FACTORS AFFECTING PLANT DISTRIBUTION 



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grand centers of action" of the atmosphere. They are understood 

 to be closed areas of high and low pressure, or hyperbars and in- 

 frabars as they are commonly called, which have a certain perma- 

 nence in a given season and give the leading features to the gen- 

 eral circulation. In fact, the flow between these centers exerts the 

 dominating influence on the circulation of the entire atmosphere. 



Fio. 1. Chart showing the grand centers of climatic control. After Buchan. See also 

 Adams's, "An Ecological Study of Prairie and Forest Invertebrates." Ills. Bull. Nat. Hist, 9 :70. 



The centers of action come to have a motion of their own, due 

 to the earth's rotation. In the northern hemisphere the cyclones or 

 columns of ascending air have a counter-clockwise rotation. In the 

 anticyclones or descending air streams the motion is clockwise. In 

 the southern hemisphere the rotary motion of the cyclones and anti- 

 cyclones is just the opposite of the motion in the northern hemis- 

 phere. 



The accompanying diagram (Figure 1), modified from Bu- 

 chan, shows the position of the permanent centers of action which 

 exert a controlling influence over the summer climate of North 



