176 OLAF GALLOP: 



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, lor 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 free/ing-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 

 \\ith 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 instability 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 role 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 role 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. 



