IOWA ACADEMY OF SCIENCES. 161 



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

 isothermiil lines did not express the true conditions, since 

 two points of the same mean annual temperature may 

 show wide differences in the extremes of annual, monthly 

 or (^aily 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. 



Merriam established his life zones on another principle, 

 namely, that it requires a deiinite amount of heat to 

 accomplish the life* 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: 



r Arctic or arctic alpine. 



\1) Boreal Region. -- Hudsonian zone. 



( Canadian zone. 

 I Alleghanian. 

 Transition zone. . . ■. Arid transition. 



( Pacific coast transition. 



/•o\ A k 1 r> • TT .1 ( Carolinian area. 



(2) Austral Region. Up oer austral Z3ne - tt o 



^ ^' ( Upper Sonoran. 



J L 1 { Austroripari.in. 



Lower austral zone ■ j ^ 



/ Lower Sonor .n. 



(3) Tropical Region. 



All('(j]in)iian area. This area reaches its greatest devel- 

 opment in this state along the Mississippi and reaches over 

 to the Missouri river extending further eastward in south- 

 western Iowa, thence further north along the river. The 

 the representative plants are: 



Juniperus Virginiana (northward). Tilia Americana. 



Quercus macrocarpa. Sanguinaria Canadensis. 



Cory /us Americana . Negundo aceroides . 



Rhus glabra. Ulmus Americana. 



Prunus America7ia. Acer saccharin um. 



Dicenta cucullaria. Acer nigrutn (Des Moines basin.) 



Solidago serotina (northward) . Aster Novce Anglice (northward). 



Carolinian. This area reaches its greatest extension in 

 southeastern Iowa, spreading northward to Dakota, with a 

 few representatives. The representative plants are: 



* Life Zones and Crop Zones of the United States. Div. Biol. Surv. U. S. Dept. Agrl. 

 10. Yearbook U. S. Dept. of Agrl. 1897: 115. 1894: 203-214. 



