The Botany of Iceland - 01.12.1914, Side 150

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THOROIJDSEN
streams there is a good opportunity of studying the development of
the different plant-formations and plant-societies because, at the be-
ginning, every lava stream is virgin soil, where plant-life must break
entirely new ground. The first establishment of vegetation is due
to rnosses and licliens; on the 23-years-old Krakatindshraun Dr.
H. Jónsson found twelve species of mosses, three of lichens and
one alga. Two of the liclien-species, Stereocaulon alpinum and
Squamaria gelida, were widely distributed on lava-domes, the rnosses
usually occurred in clefts, but nowhere had a moss-carpet yet deve-
loped (H. Jónsson, 1905, pp. 55—56). The next stage is the
Gr/mmia-heath, which occurs as a continuous covering over the
low-lying parts of the lava, while crustaceous lichens form crusts
upon the protruding points; the Grimmias form the soil-layer which
is primarily necessary for the growth of higher plants. In this moss-
foundation occur several lichens and scattered specimens of flowering
plants from different associations. After tliis the development on the
lava-streams in the lowlands proceeds in various directions according
to the surface-conditions of the lava, the nature of the rock and
other circumstances, such as the greater or less amount of drifted
sand or humus which has settled upon the surface, and whether a
supply of water is available, etc. Consequently, in the course of
time a lava-stream may support either a heather-moor, a coppice-
wood, a “herb-flat”, or grassland, or all these plant-formations may
be simultaneously present on the same lava-stream. Rocky-flat forma-
tions proper, do not occur on lava-streams, except very rarely, when
the lava-streams become covered with gravel brought down by moun-
tain streams or glacier rivers. On the plateau the vegetation on a
great many of the lava-streams does not go beyond the lichen-stage,
on others a considerable number of Grimmias are present, but al-
most never as a continuous covering, such as they form in many
places in the lowlands and especially on the peninsula of Reykjanes.
On the other hand the lava-streams of the plateau are frequently
covered with drifted sand and support a sand-vegetation which at
higher levels consists of Elymus arenarius and at lower levels of
different Salices. There are a few instances of old, partially blocked
and sand-covered lava-streams in the valleys, in localities where
water was abundantly present, forming the substratum of swampy
grassland with peat. Thus, in the course of time, a lava-stream
may give rise to almost any formation1.
1 For further information regarding plant-life on lava-streams see H. Jóns-