64 THE SCOTTISH BOTANICAL REVIEW 



drought that has to be endured by shallow-rooted vegetation 

 is increased by this poverty in humus. It is on such surfaces 

 that the most typical group of the so-called calcicole flora is 

 developed. But even at its closest contact the influence of 

 limestone on plant-association is probably very complex, and 

 may be different in different species at very short distances. 

 Certain species that are common in the clefts of limestone 

 pavement, e.g., Oxalis Acetosella, are also abundant on the 

 most acid type of humus, provided they are shaded from the 

 sun. Others, as Teuci'ium Scorodonia, are abundant both on 

 limestone and on leached rocks which accumulate humus, 

 provided they are well drained. Erica cinerea, Calluna 

 vulgaris and Arctostaphylos Uva-iirsi grow on bare limestone 

 in some places, and on leached rocks and acid humus else- 

 where (27). The species considered most distinctive of lime- 

 stone do not, however, often grow in situations developing 

 acid humus. They appear to be chiefly confined to dry 

 limestone soils, where the accumulation of humus is very 

 scanty. 



The reason for this preference for limestone is unknown, 

 but their restriction to dry limestone soils, where humus fails 

 to accumulate, and where they may sometimes be found 

 associating with some of the plants characteristic of very dry, 

 heathy, or acid soils, suggests that there may be something 

 in common between such apparently diverse habitats. What 

 does the Bearberry find in common in such habitats as dry 

 limestone slopes in the Alps, and the most acid type of peat 

 bogs in the north of Scotland, other than physical and 

 physiological drought ? In the one habitat it finds abundant 

 lime and little humus, in the other abundant acid-humus and 

 practically no lime. In this connection certain statements 

 by Schroter in regard to the habitats of Pinus montana are of 

 peculiar interest. He writes : " How fundamentally diverse 

 are the habitats of the mountain pine, which can grow on the 

 dry, loose, calcareous talus of a hot, southern slope, and on 

 high moors, which are mainly composed of bog-mosses and 

 are subalpine bogs dripping with water but poor in mineral 

 matter. The former soil is poor in humus, rich in mineral 

 matter, and dry ; the latter is a substratum rich in humus, 

 poor in mineral matter, and always saturated with water. 



