176 OLAF GALL@E 
that the conditions of moisture are essentially dependent upon it. 
When the ground thaws in spring, there is a time in which the 
upper soil-layers are, for the time being, very wet, because the 
melting snow and ice cannot sink into the ground, owing to the 
sub-surface ice. We may be justified in saying that, taken as a 
whole, the yearly growth-period as compared, for instance, with 
that in Denmark, is considerably shortened by the low temperature 
of the soil. How long this period lasts, upon the whole, as regards 
the lichens, which, as we know, assimilate as soon as the tempera- 
ture is above freezing-point, has not been investigated. But I assume 
that it is far shorter than in Denmark, where it lasts almost all 
the year round. 
(4) The water-contents of the soil in Iceland, owing to the 
great amount of precipitation and the slight evaporation, are far 
greater than in Denmark. In the surface features of the landscape 
this is shown by the frequent occurrence of bogs. But, naturally, 
there is a great difference in the amount of water contained in the 
various soils, all conditions being found intermediate between boggy 
soil saturated with water, and dry sandy soil, and soil fine as dust 
which is so dry in numerous places that it drifts with the wind at 
every opportunity. The wettest soil, which is continually saturated 
with water (the bogs), is devoid of lichens; this is also the case . 
with the driest, drifting soil, not on account of its dryness — for 
it is well-known that lichens chiefly imbibe directly- precipitated 
moisture, and are fairly independent of other water-supplies, — but 
on account of the unstability of the drifting soil. Lichens grow on 
soil intermediate, with regard to dampness, between these two ex- 
tremes; they grow in association with other plants, as will be fully 
described below. 
(5) All drifting soil is devoid of lichens. 
(6) The rôle which burrowing animals play in Iceland is 
not known very particularly; it is however in all probability quite 
insignificant, while in Denmark, as is well-known, it is very great, 
especially in the woods. | 
(7) Leaf-fall. The layer of decaying leaves which in Den- 
mark, during autumn, buries all the small plants of the wood-floor, 
plays, as a matter of course, a similar rôle in Iceland. When the 
trees or the shrubs (willow, birch) stand very close, the ground is 
frequently devoid of lichens, and this is no doubt partially due to 
this leaf-covering. 
OT 
