34 



OUTLINE OF PLANT GEOGRAPHY 



(Hieracium) , orchids, and other forms absent from the extreme 

 arctic regions. 



In the high northern latitudes, elevation above sea-level, ap- 

 parently makes little difference in the temperature, many plants 

 growing at altitudes of 500-600 metres or even more. 



The severity of the arctic climate, and the very brief growing 

 season, make any sort of agriculture impossible, and except for 



Fig. 3. — Tundra vegetation, basin of Colville River, North Alaska. 



Photo., Dr. Philip S. Smith. 



the grazing afforded reindeer and musk ox, and a few edible lichens, 

 berries and roots, the plant-life of the arctic regions offers little 

 help to man in supporting life. 



The Sub-arctic 



The northernmost extension of tree-growth, the timber-line, 

 marks the southern limit of the strictly arctic vegetation. Be- 

 tween the timber-line and the middle regions of the boreal area, 

 lies a rathor indefinite zone, the sub-arctic. 



The number of tree species is limited, but they are often gre- 

 garious, and may form forests of great extent. Coniferous forests 

 are a marked feature of the sub-arctic, comprising firs, spruces, 

 pines and larches, and low-growing junipers. Of deciduous trees, 



