Tracer 



A Radioisotope isotope (q. v.) used to follow the course 

 of or to determine the location of a normal element in an 

 organism. 



Trachea 



(1) A Vessel in the Xylem (q. v.) in plants. (2) The tube 

 from the throat to the lungs in Vertebrates. (3) One of the 

 small tubes that conduct air in the bodies of most Ar- 

 thropods, especially in insects, opening to the exterior 

 through Spiracles (q. v.). 



Tracheid 



A long, thick-walled cell, without perforations, with 

 tapering ends, that conducts water and gives support, located 

 in the Xylem of plants, especially Conifers. 



Tracheophyte 



A member of the Tracheophyta (vascular plants) com- 

 prising four subphyla; Psilopsida (the most primitive), 

 Lycopsida (the clubmosses), Sphenopsida (the horsetails), and 

 the Pteropsida (the ferns, conifers and their allies, and the 

 flowering plants). 



Trade Winds 



Winds that blow regularly from subtropical areas of high 

 pressure towards areas of low pressure along the equator, 

 in the northern hemisphere from the northeast, in the south- 

 ern hemisphere from the southeast; important in producing 

 ocean currents. 



Transad 



A species, or closely related species, that exist on both 

 sides of a barrier and consequently must have extended across 

 it at one time. 



Transect 



A long, narrow sample area, or a line, used for analyzing 



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