34 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



It is generally admitted that heat is the most potent factor in 

 deterniinino: the distribution of plants, and that in general the old 

 division into Arctic, Temperate, and Tropical zones approximates the 

 real truth. . Such zonation, depending on heat, is far more wide- 

 reaching than one depending mainly on moisture. While the former 

 gives rise primarily to what we in general know as Arctic or Trt)p- 

 ical zones, the factor of moisture determines the opposed conditions 

 we distinguish as arid and moist. Diiferences in the heat factor 

 are universal, resulting in the whole earth being divided into more or 

 less well-marked zones, corresponding in general with isothermal 

 lines. Differences in the moisture factor are relatively local, so 

 that deserts may occur in the midst of the most varied surroundings. 



While the larger zones dej^ending on heat are strikingly ditlerent, 

 yet each passes gradually into the contiguous ones. Determination 

 of such zones is therefore more or less arbitrary. The scheme of life 

 zones, so far as North America is concerned, that has resulted from 

 the studies of Merriam has been generally adopted. It is as follows: 



{Arctic or Arctic- Alpine zone. 

 Hudsonian zone. 

 Canadian zone. 



(Alleghenlan area. 

 Arid Transition area. 

 Pacific or Humid Transition area. 



Austral region.- Upper Austral zone (Carolinian area. 



lUpper Sonoran area. 



.Lower Austral zone J Austroriparian area. 



iLower Sonoran area. 



Tropical region f Humid Tropical area. 



lArid Tropical area. 



In this scheme zones are based primarily on the distribution of 

 plants and animals as determined by the heat factor. The subdi- 

 vision of the zones or areas depend mainly on the differences due to 

 the moisture factor. As may readily be imagined, all possible combi- 

 nations of these two factors occur, so that regions of mixed character 

 are found wherever zones or areas are contiguous. This overlapping 

 of contiguous zones that are usually well marked is perhaps more 

 pronounced in the Pacific northwest than elsewhere in North 

 America. It has been ascribed to the very equable temperature of 

 the region. In consequence of this peculiarity the determination of 

 the life zones in Washington, so as to coordinate them with adjacent 

 regions, presents unusual difficulties. Six life zones or life areas are 

 represented, namely, the Arctic, the Hudsonian, the Canadian, the 

 Humid Transition, the Arid Transition, and the Upper Sonoran. 



The first-mentioned zone, the Arctic, is sharply marked, consist- 

 ing of the alpine flora above timber line. These alpine meadows 



