Threshold 



The duration or intensity of a stimulus that is required 

 to produce response in an organism. 



Thrombosis 



The coagulation or clotting, as of blood, in the vascular 

 and lymphatic systems of living animals. 



Thyroid 



One of the Endocrine glands (q. v.) found in Vertebrates, 

 secretes a Hormone containing iodine. 



Thyroxine 



The substance containing iodine produced by the Thy- 

 roid gland. 



Tick 



An animal in the order Acarina, class Arachnida, which 

 sucks blood, e.g., the fever tick on cattle. Also used for some 

 parasitic dipterous insects such as the sheep tick. 



Tidal Flat 



An essentially barren, nearly flat, muddy area, periodi- 

 cally covered by tides, the lower parts daily, the higher parts 

 only during exceptionally high tides. 



Tidal Marsh 



A low flat marshland that is intersected by channels and 

 tidal sloughs, usually covered by high tides, with vegetation 

 consisting of rushes, grasses, and other low, salt-tolerant 

 plants. 



Tidal Zone 



The area of a shore between the levels of high and low 

 tides, syn. Intertidal zone. 



Till 



An unstratified deposit of gravel, boulders, sand, and 

 finer materials which has been transported by a glacier, cf. 

 Drift, Colluvium. 



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