NORTHERN RANGE OF SORGHUM. 69 
during three or four months of the year. These months 
cover the whole period of growth of this cane, and where- 
ever the high summer temperature is sufficiently protracted, 
and the requisite conditions of soil and moisture are sup- 
plied, its cultivation may be successfully carried, although 
the mean annual temperature of such a region may fall con- 
siderably below that of the native country of the plant. 
Our common corn, a subtropical plant also, furnishes a 
good illustration of this unique feature of our summer 
climate in its capability of being here successfully extended 
in cultivation up to a latitude unexampled elsewhere. But 
even Indian-corn may ripen in a climate where the yield 
is too meager to render it a successful competitor with the 
hardier grasses, or to reward the toil of the cultivator. 
The same is true of sorghum. The limit of remunerative 
production will fall considerable short of the line which 
marks the northern boundary of its growth. A certain line 
of equal summer heat quite accurately fixes this limit. 
So far as temperature is concerned, the growth of any 
annual plant may be successfully carried to any geographi- 
eal limits within which the specific amount of heat neces- 
sary to the species is uniformly supplied, during a definite 
period of time. This period, which measures the whole 
life of the annual plant from the germination to the ripen- 
ing of the seed, has been found for the larger varieties of 
sorghum in those climates where it has uniformly ripened 
and attained to the fullest perfection, to average from 
130 to 140 days. The line of temperature which marks 
the northern range of the sorghum as a sugar-producing 
plant on this continent, may be defined to be the summer 
isotherm of 76° as determined by Blodget in his “ Clima- 
tology of the United States.” This line passes from the 
east through Southwestern Connecticut, the southeastern 
extremity of New York, Northern New Jersey, Eastern 
~ 
