PostclKmax 



A Climax community (q. v.) that requires more mesic 

 conditions than obtain generally in the region where it is 

 present, often considered a remnant of a former widespread 

 Climatic climax (q. v.). 



Potential, Biotic 



See Biotic potential. 



Pothole 



A hole formed in rock by the grinding action of a stone 

 kept in motion by a stream. 



Potometer 



An instrument used to measure the absorption of water 

 by plants. 



Prairie 



Grassland vegetation, particularly the extensive tract of 

 nearly level or rolling land in North America occupied by 

 tall grasses. 



Prairie Soils 



A Zonal group of soils with dark surface horizons grad- 

 ing through brown to lighter colored parent material at two 

 to five feet, formed under tall grasses in a temperate, humid 

 climate, cf. Chernozem. 



Preadaptation 



The possession by an organism of characteristics that 

 enable it to survive when exposed to new ("prospective") 

 conditions, e.g., the possession of a disease-resistant gene in 

 an organism not yet exposed to the disease, cf. Postadaptation. 



Preboreal Period 



See Boreal period. 



Precambrian 



The geological time preceding the Cambrian period, the 

 oldest period of the Paleozoic era. 



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