Or 
~T 
Mar. 1911.] BOTANICAL SOCIETY OF EDINBURGH 1 
Four associations may be recognised :— 
1. Nardus association. 
2. Agrostis-Calluna association (transitional). 
3. Pteris association. 
4. Luzula sylvatica association. 
The third and fourth are of scanty occurrence. 
1. Nardus association.—Oceurs usually on dry soil, but, 
from the stunted growth of accessory plants, not apparently 
very rich. Nardus dominates. Other grasses, such as 
Agrostis vulgaris and canina, Aira cespitosa, A. precox, 
and A. flexuosa, Anthoxanthum odoratum, Holecus lanatus, 
and Trioidia decumbens, occur in varying proportions. 
Calluna is often sub-dominant. Juncus squarrosus may 
be so in places. The accessory plants are those of dry 
heath—Potentilla tormentilla, Erica cinerea, Empetrum, ete. 
2. Agrostis-Calluna association.—On richer soils. Also 
entirely of artificial origin. Dominant, Agrostis vulgaris. 
Other grasses, Agrostis canina, Trioidia decumbens, Antho- 
xanthum, Holcus, Aira flexuosa, A. czespitosa, ete. ete. 
Calluna in vigorous clumps usually, and Erica cinerea 
also luxuriant. These are kept down by burning periodi- 
cally. A great variety of heath and pasture plants may be 
present. 
This is a transition to the more widespread Cynosurus- 
Agrostis association of less acid soil, and scarcely merits 
the rank of association. 
3. Pteris association.—Pteris aquilina is not abundant 
in Orkney, and as a dominant plant it is rare. Its pre- 
ference for a sunny southern slope is rather peculiar in 
view of its frequently sylvan habitat in the South. This 
association may be seen in Flotta, and mingled with Calluna 
in several places e.g., Berriedale, Hoy, and Firth parish. 
What determines its presence I have not endeavoured to 
find out. 
4. Luzula sylvatica association.—Luzula sylvatica often 
dominates on steep slopes. Calluna is frequently present ; in 
other cases grasses are associated with the dominant plant. 
Sub-Formation II.—Neutral Pasture. 
When the agencies so frequently referred to have acted 
so long or so thoroughly as to banish all peaty character 
