The Problems of Plant Distribution 243 



FACTORS CONCERNED IN PLANT DISTRIBUTION 



The factors that have been active in determining 

 the distribution of the existing floras of the earth 

 are many and complicated. Some of them are suf- 

 ficiently clear, but of many we are quite ignorant at 

 present. Of the most obvious conditions, probably 

 climate, i.e., temperature and moisture, is of most 

 importance; but several other very evident factors 

 may be mentioned which play parts quite as impor- 

 tant in the distribution of plants. These are the 

 continuity of land areas, composition of the soil, and 

 the exposure of the soil. But perhaps most impor- 

 tant of all are the individual characters of the plants 

 concerned, some being especially adaptable and pro- 

 vided with ready means of transportation, others 

 very particular as to their requirements of growth, 

 and therefore confined to extremely limited areas. 

 It is the underlying causes of these great differences 

 in the very constitution of different plants, that 

 are the most obscure and little understood factors 

 governing plant distribution. 



Uniform Flora of Northeastern United States 

 Where there are extensive areas with uniform cli- 

 matic conditions, and no barriers to prevent ready 

 communication, the flora will be found to be very 

 similar, varying only with the local peculiarities of 

 soil or elevation. The Northeastern United States 

 is an excellent example of such an area. A very 

 large number of species occur throughout the entire 



