274 LIVINGSTON— CLIMATIC AREAS [April i8, 



tion data upon which the present study has necessarily been based 

 renders this chart of doubtful accuracy as a picture of normal con- 

 ditions, but it serves its purpose admirably, of illustrating something 

 of what may be possible in the way of quantitative vegetational 

 climatology, whenever the attention of climatologists may be seri- 

 ously attracted to this aspect of the application of their science. 



The relation- of vegetation to climate. 



In order to study vegetational distribution as this is related to 

 such climatic areas as have been brought out on the charts here pre- 

 sented, it is of course necessary to have recourse to corresponding 

 charts showing the distribution of natural or cultivated plants. It 

 ivould be beyond the scope of the present paper to attempt to show 

 by examples how the area occupied by any plant may thus be climat- 

 ically characterized, and such examples will not be brought forward 

 here. It may be repeated, however, that the obvious and visible 

 characters of the great vegetational types (such as those of conifer 

 and deciduous forest, grassland and desert), while exhibiting an 

 unequivocal relation to moisture conditions, still bear no relation to 

 conditions of temperature. Only when the thus far practically in- 

 sensible physiological characters of plants may be considered will it 

 become possible to relate their distribution to temperature conditions. 



The student of the climatic relations of plants must bear in mind 

 the extremely complex naturej)f the conditional complex which must 

 determine plant distribution. Aside from climatic conditions, the 

 nature of the soil usually plays an important part, as has been em- 

 phasized. Furthermore, numerous mechanical and other factors- 

 may have determined, in the past, whether or not a given plant form 

 may ever have reached a specified locality. Because of this historic 

 factor in plant geography, the climatic and soil conditions cannot be 

 taken as limiting distribution unless we are certain that the plants 

 thus limited have been tried throughout the region under discussion. 

 After they have been tried the historic factor vanishes from our con- 

 sideration. Nevertheless, without recourse to this removal of the 

 historic factor from the argument, it is still quite possible and logic- 



