ROBERT SMITH ON PLANT ASSOCIATIONS OF THE TAY BASIN. 21 $ 
RELATION TO THE NATURAL BOTANICAL SUB-DIVISION OF 
THE TAY BASIN. 
It is now the feeling of many botanists that the most natural 
botanical sub-divisions of any region must be those marked out by the 
most important social species. Humboldt emphasized this very 
strongly, and many after him have applied it to different countries. 
This is the basis of the late Dr. Buchanan White’s natural sub-division 
of Perthshire into alpine, sub-alpine, lowland, and littoral districts. 
Watson was partly following the same lines when he instituted his 
arctic and agrarian zones, determined by such plants as Pteris 
aquilina^ Erica Tetralix^ Calluna^ etc. 
Following the divisions laid down by Dr. Buchanan White, we 
shall find that each can be subdivided naturally into a certain definite 
number of associations. 
In the Alpine district, although such species as Salix herbacea^ 
L., Vaccmium Myrtillus^ L., Empetrum nigrum, L., Alchemilla 
alpina, L., etc., form fairly large and well-defined associations, still 
the conditions of life are such that it is more natural to limit the 
associations by the climatic changes than by the area covered by any 
one plant, for competition between each other is of much less 
importance than the struggle with the extreme conditions of the 
weather. Thus, in these regions the subdivisions into plant associa¬ 
tion of the alpine plateau and plant association of the alpine crags 
seems the most natural. 
But in the Sub-Alpine district there is much less difficulty. Here 
Calluna and grass associations extend over most of the area, and can 
often be sharply defined from each other. The largest grass associa¬ 
tions ar-e those of Nardus stricta, L., Agrostis vulgaris, With., Fesiuca 
ovina, L., and Molinia varia, Schrank. The tree associations of this 
region are pine, birch, and oak. The bracken is characteristic of its 
lower border. 
In the Lowland district the associations are formed chiefly by the 
cultivated plants introduced by man, by whin and broom, by broad¬ 
leaved perennial herbs, and by woods of mixed trees, including those 
of beech. 
The Littoral district is inhabited by the salt-loving plants, and by 
the marine associations. The former include Carex arefiaria, Ammo- 
phila, Elynius, Salicornia, Triglochin, etc., and the latter Zostera 
marina, L., and the social-living seaweeds. 
The social communities in fresh water are partly outside such an 
arrangement, for their conditions of life are comparatively uniform 
and vary only slightly with the altitude. 
Each of these four series of associations, Littoral, Lowland, Sub- 
