34 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



It is ivenorally adniittod that heat is the most potent factor in 

 determining the distril)iiti()n of phmts, and that in o-eneral the old 

 division into Aivtie. Temi)erate, and Tropical zones approximates the 

 real truth. Such zonation, dependino- 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 Trop- 

 ical zones, the factor of moisture determines the opposed conditions 

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

 are universal, resulting in the whole earth l)eing 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 dependmg on heat are strikingly different, 

 yet each passes gradually into the contiguous ones. Determination 

 of such zones is therefore more or less arl)itrary. The scheme of life 

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

 the studies of Merriam has been generally adoi)ted. It is as follows: 



f Arctic (iV Arctic- Alpine zone. 

 Boreal regi m -- Hndsonian zone. 



I ( 'anadian zone. 



f Alleghenian area. 



Transition zone - - - . Arid Transition area. 



i Pacific or Hnmid Transition area. 



Austral region •, Upper Anstral zone fCarolinian area. 



lUpper Sonoran area. 



Lower Austral zone f Austroriparian area. 



iLower Sonoran area. 



_, . T • (Humid Tropical area. 



Tropical region i / 



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 usiuilly 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 consecpience 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 



