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It was early learned that the whole of North America could beconveniently divided into seven transcontinental belts or life-zones,based upon the length of the growing season, which has been defined as the period between the date in the spring when the normal meandaily temperature rises to 43 degrees Fahr., or above, and the date inthe autumn when it falls to below that figure. (The reader should con-sult Bulletin No. 10 of the U. S. Department of Agriculture, Division ofBiological Survey, entitled "Life Zones and Crop Zones." Part IIIof that Bulletin is especially interesting). These life-zones are, asnoted above, adapted to plants requiring growing seasons of similarlength and temperature range. Thus, if soil conditions, exposure, andamount of annual rainfall are alike in two distant portions of a zoneplants which succeed in one portion may be expected to succeed in theother portion. They may in any event be tried out with considerableconfidence when all the conditions are known to be the same, asdescribed above. |
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