The Botany of Iceland - 01.12.1914, Blaðsíða 142

326
THORODDSKN
plateau patches of Anthelia niualis occur. The rocky-flat-formation
appears to be an original, late Glacial formation from which a great
many distinct formations have developed, the different species having
become associated according to their conditions of life. The outer
limits of the rocky flat are the Grimmia-heath and the “herb-flat,”
but transitional stages to heather-moor and grassland often occur.
As sub-divisions or nearly related formations the following majr be
mentioned: — gravelly flats (melar), stone-covered ridges (holt); fallen
blocks and debris upon mountain slopes (urd, pl. urdiij, steep cliffs
(hamrar), gravelly river-plains and river-terraces (eyraij, sandy tracts
of various kinds, clayey flats and lava-streams.
Gravelly flats (melar) also occupy large areas in the lowlands;
the soil-conditions differ somewhat, but generally the gravel is mixed
with clay and then the surface often cracks into polygonal cakes
and forms a “rudemark” (p. 257). These “rudemarks” greatly influence
plant-distribution, as the plants generally resort to the gravel bands
between the cakes, where they fmd shelter and protection. Some-
times gravelly flats are so poor in plant-life tliat they appear quite
bare and naked; sometimes they are so densely covered as alinost
to form a “herb-flat.” The most common plants on gravelly flats
in the lowlands are Cerastium alpinum, Arabis petrœa, Draba hirta,
Silene acaulis, S. mariiima, Armeria maritima, Salix herbacea, Sagina
nodosa, Spergula arvensis, Arenaria ciliata, Alsine verna, Thgmus
serpyllum, Dryas octopetala, Papaver nudicaule, Oxyria digyna, Rumex
acetosella, Trisetum subspicatum, Poa glauca, Festuca ovina, Agrostis
alba, Luzula multiftora, L. spicata, etc. Naturally all the above-
mentioned species do not occur together; in some places a great
many of them may oceur, while in other places a very few, perhaps
only three or four, may be found. The vegetalion is also somewhat
dependent upon neighbouring plant-formations. Usually mosses or
lichens are very sparsely present upon these gravelly flats in the
lowlands; only here and there small Grimmia-cushions occur.
On stone-covered ridges (holt) there is usually a greater
variety as regards soil and situation than on the gravelly flats, and
the vegetation there is sometimes fairly luxuriant and conspicuous
especially in early summer when Silene acaulis, Dryas octopetala and
Thymus serphyllum are in bloom; these are very common tliere,
and also the majority of the plants of gravelly flats. Moreover, the
following are noteworthy: Alchimilla alpina, Saxifraga cœspitosa
and S. oppositifolia, Viscaria alpina, Empetrum nigrum, Sedum acre