major importance in the secretion of Hormones, many of 

 which control the activity of other Endocrine (q. v.) glands. 



Plagioclimax 



A type of Climax (q. v.) vegetation which is the result of 

 man's activity, a biotic climax, e. g., an apparently stable 

 community caused by continued mowing or grazing, cf. Dis- 

 climax, Proclimax, Sub climax, Plagiosere, Ser climax. 



Plagiosere 



A Sere (q. v.) deflected from its undisturbed course by the 

 constant intervention of man in such activities as burning, 

 grazing, and mowing, resulting in a Plagioclimax. 



Plain 



An extensive tract of nearly level or gently undulating 

 land that is usually occupied by grassland vegetation. 



Plane Table 



A surveying , instrument used for making a sketch-map 

 of a small area. 



Planimeter 



An instrument used to determine the area of a plane fig- 

 ure or object such as a leaf by tracing the boundaries. 



Plankter (Plankt, Planktont) 



An individual organism in the Plankton (q. v.). 



Plankton 



The floating or weakly swimming animal and plant or- 

 ganisms occurring at any depth in lakes, ponds, streams, or 

 seas; often microscopic in size. 



Planosol 



An Intrazonal group of soils with eluviated surface hori- 

 zons underlain by clay pans or fragipans, developed on nearly 

 flat or gently sloping uplands in humid or subhumid cli- 

 mates, cf. Eluviation. 



265 



