GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



39 



summer, but of warmth in winter. Hence, Humboldt arrives at this con- 

 clusion : " That the lines of equal mean heat, which may be called isothermal, 

 are not parallel with the equator, but intersect the geographical parallels at 

 a variable angle." The mean annual heat of the same latitudes, in the new 

 and old worlds, are shown in the following table : 



Thus it is found that the old world is warmer than the new, and that the 

 heat of America does not decrease from Florida to the Gulf of St. Lawrence 

 iu the same ratio that it does in Europe from Egypt to Scandinavia. In 

 general, the summer temperature of North America, as far as 40 N. lat., 

 is about 4 higher than in Europe, under the same isothermal parallel ; 

 which accounts for Magnolias, Rhododendrons, Anonas, and other trees 

 extending so far to the north as latitude 36, where the summer heat scarcely 

 differs from the mean annual heat of the equator. 



138. A certain degree of difference is sometimes found in the vegetation 

 of a country according to its longitude ; but as this is occasioned almost 

 entirely by the nature of the face of the country, or its situation relatively 

 to the ocean, longitude by itself cannot be considered as having any influence 

 whatever either upon temperature or vegetation. 



139. The mean heat of any situation does not enable us to judge of what 

 particular species of plants will live there ; for the mean temperature found 

 may be deduced from such extremes of heat and cold as would suit but few 

 plants, as in the case of certain northern regions ; or it may be made up 

 from moderate limits in which many plants will live ; as, for example, from 

 the summers and winters of Ireland, or of the sea-coast of the middle of 

 Europe. Thus the constitution of a plant which may be very well suited 

 to the mean temperature of a place, may not be adapted to its extreme dif- 

 ferences. Hence many plants which will live in the open air at Belfast, 

 would perish in the winters of Edinburgh ; and many which would live there, 

 owing to the dryness of the air, and the moderate degree of cold from the prox- 

 imity of the sea, would perish in Yorkshire, where the air is not only more 

 highly charged with moisture, but much colder. Hence the mean annual 

 temperature of any place is of much less consequence with respect to the 

 stations of plants, than the mean monthly temperature, and the extremes of 

 each month. In general, " the western parts of continents are more nearly 

 equable in their temperature throughout the year than the eastern, and the 

 southern hemisphere than the northern ; and evergreens are found to affect 

 the former, and deciduous trees the latter description of climate." (Henslow.) 

 In all those parts of the world where the sea never freezes, the temperature 

 is higher, and much more equable than the temperature of inland situations 

 in the same degree of latitude ; and hence plants which mature their fruit 

 or ripen their wood at Edinburgh in the open air, require protection at 



