in temperature or any increase in water vapor will result in 

 condensation of the vapor into liquid water; at this point 

 the Relative humidity (q. v.) is 100 per cent and the Satura- 

 tion deficit (q. v.) is zero. 



D Horizon 



The stratum in the soil below the depth of weathering, 

 composed of undifferentiated and unconsolidated parent ma- 

 terials, immediately below the C horizon, cf. A horizon, B 

 horizon. 



Diameter Breast High 



See Breast height. 



Diapause 



A period of suspended growth or development and re- 

 duced metabolism in the life-cycle of many insects, in which 

 the organism is more resistant to unfavorable environmental 

 conditions than in other periods. 



Diaspora 



A portion of a plant such as a seed, spore, bud or other 

 part that undergoes dispersal and can give rise to a new 

 plant, cf. Disseminule. 



Diastrophism 



Dislocation of the earth's crust such as folding, resulting 

 in the formation of mountains, sea basins, etc. 



Diatom 



A one-celled, microscopic alga in the class Bacillariaceae, 

 with siliceous walls. 



Diatomaceous Earth 



A deposit of the siliceous remains of diatoms. 



Diatomaceous Ooze 



Material consisting of siliceous remains of diatoms found 

 in cold seas. 



108 



