Psammosere 



All the stages of a successional series or Sere (q. v.) orig- 

 inating in sandy soil. cf. Hydrosere. 



Psychrometer 



An instrument used to measure the Relative humidity of 

 the atmosphere by means of the effect of temperature differ- 

 ences of the wet and dry bulb thermometers, cf. Hygrometer. 



Pteridophyte 



A Vascular plant in one of the subphyla of the phylum 

 Tracheophyta, exclusive of the seed plants, e.g., fern, club- 

 moss, horsetail. 



Public Domain 



Land for which the title was originally vested in the 

 Government of the United States by virtue of its sovereignty. 

 More than half of the original public domain has been 

 granted to states, homesteaders, railroads, or has been sold. 

 The remainder is set aside for national forests, national 

 parks, national monuments, Indian reservations, grazing dis- 

 tricts, and similar purposes. 



Puddled (Soil) 



Dense, massive soil artificially compacted when wet, hav- 

 ing no regular structure. 



Puddling 



The process of destroying the structure of the soil during 

 which the porosity and permeability are reduced. 



Pulse 



The sudden appearance of a great abundance of plant 

 Plankton. 



Puna 



The cold, bleak portions of the high plateau region in 

 the central part of the Andes in South America. 



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