GEOGRAPHY OF PLANTS. 477' 



Fot the purpose of discovering these correspondences of climates De Can- 

 dolle calculated the days on which the temperature 1°, 2°, 3°, &c., up to 8° 

 centigrade commence and end at certain European localities ; these all being 

 above the freezing point, and correspond, respectively, to 33.8°, 35.6°, 37.4^, 

 &c., up to 46.4° Fahrenheit. At the lowest of these temperatures some forms 

 of vegetable life are called forth from their winter sleep. By associating with 

 each locality noted, the product indicating the heat received in excess of each 

 of these degrees, and ap]>lying these figures to the facts of vegetation, highly 

 o'atisfactory results are obtained. One instance we may adduce in illustration 

 of this method. 



The Ali/ssum calycinum, an annual plant, grows on the eastern coast of 

 Britain, as high as Perth. It is not found on the western coast of England, 

 nor in Ireland, in Jersey, in Guernsey, nor in Brittany, which may be attributed 

 to the constant humidity of those regions, for this plant loves a dry atmosphere, 

 and a plant that grows in Scotland cannot be said to find the heat of Brittany 

 insufficient for its existence. On the continent, the Ahjssum cahjcinum spreads 

 to the northwest as far as Holstein and the Baltic sea, but not into Scandinavia, 

 and on the northeast as far as Moscow, but is not found in Kazan in about th« 

 same latitude, though further cast, and reappears in the governments of Pensa 

 and Simbrinsk, but little south of Kazan, and again in the Steppes between 

 the Volga and the Ural. The limits where its extension may be thought to be 

 determined solely by temperature stretches, therefore, from near Perth, in Scot- 

 land, under the parallel of 5Qh degrees of north latitude, passes along the 54th 

 degree in Holstein, and thence oscillates in Russia between the 55th and 56th 

 de^-rees. This line varies widely from any line founded on equality of temper- 

 ature for any season or month of the year. On examining a table of daily 

 temperatures, it appears that at or near Perth, in Scotland, the temperature of 

 7" centigrade, about 45° Fahrenheit, or upwards, continues from the 18th of April 

 to the 3ist of October, and that, during this time, the product of the days bj 

 „iic mean temperature amounts to 2,281° centigrade. At Koenigsberg, on the 

 Baltic, the temperature of 7° centigrade and upwards is of shorter continuance, 

 but the summer being hotter, the product amounts to 2,308^. As the limit of 

 the species is about sixty miles to the north of Koenigsberg, this figure must be 

 reduced, and thus more nearly resembles that found for Perth, in Scotland. At 

 Moscow the mean of 7^ centigrade commences on the 22d of April, and termi- 

 nates on the 5th of October, the product in consequence of the heat of summer 

 rises to 2,473°. This is more than is necessary for the Ahjssum which probably 

 flourishes more than one hundred miles north of JIoscow. At Kazan the figures 

 fall to 2,196°, so that it is not surprising that the Ahjssum cahjcinum has dis- 

 appeared. Thus the hypothesis of 7° centigrade for an initial temperature or 

 miLiimnm at which this plant commences to vegetate, and 2,280° to 2,300° for 

 the quantity of heat demanded, accord completely with the facts. 



Another instance may be adduced of conformity of facts with figures derived 

 from temperature. The Euonymus Europceus, or European spindle-tree, nearly 

 related to the burning- bush of our gardens, has for its northern limit the north 

 of Ireland, Edinburgh, in Scotland, the north of Denmark, the south of Swe- 

 den, the isle of Aland ia the Gulf of Bothnia, latitude 60° north, Moscow, 

 southeast to Penza, in Russia, in latitude 53°, through a latitude of seveii 

 degrees. The mean annual and mean summei* temperatures vary several 

 degrees at these places when they are respectively compared, nor do the 

 monthly temperatures agree much more nearly. The Euonymus requires a 

 temperature of 2,480° centigrade, the sum of the daily temperatures between 

 the two epochs of the year when the curve of mean temperature ascends 

 above 6° centigrade. At Edinburgh this is found to be 2,482°. At Stock- 

 holm, beyond the limit, the sum is but 2,268°. At St. Petersburg, like- 

 wise beyond it, it is too small. At Aland there may exist a more elevated 



