388 



GRASSES OF IOWA. 



zones. It was found, however, that zones established on isothermal lines 

 did not express the true conditions, since two points of the same mean 

 .annual temperature may show wide differences in the extremes of an- 

 nual, monthly or daily temperatures. It was found that life processes 

 depend on these more than on the mean, hence some other basis must be 

 established for the life zones. 



Pig. 262. Wooded bluffs near Mississippi river. The bare slopes covered with Andropogon scoparius, 

 A. provincialis, Bouteloua racemosa, Lithospermum canescens, Caslilleia sessiliflora. In the woods 

 Calamagrostis Canadensis, Fesluca nutans, Bromut purgans. (Charlotte M. King, Photo. ) 



Merriam establishes his life zones on another principle, namely, 

 that it requires a definite amount of heat to accomplish the life 1 cycle of 

 the plant from the time of germination to maturity. That for a given 

 species this is the same, being the sum of the mean daily temperatures 

 during the cycle of vegetation. This is the physiological constant. 



Dr. Merriam recognizes the following classification: 



(1) 



(2] 



(3) 



{Arctic or arctic alpine. 

 Hudsonian zone. 

 Canadian zone. 



f Alleghanian 



< Arid transition . 



I Pacific coast transition. 



Transition zone. 



Austral Region. Upper austral zone. 

 Lower austral zone 

 Tropical Region. 



I Carolinian area. 

 I Upper Sonoran. 

 J Austroriparian. 

 I Lower Sonoran. 



