STATE HORTICULTURAL SOCIETY. 157 



Michigan, and forminfi^ a great recess in the western line of the 

 heavy forest of the Athmtie region with a depth of nearly 6(H) miles. 

 The transition from the heavy forest of the eastern and central por- 

 tions of the Atlantic region to the treeless plateau is gradual. The 

 change occurs within the prairie region. Here is the strip of dehat- 

 able ground, where a continuous struggle l)etween the forest and the 

 plain takes place. There is here sufficient precipitation of moisture 

 to cause, under normal conditions, a growth of open forest; but so 

 nicely bahuiced is the struggle that any interference quickly turns 

 the scale. Trees planted within this prairie belt thrive if protected 

 from tire and the encroachment of the tough prairie sod, and so ex- 

 tend the forest line westward. If the forest which fringes the east- 

 ern edge of the prairie is destroyed it does not soon regain possession 

 of the soil, and the prairie is gradually i)ushed eastward. 



" The eastern line of the plain where arborescent vegetation is 

 confined to the river bottoms, and which divides it from the prairie 

 where trees grow naturally to some extent, outside of the bottoms, 

 and where they may be made to grow under favorable conditions 

 everywhere, is determined by the rainfall enjoyed by this part of the 

 continent. The extreme eastern point reached by this line is found, 

 upon the fortieth degree of north latitude, near the northern boun- 

 daries of the State of Kansas. North of the fortieth degree it grad- 

 ually trends to the west, reaching the eastern base of the Rocky 

 mountains, in about latitude 52°. This northwestern trend of the 

 eastern plain line may be ascribed to the comparatively small evap- 

 oration which takes place during the shorter summer of the North, 

 and to a slight local increase of spring and summer rainfall. South 

 of the fortieth degree the plain line gradually trends to the south- 

 west under the influence of the Gulf of Mexico, reaching its extreme 

 western point in Texas upon the lOOth raeredian. 



" Other causes, however, than insufficient rainfall and a nicely 

 l)alanced struggle between the forest and the plain have prevented 

 the general growth of trees in the prairie region east of the ninety- 

 fifth meridian. The rainfall of the region is sufficient to insure the 

 growth of a heavy frost. The rain falling upon the prairies of Min- 

 nesota, Wisconsin. Iowa. Illinois and Missouri, equals in amount that 

 enjoyed by the Michigan jieninsular and the whole region south of 

 Lakes Ontario and Erie, while prairies exist within the region of the 

 heaviest forest growth. It is not want of sufficient heat, or of suffi- 

 cient or equally distributed moisture, which has checked the general 

 spread of the forests over these prairies. The soil of which the 

 prairies are composed, as is shown by the fact that trees planted 

 upon them grow with vigor and rapidity, is not unsuited to tree 

 growth. It is not, perha])s, improbable that the forests of the Athin- 

 tic region once extended continuously as far west, at least, as the 

 ninety-fifth meridian. Although circumstantial evidence of such a 

 theory does not exist; and the causes which first led to the destruc- 



