PLANT MIGRATION. 505 



rich in humus; between alkaline areas and those with a small 

 salt content, and so on. A study of the vegetation elements 

 along these lesser lines of stress is possible in any locality where 

 one may see the struggle along the border line between plants 

 occupying different kinds of territory. 



973. Total heat as limiting factor in north and south migra- 

 tion. According to Merriam plants and animals are limited in 

 their northward distribution by the sum total of heat (above 

 6 C.) during the period of growth and reproduction (the sum 

 of the effective temperatures), but they are limited in their south- 

 ward distribution by the mean temperature of the hottest part 

 of the year. Since some plants and animals require a larger 

 sum total of heat than others, they are drawn into these different 

 life zones in accordance with their temperature requirements. 

 But singularly on the Pacific slope there is greater mixing of 

 boreal and austral forms, the boreal forms extending far south- 

 ward and the austral forms far northward. This is due to the 

 more equable temperature during the reproductive period. The 

 comparatively low mean temperature during the summer per- 

 mits northern forms to go far southward; while the high sum 

 total of effective heat permits southern forms to go far north- 

 ward (see characters of the Pacific coast transition area, Chap- 

 ter LVII). 



974. Limiting factor for east and west distribution in United 

 States. On the other hand, plants and animals are limited in 

 their east and west distribution in the austral and tropical regions 

 of North America by difference in rainfall and humidity, the 

 great arid area in the centre of the continent affording an effec- 

 tual barrier to the west or east movement of the east or west 

 forms, while the forms of the arid area are prevented from mov- 

 ing east or west of this area partly by the dominance of the forms 

 in the humid area and partly by the specialized character of arid 

 area plants unfitting them for humid climates. The mountain 

 ranges also to some extent limit east to west distribution, but 

 the factor here is heat, and these areas as pointed out above are 

 southward extensions of the general transcontinental zones. 



