GEOGRAPHY OF PLANTS. 



475 



ON THE RANGE OF CERTAIN ANNUAL PLANTS. 



It has been the custom to attribute the disagreement between the limits of 

 plants and the lines of equal temperature to errors of observation on the locality 

 of species, to uncertainty respecting the thermometric mean, or to varying de- 

 grees of dryness and humidity. But certain facts and calculations upon the 

 heat requisite for culture in different countries have awakened doubts of the 

 truth of these explanations, and have led to more minute investigations. 



As each species of plant can bear a deiinite range of temperature, indeed, 

 requires a certain amount during a given period of time to enable it to perform 

 all its functions properly, the only true indication of climatic adaptation is, 

 that the plant, under the conditions assumed, perfect its seed and produce its 

 various secretions ; and more recent research has demonstrated that neither the 

 annual mean, nor that of the summer season, accurately indicate the northern 

 limits of the extension of the range of our annual plants. An extended series 

 of comparisons of the i-ange of many European plants, with the tables of monthly 

 temperatures and seasons over the districts where these plants naturally appear, 

 has shown that in no case does the limit of a species exactly coincide with a 

 line of equal temperature for any one period of the year. Also, which will ap- 

 pear moi-e worthy of remark, that the limits of annual plants in the plains of 

 Europe cross one another with considerable frequency, and that the limits of 

 perennial and ligneous species also cross each other in different directions, and 

 both are far from being parallel when they do not thus cross. 



The lines of mean temperatui'c are derived from a consideration of fixed pe- 

 riods whether of a month or of a season, while the vegetation of annuals lasts 

 during periods which are variable, the vegetation of the plant having its com- 

 mencement and completion at difterent epochs of the year dependent upon the 

 condition of greater or less excess of heat when compared with the mean of the 

 growing season. There can obviously be but small agreement between these 

 two classes of facts which have no necessary connexion or relation. 



A. de Candolle has taught that if we would estimate the heat really useful to 

 a plant we must consider in our calculation those values only which are above 

 a certain degree of temperature, which degree varies with the species. This he 

 adds is a field hitherto unexplored, and proceeds to explain the application of 

 the above principles in combination in European climates, by which are brought 

 about similitudes or dissimilitudes to which the means ordinarily employed 

 furnish no key. 



London and Odessa are certainly not under the same lines of temperature. 

 The mean summer heat at Loudon is Gl° Fahrenheit, at Odessa 70^ Fahren- 

 heit ; while in winter the difference is rather greater. In the monthly means 

 these two climates have but imperfect analogy. The monthly means at London 

 and Odessa appear to be as follows, in degrees of Fahrenheit : 



For greater convenience we will convert the above into degrees of centigrade, 

 or the French mode of dividing thennometers, which shows but 100 degrees 



