V. THE HORIZONTAL DISTRIBUTION 



OF THE SPECIES. 



IN order to obtain a correct notion of the importance of each plant- 

 species within a floral district it is necessary to know not only 

 their distribution in the different parts of this district, but also their 

 relative abundance (number of individuals) in comparison with other 

 species. The following Table gives a summary of all the Bryophyta 

 found in Iceland, with the exception of the species growing in warm 

 soil, since, as regards these species, it is chiefly the heat of the soil 

 and not the climatic conditions which determine their distribution. 

 The frequency within the different districts is indicated by the figures 

 1--4, so that 1 indicates rare or very scantily occurring species; 2. 

 those that occur here and there or only in a comparatively few 

 localities and not in abundance; 3 frequent, but as a rule playing 

 no important part in the moss-covering, or occurring only locally 

 in abundance; and 4 the common and abundantly occurring species* 



The majority of the species are not equally distributed in all the 

 different altitudinal regions, and the frequency is therefore reckoned 

 relatively to the region in which each species is most widely distri- 

 buted. As regards species which grow only in quite definite areas 

 (for instance littoral species) the frequency is reckoned separately. 



The figures in the table are, however, in many cases somewhat 

 doubtful, since many districts are too superficially investigated to 

 allow one to form an opinion as to the frequency of the less com- 

 monly occurring species. This is the case, for instance, as regards 

 the majority of the districts of East Iceland situated above an alti- 

 tude of 300 metres, because in June, when I travelled through this 

 part of the country, the mountains were still for a great part snow- 

 covered, which in connection with continual fog made investiga- 

 tion almost impossible. 



It must also be noted that the table is based only on the 



