ceived by the human eye; on a clear summer day it is 8000 

 to 10,000 foot-candles at noon. cf. Irradiance. 



Illuviation 



The accumulation of material in a soil horizon by precip- 

 itation from solution or from suspension from a layer above, 

 cf. Eluviation, B horizon. 



Imago 



The adult, sexually mature stage of an insect. 



Immature Soil 



A soil in which development is not complete because of 

 insufficient time since deposition or exposure to the action 

 of the physical environment and organisms on the Parent 

 material (q. v.). 



Immigrant 



A plant or an animal of a species which has recently 

 invaded an area, whose role is still uncertain, cf. Emigration. 



Immigration 



The Migration (q. v.) of an organism into an area where 

 it did not occur previously, cf. Emigration. 



Immunity 



The capability of an organism to resist infection by a 

 Parasite or one of its products. 



Imperfect Flower 



A flower lacking pistils or stamens. 



Impervious Soil 



Refers to soil or a certain layer in which the penetration 

 of water, and usually air and roots as well, occurs slowly 

 or not at all. 



Impoundment 



An artificial lake or pond. 



185 



