southern limit marked by a normal mean temperature of 

 50 F. during the six hottest weeks of summer. 



Arctogea 



The faunal realm which includes the Ethiopian, Oriental, 

 Palearctic, and Nearctic regions (q. v.). syn. Megagea. 



Area 



The total territory or range in which a Taxon or Com- 

 munity occurs, cf. Basal area, Coverage. 



Areg 



A sand desert. 



Arenaceous 



Refers to a sandy substratum. 



Arenicolous 



Refers to organisms inhabiting sandy substrata. 



Areography 



The study that deals particularly with area. 



Areole 



A small area of a leaf surrounded by intersecting veins. 



Argillaceous 



Refers to clayey material. 



Arid 



Refers to regions or climates which lack sufficient mois- 

 ture for crop production without irrigation; precipitation 

 10 inches or less in cool regions, up to 15 or 20 inches in 

 tropical regions, cf. Semiarid. 



Aridity 



The condition of dryness. See Arid. 



Arid Transition Life Zone 



The western part of Merriam's Transition zone, lying 

 west of the 20-inch annual precipitation line. 



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