ON THE PLANT REMAINS IN THE SCOTTISH PEAT MOSSES. 61 



Some of these are plants of wide distribution from temperate to arctic regions ; 

 others, like Betula nana and Salix Arbuscula, have an entirely arctic or sub-arctic 

 distribution and do not now occur near sea-level in Britain ; but these basal beds 

 suggest less rigorous conditions than those in the Shetlands, for bog plants of wide 

 distribution are the most abundant ; whilst in the Shetlands, Salix reticulata is the 

 characteristic plant. 



The plants of the First Arctic Bed must have immigrated to Scotland, either across 

 the North Sea, or through England. The incoming flora would probably pass along 

 the higher ground, while the valleys and lowlands were still covered with the shrinking 

 remnants of the local ice-sheets. The evidence from the mosses in Lewis certainly 

 suggests that the incoming arctic flora barely established itself in the extreme west of 

 Scotland before the Lower Forest overspread the peat bogs, as the typical arctic plants 

 represented in Shetland are either absent or poorly represented in the corresponding 

 zone of the Hebrides. 



The Lower Forestian. — Considerable interest attaches to the flora of this zone, owing 

 to the absence of any arctic or sub-arctic plants, suggesting that a complete change 

 of conditions took place between the first and second arctic beds during which the 

 arctic flora withdrew from the peat areas. 



In the Southern Uplands, Hebrides and the Shetlands, the Lower Forest is well 

 preserved, the trees being Betula alba, Corylus Avellana, Alnus glutinosa. The list 

 of accompanying plants is not large, as the peat is chiefly formed of wood debris and 

 bark. Inferences as to temperature drawn from these trees alone are inconclusive, but 

 certain features presented by the distribution of the forest-bed bear directly upon the 

 general climatic conditions of that time. The presence of a buried forest extending to 

 the western coast of Shetland points indubitably to the fact that the path of the Atlantic 

 cyclonic systems must then have been different to that of the present day. Trees are 

 now entirely absent from Shetland, although many attempts have been made to form 

 plantations in sheltered spots. Planted in gardens, ash, rowan and birch grow as high 

 as the surrounding walls, and are then stunted by the winds, so that the trees appear 

 to be shorn off at the level of the protecting wall. If this takes place at the inland 

 and more sheltered spots, it is still more difficult to account for the presence of the 

 buried forest near the cliffs of the west coast, where the full force of the Atlantic gales 

 is felt, as they succeed one another during the greater part of the year. As the mean 

 annual temperature of the Shetlands is perhaps higher than many parts of Britain 

 which are tree-clad, the absence of trees in these islands must be due, not to tempera- 

 ture, but to the force of the salt-laden winds. 



The accompanying flora of the Lower Forest contains many plants of wide distribu- 

 tion which do not give any definite evidence with regard to temperature. The zone 

 also contains other plants, which are not usually found accompanying arctic plant 

 associations, such as Ajuga reptans, Ranunculus lingua, Lychnis diurna, Salix 

 purpurea. 



