298 PLANT LIFE. 



1. The Arctic-alpine association, in the broadest sense, 

 which is only found at great altitudes or on the sea 

 coast (see p. 195). The level varies in different parts of 

 Britain. Thus plants of this character occur at much 

 lower levels near Helensburgh than they do in Dumfries- 

 shire {Oxyria digyna occurs at 600 feet on Benchaorach 

 and at nearly 2000 feet near Moffat). Their lower limit 

 is often much higher on the southern side of a mountain 

 than on its northern flanks. Moreover they are often 

 carried down the steep slopes of the burns and colonise 

 shingle beds and sandbanks a long way below their 

 proper position. Hence their correct range is very 

 difficult to determine. Oxyria digyna^ Alcheinilla 

 alpina^ Silene acaulis, and Gnaphalium supinum are 

 good types of this flora. 



2. The Pine-bracken flora has been so much altered 

 by the grazing of sheep, and the destruction of the 

 Pine-woods, that it is almost impossible to find any 

 typical examples. Parts of the Black Wood of Ran- 

 noch, the older parts of the pine forest near Rothie- 

 murchus and Spey Bridge, and Achnacarry forest are 

 supposed to be as nearly primaeval as we can find in 

 this country. The Sylva Caledonica of the Romans 

 is supposed to have been this Pine forest, and traces 

 of it may be found in many peat bogs below the 

 stratum of bog oak. In Denmark it seems to have 

 covered the country when man was in the Neolithic 

 stage of civilisation. 



3 and 4. The Oak-forest association and that of the 

 Beech covered the greater part of the British Islands at 

 the time of the Roman invasion. The country has 

 been completely cleared of these woods, and the ground 

 has been so much altered by replanting and by cultiva- 

 tion, and possibly also by climatic changes, that it is 



