Dominant (Species) 



A species that manifests Ecologic or Social dominance. 

 cf. Secondary species. 



Donga 



A gully with steep sides or a dry watercourse, a term used 

 in South Africa. 



Dormancy 



The condition in an organ or in an organism where 

 metabolic processes are relatively inactive as a result of in- 

 ternal causes, e. g., many kinds of seeds, overwintering stages 

 of insects, cf. Hibernation, Estival, Diapause. 



Double Fertilization 



The process, unique in Angiosperms, in which one male 

 nucleus fertilizes the egg nucleus to form the Zygote which 

 develops into the Embryo, and the other male nucleus joins 

 with two other nuclei in the embryo sac to form the Endo- 

 sperm, e. g., in corn and other grasses. 



Down 



An area of open, treeless upland with a thin covering of 

 soil, used mostly for sheep grazing; especially the chalk hills 

 of southern England. 



Downland 



Temperate grasslands in Australia and New Zealand. 



Drainage Basin 



The largest natural drainage area subdivision of a con- 

 tinent, such as the Mississippi, Columbia, and Colorado 

 basins, cf. Watershed. 



Drainage Terrace 



A Graded terrace constructed to have a relatively deep 

 channel and a low ridge primarily for drainage of a hillside. 



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