70 INFLUENCE OF CLIMATE. 
and Western Pennsylvania, north of Reading and Pitts- 
burg, through Northern Ohio, touching the shore of Lake 
Erie, Southern Michigan, including Northern Indiana and 
Illinois as it passes around the southern extremity of Lake 
Michigan, and thence through Southern Wisconsin, and 
Minnesota. North of this line, the cane fails to ripen. 
South of it, it ripens generally everywhere when planted 
early, but this is rendered more certain, and the growth of 
the cane is more vigorous when the summer line of 72° is 
reached. There it attains its fullest development. The 
line of 72° crosses the State of Delaware going westward, 
extends through Central Maryland into Virginia, where it 
passes up the Shenandoah Valley, being sharply deflected 
southward by the mountain chains, thence through Westera 
Virginia, Southern Ohio, Central Indiana, Illinois north 
of Springfield, and Southern Iowa. North of this line, the 
period of growth is necessarily extended, and the summers 
are gradually shorter and cooler as we approach the line of 
70°. There one hundred and thirty days from the time 
of planting are not sufficient, and the necessary amount of 
heat which is lacking must be added in September to ripen 
the plant. Beyond this point, the period of growth is ex- 
tended to one hundred and fifty or one hundred and sixty 
days, but before that time has elapsed, the early frosts 
suddenly intervene, and cut off the crop. Occasionally 
the same thing occurs far to the southward, but it is hap- 
pily quite unfrequent. 
/ In the vast extent of country between the line of 72° 
| and the cotton and cane regions of the South, there is 
certainly no climatic obstacle to the successful growth of 
sorghum. The future sugar district of the United States, 
instead of being limited to a narrow border along the 
Gulf of Mexico, may now be defined to be a great conti- 
-nental belt of irregular outline, having for its northern 
\boundary tne summer line of 72° Fahr. 
