CLIMATIC ZOXI :s 



35 



birches, alders, willows and poplars characterize the more northern 

 portions of the zone, while toward the south, oaks, beeches, horn- 

 beams and chestnuts mark the beginning of more temperate con- 

 ditions. 



Like the arctic zone, the vegetation of the sub-arctic is very 

 much alike in both hemispheres, due in part to a large intermixture 

 of arctic species, but also to the fact that most of the sub-arctic 



Fig. 4. — Northern limit of trees; white spruee, Unakserak River, lat. OS . 



Photo., Dr. Philip S. Smith. 



genera, and a good many species are common to Eurasia and 



America. 



Throughout the zone one encounters a very characteristic 

 forest and bog flora composed of identical or closely related species, 

 and this flora is met with far south of the sub-arctic in the moun- 

 tains, and in the colder parts of the temperate zone. 



A notable feature of these northern latitudes is the abundance 

 of club-mosses of the genus Lycopodium, most of the species 

 occurring both in Eurasia and America. These are undoubtedly 

 very old forms, and the same may be said of the horse-tails (Equi- 

 setum), and a number of wide-spread species of sub-arctic ferns. 



Carpeting the floor of the northern evergreeD forests are several 

 familiar and attractive plants of the wintergreeD family (Pyro- 

 laceae). Several species of Pyrola, the dainty Bweel scented one- 



