severe conditions in the physical environment because of 

 inherent qualities that it possesses. 



Respiration 



The complex series of chemical reactions in all living 

 organisms by which the energy in foods is made available for 

 use. In aerobic respiration free oxygen is utilized and carbon 

 dioxide is liberated; in anaerobic respiration, free oxygen is 

 not required. 



Respiratory Quotient 



The ratio of the volume of carbon dioxide given off by 

 an organism to the volume of oxygen used in a unit of time. 



Restocking 



The state in a forest in which replacement of important 

 trees is taking place by natural or artificial means, cf. 

 Reforestation. 



Retrogression 



The change from a more highly organized individual, 

 group, or state of organization to one on a lower level, as in 

 a Succession (q. v.) that recedes from the Climax. 



Revetment 



A structure or obstacles placed along the margins of a 

 stream in order to protect the bank from erosion. 



Rheology 



The division of Limnology (q. v). dealing with running 

 waters, their physical, chemical, and biological conditions 

 and interrelationships, cf. Lotic. 



Rheophyte 



A plant that grows in running water, cf. Hydrophyte. 



Rheotaxix 



The orientation in the locomotion of organisms in 

 streams with reference to the current. 



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