Fishway 



A sloping structure over which water flows, to enable 

 fish to ascend a stream around a dam or other obstruction; 

 a fish ladder. 



Fission 



(1) Reproduction of a unicellular organism by simple 

 division into two parts. (2) Refers to atomic bombs in which 

 elements such as uranium and plutonium are split into 

 products as, for example, Sr 90 , I 131 , and Cs 157 , which are 

 formed during the explosion. 



Fitness 



The degree of Adaptedness (q. v.) to the environment 

 that an organism possesses. 



Fitness of the Environment 



The suitability of environmental conditions such as the 

 nature of the water, gaseous composition of the atmosphere, 

 and temperatures for the maintenance of life or for the 

 activities of a certain organism. 



Fixation (in Soil) 



The conversion of a soluble substance such as phos- 

 phorus from a soluble or exchangeable form to a relatively 

 insoluble form. 



Fjaeld (Fjeld) 



A more or less barren upland area (Scandinavia). See Fell. 



Flachmoor 



A moor with a flat or even slightly concave surface and 

 soil poor in salts and acid in reaction. 



Flatworm 



A member of the phylum Platyhelminthes, e.g., flukes, 

 tapeworms. 



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