The Botany of Iceland - 01.12.1914, Side 149

PHYSICAL GEOGRAPHY
333
sected with clefts, rent bv frost, which are filled with Grimmias and
Salix herbacea.
The ílora of the lava-streams cannot be referred to any single
plant-formation, because according to the age and the progessive
development of the vegetation, the lava may bear on it all possible
kinds of plant-formations. Nevertheless in Iceland several plants
Fig. 33. Lava-íieltl in Nordrtírdal in the district of Borgarfjord (Aug. 3, 1909).
The lava is covered to a deptli of one foot with a layer of Grimmia hypnoides.
Projecting parts of the lava are covcred with crustaceous lichens. Here and
there a tuft of Fcstuca ovina. Birch coppice in the background.
(Pliot. A. Hesselbo.)
are more particularly associated with lava-streams and have there
lound shelter in the numerous clefts and depressions, where condi-
tions of life are especially suitable for them. Paris quadrifolia is
found only on lava-streams, and also ferns are found, in abundance
and often as very large specimens. Aspidium fúix mas, A. spinulo-
Sl»n, A.lonchitis and A. phegopteris very rarely occur in other habi-
tats than lava-clefts; Athyrium filix femina, Woodsia ilvensis, Poly-
podium vulgare, Aspidium dryopteris and Cystopteris fragilis are also
cornmon in lava-clefts, although they are also met with fairly often
■n other localities, between blocks of rock and in rock-clefts.
Milium effusum occurs also most fre([uently in lava-clefts. On lava-
22*