INFLUENCE OF CLIMATE ON VEGETATION. 659 



mountains, desert tracts, prevent their spreading indefinitely : 

 some of these may be overcome by accidental influences, such as 

 seas intervening between countries, and the like ; others can 

 neither be conquered nor evaded, such as climate, which fixes un- 

 alterable limits to the stations which can be permanently occupied 

 by species. 



That Cold and Heat, Damp and Drought, intensity and duration 

 of Light, the chief constituents of what we call Climate, are the 

 most important of the external influences acting upon plants, is a 

 fact manifest not merely from the conclusions at which we have 

 arrived in the study of Vegetable Physiology, but one which is 

 revealed by the most slender experience of horticulture and the 

 most superficial acquaintance with physical geography. 



The nature of the soil doubtless has also much influence on the 

 distribution of plants, dependent less, probably, on its chemical 

 composition than on its mechanical constitution and hygrometric 

 state. 



Every species of plant flourishes best within a certain range of 

 temperature, beyond which, on both sides, it either suffers from 

 summer heat or is killed by winter cold. 



If the earth's surface were of uniform character, we might 

 expect to find forms of vegetation arranged in bands or zones suc- 

 ceeding each other from, the equator toward the poles, each occupied 

 by plants " hardier" than those of its equatorial neighbour. Such 

 zones of vegetation have in fact been laid down by botanical geo- 

 graphers. Meyen drew up a plan, in which a number of zones 

 were marked, defined on each side by lines passing round the earth 

 at certain parallels of latitude, between which a certain average 

 climate was assumed to exist. 



Isothermal Lines. From the want of uniformity of the surface of the 

 globe, the isothermal lines, i. e. lines passing through spots which have 

 an equal annual temperature, by no means correspond to parallels of 

 latitude the distribution of land and sea, and the alternation of plains 

 and mountains, deflecting such lines to the north and south, sometimes to 

 a very great extent. In addition to this, from the diversity of habit of 

 plants, they are differently affected by heat and cold, and the distribution 

 of species is far more influenced by the summer and winter temperature 

 than by the entire amount of heat received during the year. 



Nevertheless, as a general rule, plants have a polar and an equatorial 

 limit, fixed by temperature. 



Temperature as regards plants may he divided into useful, useless, and 

 injurious or destructive. Each plant, as we have seen, requires a certain 

 sum of heat to live, so much more to flower, more still to ripen its fruit, 

 &c. Within certain limits it is immaterial whether the plant get this 

 heat in a short period or diffused over a longer time. Temperatures below 

 C. are useless to most plants (that is to say, vegetative action is 



