GRASSES OF IOWA. 



387 



PRECIPITATION TABLE— Continued. 



The regions here considered, naturally, would not show very much 

 variation in temperature except such as is due to latitude. But the north- 

 western portion of the state, because of its higher altitude and open 

 prairies, is somewhat cooler than the more thickly wooded portion of 

 si utheastern Iowa. It is an undisputed fact, however, that thermal helts 

 extend along the Missouri and Mississippi rivers. Certain varieties of 

 apples and cherries may he grown along the Missouri that will not suc- 

 ceed further eastward in the same parallel of latitude. Mr. Greene 1 , in 

 a paper read before the Iowa State Horticultural Society, has very strik- 

 ingly brought out these facts in studying fruit bloom. He says, "The 

 maps for April, 1899 and 1900, show the average temperature on the 

 morning of the first day of May. In 1899 there were nine thermal lines 

 presented, 54^ to 46 , inclusive, and in 1900, eight, 56 : to 49 . The 

 normal temperature for April is |<;.s , so that April, 1900, was 2.7 

 above the average temperature for that month and ought to be three days 

 earlier. And so it proved to be by the tree record. Last year it was 

 reported from the same place on the 26th, just three days earlier. 



In 1899, the isotherm 51 ran through Clinton, Linn, Van Buren, 

 Fremont and Pottawattamie counties, on April 30th. In 1900 it was 

 in Allamakee, Buchanan, Kossuth and Lyon counties. The eastern part 

 was colder, relatively, in the spring of 1900 than in [899, as compared 

 with the central and western part of the state. 



Heat is an important factor in the development of plants. The 

 plant zones of Humboldt were established by connecting the points hav- 

 ing the same mean annual temperature. He called these isothermal lines. 

 On this basis there were established the Boreal, Austral and the Tropical 



1. Greene, W. , Report la. State Hort. Soe. 1900: 222. 



