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and barley ripen ; but wheat will not ripen, since the summer 
temperature never rises sufficiently high for this purpose. 
What this temperature is I shall inquire immediately. 
So with the vine, which requires, in order to produce 
good wine, at least five months of a mean heat of 60 deg. 
Fahrenheit. If September and October have not this degree 
of heat, the wine is sour. For these reasons, therefore, that 
the temperature adapted for wheat is not indicated by lati- 
tude, nor by isothermal lines, but simply by a country 
having a certain number of days of a certain summer heat, 
or by the isotheral heat of Humboldt, let us examine its pro- 
duction in the three astronomical zones of the earth, viz., the 
Torrid, Temperate, and Arctic circles — for its cultivation 
fails north of 60 deg. These three zones, however, are too 
extensive, and include so many forms of vegetation, that 
Meyen has marked out smaller ones, and has divided each 
hemisphere into eight smaller zones. 
To begin the description with the Torrid Zone, called, by 
Meyen, the Equatorial Zone. 
The Equatorial Zone embraces, on both sides of the 
Equator, a zone of 15° of latitude, and a mean annual tem- 
perature of 80°, a heat which, in union with as high a degree 
of atmospherical moisture, calls forth an extraordinary pro- 
fusion of vegetation. In several tropical countries wheat 
and the northern cereals are grown in winter, often in the 
very places where tropical fruits are grown during the wet 
summer months. Meyen saw this in the neighbourhood of 
Canton, and Royle mentions it for India, where, in some 
parts, in winter, the vegetation has a perfectly European 
aspect, and many species of true European genera make 
their appearance. 
The low elevations at which wheat is grown in some parts 
of the tropics is remarkable ; at Victoria, in the province of 
Caraccas, with 10° of north latitude, at the height only of 
