GEOGRAPHY OF PLANTS. 487 



to the cultivation of the grape in the United States east of the Mississippi. 

 DeCandoUe has sought for the causes which have prevented the successful cul- 

 ture of the grape in the middle regions of the United States, and has failed to 

 discover the reason why it has heen abandoned and its place supplied bj na- 

 tive varieties. He concludes that there is nothing either in the means or the 

 sums of the temperatures or the number of days of rain that can be shown to be 

 injurious to the vegetation of the vine, but suggests that in abundant rains, 

 especially at the period of blooming, which are known in Europe at those 

 places where the vine has not prospered, combined with other circvxmstances 

 which have eluded his research, such, perhaps, as diurnal variations of tempera- 

 ture or the humidity of the soil, we may find the true obstacle to the successful 

 culture of the Eui'opean wine grape in the United States. 



It is doubtless to the variable quantity of atmospheric humidity which char- 

 acterizes the climate of the eastern United States that we ought to attribute 

 the failure of attempts to cultivate the wine grape. Periods of excessive heat 

 and saturation continuing from two to eight days are common in the American 

 summer, but are rare in Europe. The maximum of 100° in the shade is fre- 

 quently observed in Canada, and south of Boston the mercury may be confi- 

 dently expected to make one or two visits to the region of 95° and 98° at one 

 or more times during " the heated term." This high heat of 95° is sometimes 

 attended by saturation with moisture, and at such times becomes destructive to 

 human life by causing exhaustion or depression of the vital powers through 

 the joint action of heat and humidity. When the highest measure of 100° to 

 104° which have been observed occur, when the air is very dry, equally inju- 

 rious consequences must follow to tender vegetation from the destructive dry- 

 ing of the tissues. In the eastern United States there often occur periods of 

 several days which are burdened with excessive moisture alternated with pe- 

 riods as marked by excessive dryness to which we find no parallel in the wine 

 districts of Europe. Our average of atmospheric humidity is, however, very 

 low, as is indicated by the prevailing forms of vegetation. Our forests do not 

 abound in mosses except in elevated districts. The grasses furnish evidence 

 of aridity which cannot be readily overlooked, and fail to cover the earth with 

 perennial verdure, as in the moister climate of England and western Europe. 

 South of the parallel of 38° north latitude the introduced grasses cannot be 

 cultivated. 



Though the uplands of Georgia and the interior northward to Pennsylvania, 

 districts in which the cultivation of the European wine grape has been at- 

 tempted without success, have many points of resemblance with those of 

 France as respects capacity for cultivation, in the extremes of dryness and hu- 

 midity they present no features in common. It is to these extremes we must 

 ascribe this wide disparity, for in our country it may be said to be alternately 

 too dry or too wet, too warm or too cold, to conform to the corresponding pe- 

 riods of the life of the vine as experienced in France.* 



It is exceedingly difficult to obtain observations which may be properly 

 compared because of the different local conditions of the experiments. A de- 

 cisive mode of comparing the humidity present in the atmosphere of America 

 and England may be found by measuring the quantity of water naturally 

 evaporated from an exposed surface. The following will serve to show the 

 difference between the evaporating powers of the air in England and in the 

 United States :* 



*Blodget's Climatology of the United States, pp. 226, 229. 



