146 
J. R. Matthews. 
to the depths of a few centimetres lies on the surface of the humus, 
and in this the bogbean flourishes. With the annual addition of 
sub-aqueous humus the surface of the substratum is raised, and the 
Carex-Menyanthes community becomes invaded by Comarum. The 
competition between the Comarum and the Menyanthes is really a 
struggle between two communities, and the result is the establishment 
of Comarum palustre to form what has been described as the 
Comarum palustre sub-association. Mosses now appear, the 
community becomes one in which the water-level is a little below 
the surface of the soil, and social species are much more abundant 
than in the Carex-Menyanthes community where the water-level is 
above the surface of the soil. 
The alder-willow association is confined to a fringe round the 
land margin where the accumulation of humus is greatest. As 
already mentioned, the trees have been planted, but the habitat is 
quite a natural one and the ground vegetation consists of typical 
marsh plants. Of these Spircea Ulmaria is dominant, and at 
places is beginning to spread over the herbaceous marsh. 
One outstanding feature regarding the vegetation of the White 
Moss compared with other similar habitats is the paucity of species. 
The plants of the highland loch— Lobelia, Subularia, and Isoetes — 
do not occur, but it is more surprising that many lowland loch 
plants are also absent, e.g., species of Ranunculus , Nymphcealutea , 
Castalia alba, Hippuris vulgaris , Epilobium hirsutum and others. 1 
Nymphcea lutea occurs in the back-waters of the Earn at no great 
distance from the loch. The writer is inclined to believe that the 
paucity of species is due indirectly to the raising of the humus level 
—the biotic factor which makes possible both the invasion of a 
community by the succeeding one and the centripetal encroachment 
of the vegetation as a whole—and directly to the actual suppression 
of the rarer species by those that are dominant in each successional 
stage. Competition as a biotic influence has already been mentioned 
in connection with the struggle between Comarum and Menyanthes. 
Other instances have come under the writer’s observation where 
the competition has ended in complete suppression. Lindman* has 
given interesting examples of plants suppressed by other plants, and 
the cases which have been noticed at the White Moss may be cited. 
About fifteen years ago an artificial curling pond was made 
near the S.E. corner of the loch (rectangular figure in map, Fig. 1). 
1 Cf. lists given for lowland lochs by R. and W. G. Smith, l.c. 
2 C. A. M. Lindman. “ Some cases of plants suppressed by other plants.” 
Nkw Phytologist, Vol. 12, 1913, p. 1. 
