n6 ROBERT SMITH [February 



in so far that a number of the casual plants in this association are 

 weeds of cultivation, man has not modified it much. 



The grassy links are on low -lying, more sheltered, and better 

 irrigated parts, but still exposed to the sweep of the winds. The 

 grasses (Agrostis, Air a, etc.) and herbs inhabiting the ground are 

 perennials — low -growing, turf - forming, wiry species. Competition 

 between the members is very severe, and very few plants can fight 

 their way through the close turf which covers the ground. Sheep 

 are pastured on the links, and rabbits in abundance nibble there. Only 

 such species can exist that withstand this continual cropping. The 

 manure from the animals enriches the soil, and here and there enables 

 finer species of grass, clover, etc., to enter. 



The association of Calluna is especially found on the dry, mossy 

 knolls, where variations of water-supply must be considerable. The 

 adaptations of Calluna, Erica, etc., to these severe conditions of life 

 are well known. 1 This association is not particularly well developed 

 upon the Ayrshire links compared with other similar stations where 

 the association is protected and increased by being regularly burned 

 by man. The immediate effect of burning is usually to allow a growth 

 of grasses, Vaccinium Myrtillus, or other plants — secondary social 

 species, which for a while dominate — but later the young heather 

 comes up and peoples the area more fully than before. 



The pine wood may be regarded as the natural arborescent vegeta- 

 tion of the fixed dunes or links, although here the present woods have 

 certainly been planted by man. But if grazing animals are excluded, 

 seedlings spring up naturally — as can be seen in some parts of those 

 woods — so that when the wood becomes old, its vegetation is essentially 

 that of a naturally-sown wood. Its situation includes ground similar 

 to the grassy links and to the heathy knolls, and since these particular 

 woods are fairly well lit, the subordinate vegetation is in part similar 

 to that of these exposed stations. The pine association is complicated 

 in its constitution. Sub-associations, dominated over by Betula, Rubus, 

 Uler,, Calluna, various grasses, etc., occur in its clearings or under its 

 canopy, according to the differences in degree of moisture, shade, or 

 humus. Quite a hierarchy of forms can be grouped as constituting 

 this association: — trees, shrubs, herbs, mosses, and thallophytes — each 

 forming a different layer of vegetation. Certain forms characteristic of 

 woods are present : — saprophytes and shade plants (Goodycra vcpens) ; 

 climbing plants (Lonicera pcriclymcnum) ; epiphytes (lichens and 

 occasionally Polypodium mrtgarc), etc. 



The classification of plant associations has recently been carefully 

 considered by Warming (1896). He points out how the state of 



1 See Professor L. C. Miall, "A Yorkshire Moor," Nature, vol. Iviii., August 1898, 

 pp. 377-380 and 401-404. 



