29 PHYSIOLOGICAL BOTANY. PART II. 



places where the mean annual temperature is found to 

 be the same, Humboltlt established a series of " Iso- 

 thermal" lines intersecting the parallels of latitude. But 

 these lines by no means show us what might be the 

 probable range of particular species. For an isothermal 

 line may intersect a range of country where the extremes 

 of heat and cold are very different ; and the constitution 

 of different species, which may be equally adapted to a 

 given mean temperature, may not be equally suited to 

 these differences in the extremes. Thus many plants 

 which will live in the open air at Edinburgh, would 

 perish during the severer winters of more southerly 

 regions, whilst many that can stand greater cold than 

 that to which they would he exposed at Edinburgh, 

 require also greater heat in the summer than they would 

 find there, in order to bring their fruit to perfection, or 

 even to ripen their wood sufficiently to maintain them 

 in a healthy condition. In fact, the mean distribution 

 of temperature throughout the year, is a considera- 

 tion of much less importance than the distribution 

 per month, which perhaps most effectually regulates 

 the range of species. As annuals cannot maintain 

 their footing in any climate without yearly perfecting 

 their seeds, they are necessarily limited to more tem- 

 perate habitations than certain perennials ; it is suf- 

 ficient for the latter, if they occasionally meet with 

 a season in which they may be able to do so. It has 

 been remarked that 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 that evergreens affect the 

 former, and deciduous trees the latter description of 

 climate. Maritime districts have always a more nearly 

 equable temperature than such as are inland. 



Besides the physiological relations which plants pos- 

 sess with regard to temperature, there are others of a 

 physical character by which their distribution is con- 

 siderably affected. Where the temperature is so low 

 that water exists only in the form of ice, it cannot be 

 imbibed by the roots, and no plants can live, there. 



