GEOGEAPHY OF PLANTS 469 



The summit of Mount Washington,* six thousand two hundred and eighty- 

 eight feet high, produces several phints which have no representatives south 

 of Labrador. These, it may be interesting to some readers to be informed, are 

 Andromeda hypnoides, Saxifraga rivularis. Rhododendron hippouicum, and 

 Diapensia lapponica. 



We are all as well aware that the mean annual temperature in this country 

 decreases as we proceed towards the north, as we are that we shall encounter 

 increase of cold on ascending a high mountain. All are not equally well 

 acquainted with the rate of variation either on the plain or on the mountain 

 slope. In central Europe the change in mean annual temperature takes place 

 at about the rate of one degree of Fahrenheit for each degree of latitude, or for 

 about every seventy miles; while in the interior of the United States, in the 

 Mississippi valley, the rate of decrease is about one degree of Fahrenheit for 

 every forty miles ; or, in other words, as we travel north or south we reacji 

 successively localities the mean annual temperatures of which are one degree 

 of Fahrenheit's thermometer lower or higher as we pass over seventy miles in 

 central Europe, or forty miles in the valley of the Mississippi. In the Alps of 

 Switzerland, in ascending or descending, the same change of one degree in 

 mean annual temperature is experienced for about every three hundred feet of 

 vertical height, so that we can pass, within the narrow limits of between six or 

 seven thousand feet, from the vine-clad shores of the lakes of northern Italy to 

 the icy fields or snow-capped mountains, whose summits are never adorned by 

 vegetation, a journey that may be made in a single day; while to descend to 

 the level in our ov>'n land and latitude, which corresponds in mean annual tem- 

 perature to that experienced at the foot of the Alps, and travel northward, we 

 shall find in the valley of the Mississippi a diminution of one degree of tem- 

 perature for every forty miles ; so that to find the region where the mean annual 

 temperature is reduced to the freezing point, we should travel over about twelve 

 degrees of latitude. This would conduct us to districts beyond the northern 

 borders of Lake Snperior before we could pass over the same range of climatic 

 changes as we would encounter in one day upon the slopes of the Alps. 



The mean annual temperature of 32^ Fahrenheit does not imply a total 

 absence of vegetation on the plains, or in the northern districts of North 

 America, nor the presence of eternal frost. The heats of summer extend far 

 beyond the limits above referred to, and CA'^en Indian corn may he ripened 

 beyond the line of mean annual frost, or of a mean annual temperature of 32° 

 Fahrenheit in districts northwest of Lake Superior, where, in the valley of the 

 Red river, in latitude 50° north, about sixty days of clear tropical summer 

 occur, which are sufficient, for reasons to be hereafter cited, to ripen some 

 varieties of this most valuable of cereal grains. 



Though the correspondence between the forest vegetation on mountain sides 

 and the distribution of trees is thus marked, there exist many circumstances 

 which show that climatic influences alone, or as at present existing, however 

 extensive, will not fully account for the geographical distribution of plants and 

 animals. Their various limits do not agree precisely with the outlines indi- 

 cating the intensity of physical agencies upon the surface of the earth. The 

 limit of forest vegetation does not, as before remarked, coincide with the 

 isotherm or line of the mean annual temperature of 32° Fahrenheit; nor is the 

 limit of vegetation in altitude on mountain sides strictly in accordance with the 

 mean temperature ; nor are the plants and animals distributed under the dif- 

 ferent zones of climate the same in their respective zones in the northern and 



*The above-cited heights are taken from Gujot's "Appalachian Mountain System," a 

 very lull detail of recent accurate measurements of most of the more elevated portions of the 

 mountains of the Atlantic border of the United States. — See Silliman's Journal, (l!iGl,) 

 XXXI, pp. 158 to 187. 



