3()0 .ions. BOii: im:tkhskn 



n. n<ul:ii Wdlls. 



Drv itxks in Icelaiid aic Ibr llie most part covered witli lichcns, 

 only the most j)rominenl aiul lluis exposed peaks are devoid of 

 visible vegetation. In ])laces wheie water often trickles over the 

 roeks, eitlier wlien Ihe snow is thawing or alter rain, an algal vege- 

 tation is mel wilh. Where llie moislure is so slighl Ihal Ihe rock 

 seems ((nile dry in periods of desiccation , \ve meet wilh the algal 

 vegetation which has l)een called »Tinlenstriche« , often mentioned 

 ))ecause il is so conspicuous in the landscape. Tn Iceland I observed 

 sudl an algal vegetalion on the steep hasalt Nvall that hounds the 

 Fljotsdalur (E. Iceland) on the north-west. Sample 89 is derived 

 from the driest localily there. The algæ formed a Ihin hlack layer 

 on the rock and conld only be scraped olT in the shape of a powder 

 in which I found the following species: Calothrix parielinn , (iloeo- 

 capsa alpina, Sosloc sp., Schizothrix Hen/leri. Scijtonema criisiaceum, 

 that is to say, all C.yanophyceæ, and all wilh well developed sheaths, 

 brown or violet in colour. Under almost similar conditions I found 

 a growlh of Sligonema niinntiun and S. j)anniforme on a wall formed 

 of old lava-streams in Almannagja (see Thorodd sen 1914, tig. 9). 



In somewhat nioister piaces the algal vegetalion grows richer 

 in species, and the individuals reach a far betler develojimenl Ihan 

 in the driest localilies. In the summer the moislure will no doubl 

 be slight in periods of dry weather, and the algæ growing herc niay, 

 I think, be classed as aerial algæ. These localities correspond in 

 the main lo Stroms »moist rocks« (Strøm 192(5, p. 18). Among 

 these I class samples 85, 87, 99, 101, 127, 128, 129. In addition to 

 Cyanophyceæ, numerous Diatoms occur here. The chief C.yano- 

 phyceæ are Scfilonema Mijochrous. which may form large continuous 

 felted layers (87), Calothrix f xi rietina {li)l, 127), besides many ('hroo- 

 coccacea' (Chroococcus turgidus, Glococapsa alpina. G. Magma. G. rn- 

 pestris, Gloeothece rnpcstri.sj. The most characleristic Dialoms are 

 Galoneis fasciata, Diatonwlla lialfonriana, Fragilaria Iwoissinui, \ani- 

 cnla contenta. N. perpnsilla, Nilzschia Denticnla, and A'. sinnala. 



Over some parts of the rocky walls there is a continuous llow 

 of water, and these piaces answer in the irtain lo Strøm s inun- 

 dated rocks« (192(5, p. 18). The algæ growing here are mostly hydro- 

 philous species which are snbmerged most of the time even though 

 the water layer is only thin (samjiles 8;}, 84, 8(), 88, 90, 91, ]()(), 

 102, 122). In addition lo some of the species that inhabil moist 

 rocks such as Scgtonenia Mijochrous , Calothrix parietina, Gloeo- 



