Animation 



The variation from year to year in abundance or behav- 

 iour of organisms caused often by differences in environ- 

 mental conditions, especially precipitation and temperature. 



Annulus 



A ring-like structure characteristic of certain plant parts 

 such as the stalk of mushrooms or on the spore-case of ferns. 

 In certain animals, notably fishes, a ring, arrangement of 

 rings, or other markings formed once a year and used in the 

 determination of age and rate of growth. 



Anoestrum 



The period in animals when sexual desire or breeding is 

 absent. 



Anoxia 



The condition of oxygen deficiency as in the tissues of 

 an organism. 



Antagonism 



The depressive effect of one organism upon another 

 one such as certain grasses upon the growth of alfalfa, or 

 the excretion of antibiotic substances such as penicillin by 

 a mold. 



Antarctogaea 



The Australian zoogeographical region, excluding New 

 Zealand and Polynesia. 



Antecedent Moisture 



The degree of wetness of the soil at the beginning of a 

 run-off period. 



Anther 



The pollen-producing part of the stamen in a flower. 



Antheridium 



The organ in which sperms are produced, found in 



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