when the pressure gradient force is equal and opposite to 

 the gravity force. 



Hydric 



(1) Refers to, or containing hydrogen, e.g., hydric oxide. 

 (2) Sometimes used wrongly in the sense of "wet" as a 

 substitute for Hydrophyte (q. v.). See Hygric. 



Hydrochore 



A plant whose Diaspores (q. v.) are disseminated primarily 

 by water, e.g., water lilies, cf. Anemochore. 



Hydroclimograph 



A graph in which monthly temperature data are plotted 

 against data on salinity in the form of a polygon. 



Hydrocole 



An animal living in water, cf. Hygrocole, Hydrophyte, 

 Mesophyte, Xerocolous. 



Hydrogen-ion Concentration 



The concentration of free hydrogen ions in a solution, 

 commonly expressed as the logarithm of the reciprocal of 

 the normality of free hydrogen ions in which pH 7.0 is 

 neutral, values higher than 7.0 indicate alkalinity, below 

 this acidity, cf. Reaction. 



Hydrography 



The study of natural bodies of water such as lakes, rivers, 

 and seas, especially their physical characteristics in contrast 

 to the biological qualities. 



Hydrologic Cycle 



The cycle of the movement of water from the atmosphere 

 by precipitation to the earth and its return to the atmosphere 

 by interception, evaporation, run-off, infiltration, percola- 

 tion, storage, and transpiration. 



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