Permafrost 



Permanently frozen ground in arctic and subarctic re- 

 gions. 



Permanent Pasture 



Grazing land that remains under grazing use for many 

 years, cf. Rotation Grazing, Range. 



Permanent Quadrat 



A marked sample area or quadrat (q. v.) in which the 

 vegetation is recorded over a period of many years. 



Permanent Wilting Percentage 



The quantity o water in the soil on a dry weight basis 

 when plants growing in it have reached the condition of 

 permanent Wilting. 



Permeability 



(1) The property or condition of the soil that relates to 

 the passage of water or air through it. cf. Percolation. (2) 

 The rate of diffusion of molecules of a substance through a 

 membrane, cf. Osmosis. 



Permeant 



An animal in a terrestrial community which has great 

 motility, e. g., a bird. 



Permian 



The latest geological period in the Paleozoic era which 

 began about 230 million years ago and lasted for about 25 

 million years. 



Pessimum 



The conditions of the environment outside of the Opti- 

 mum which an organism can endure, cf. Ecological ampli- 

 tude. 



Pesticide 



An agent or substance that destroys pests, e. g., a fungi- 

 cide or insecticide. 



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