48 TRANSACTIONS OF ROYAL SCOTI'ISH ARBORICULTURAL SOCIETY. 



formation a very open one and, in addition, to modify 

 the individuals considerably. For example, where plants have 

 established themselves on a ridge they assume crescentic, 

 clipped forms, while on the steeper slopes the flat scrambling 

 habit with spreading root-system, so typical of Salix serpyllifolia 

 and Azaiea procumhetis, is characteristic. Other individuals of 

 this association are Arctostaphylos alpina, Dap/we mezereum, 

 Rosa a/pina, Salix reticulata, Sa/ix relusa, Dryas octopetala, 

 Helia7ithemu)n vulgare and Globularia cordifolia. A comparison 

 of the above flora with that described by Dr Crampton ^ 

 indicates striking differences. In the latter flora the dominant 

 individuals are Vacciniutn myrtillus, Calluna vulgaris, Erica 

 cinerea, Alcliemilla alpina, Luzula silvatica, Empetrum nigrum. 

 Azalea procumbens. The only plant common to both is Azalea 

 procumbens. As will be seen later, the flora referred to by 

 Dr Crampton occurs at a lower elevation in association with 

 mountain pine {Pinus nionta7ia) and larch {Larix decidua). 



The evolution of step-formation is probably as follows: — 

 One of the above-mentioned plants establishes itself on a frost- 

 debris slope, and immediately the sliding debris begins to collect 

 behind it. This causes a rise in level and at the same time 

 a diminution in slope at that point, with the result that a 

 miniature terrace is formed. The plant then by pressure from 

 behind assumes a more or less vertical position, and, depending 

 on the species and on the robustness of growth, it may develop 

 so that the top is horizontal, while the lower part is almost 

 vertical, thus forming a step (PI. III. fig. i). Such steps may be 

 overwhelmed by falling debris during or after their formation 

 and disappear. The formation is thus a shifting one, and as 

 such its limits of occurrence cannot be definitely laid down, 

 though in this locality it may range from 7000 feet to 8000 feet. 

 As the lower limit of this formation is approached the association 

 becomes more closed. 



PiNUS MONTANA (BUSH FORM) ASSOCIATION. 



Descending through the above step-formation a change in the 

 type of vegetation is observed, and mountain pine makes its 

 appearance. As is seen from PI. III. fig. 2, the latter at its 

 upper limit is fairly open. Towards its lower limit, however, 



1 See The Vegetation of Caithness considered in Relation to the Geology, 

 C. B. Crampton, M.B., CM., 191 1, pp. 43-46. 



