary to maintain movements, the streaming of protoplasm in chloro- 

 phyll-containing cells in darkness and in an atmosphere free from oxygen, 

 and similar subjects will be found in the larger text-books on general plant 

 physiology. 



t Kidd : " Proc. Roy. Soc," B, 1914, 87, 408 ; " New Phytol., 1919. 

 18, 248. 



