34 INTRODUCTORY EXPLANATIONS. 



mountains, than is the outlet of Loch Callater. From 

 1,000 to 1,200 feet is more fi-equently the actual limit of 

 com and potatoes in the Highland counties. In one shel- 

 tered spot, in the woods of Lochnagai-, the Pferis was ob- 

 served at 1,900 feet; and in another pait of the same 

 woods at 1,700 feet. On exposed moors, in the Higlilands 

 generally, it is veiy seldom seen above 1,200 feet, unless in 

 hollow depressions or on those declivities which front to 

 the sun. On open moors the Myi'ica Gale will rise higher 

 than the Pteris ; having been seen at 1,700 feet on a bleak 

 exposm-e in the forest of Drumochter, which forms a pait 

 of the northern declivity of the central Grampians, and is 

 stamped by the impoverished or arctic character of its 

 vegetation at comparatively moderate altitudes. More fre- 

 quently the Mp-ica ceases at 1,400 or 1,500 feet. Upon 

 these two regional divisions, we may found an aiTangement 

 of species into three climatic groups, thus : — 



Arctics, — found only in the arctic region. 



Arctic-Agi'arians, — fomid in both regions. 



Agrarians, — found only in the agraiian region. 

 But the objects of botanical geography will frequently 

 require such a sub-division of these two regions, as will en- 

 able us more exactly to express the range of climatic dis- 

 tribution for each species. For instance, the Clematis 

 Vitalba and Ulex europaeus are alike limited to the agra- 

 rian region, and yet their climatic ranges are very unequal. 

 The Clematis can scarcely be deemed native in this coun- 

 tiy farther north than latitude 53, while the Ulex is certainly 

 native beyond 56, if not beyond 58 of latitude. And in re- 

 spect of elevation, the Ulex will rise to thrice the altitude 

 at which the Clematis ceases to gi-ow. Equivalent differ- 

 ences occur between the range of climate for the Arctics, 

 as also for the Arctic-Agrarians. Genista anglica, Calluna 

 vulgaris, and Vaccinium Myrtilhis will afford striking ex- 



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