LICHENOLOGY OF ICELAND 243 



has no other descriptive means with which to express his judg- 

 ment than the terms "abundant," "less abundant," etc., merely 

 relative expressions, which have no relation to any fixed and in- 

 variable unit. 



It must therefore be absolutely recommended, in future, to de- 

 termine the abundance of lichens in a country, a plant-association, 

 a zone, etc., by still two other means, viz., frequency-number 

 and mass-occurrence (in weight per unit of area) - in addition 

 to its number of species. 



In the present treatise I have as regards some of the Icelandic 

 associations as far as travelling-conditions permitted given 

 some frequency-numbers, which may be obtained, for instance by 

 demarcating small sample-areas (1 or 2 square decimetres each) with 

 equally large, intermediate spaces between them in some places 

 it is practicable to employ the smaller unit, whilst in other places 

 the larger unit is preferable - and by noting whether they contain 

 lichens. This method, as already mentioned, is RaunkiaBr's for 

 phanerogams, and is also very good for lichenological purposes. In 

 that way it is possible to determine almost all possible frequency- 

 numbers in detail, to investigate for instance the frequency-number 

 for crustaceous-lichens only, foliaceous-lichens only, etc., or the 

 frequency-number for lichens taken collectively. If it be a question 

 of wishing to know, for instance, how frequently lichens, as com- 

 pared with phanerogams, occur in the sample-areas one can just 

 take, say 100 200 or 1000 sample-areas, according to what may 

 be considered necessary in order to obtain a reliable impression of 

 the conditions, and note down in which areas lichens occur and 

 in which phanerogams (mosses, alga?, etc.). If lichens occur in all 

 the samples, then the frequency-number of the lichens is F /o 100 

 (F % stands for the frequency-percentage); if they occur only in 50 

 out of 100 sample-areas, then the frequency-number is F /o 50, etc. 

 All this has been treated of above to some extent, but here it 

 is explained more fully. - - In the case of phanerogams, mosses, etc. 

 exactly the same method is employed. 



If one should wish to determine how frequently crustaceous, 

 foliaceous and fruticose lichens occur among themselves, one must 

 note down, with regard to each of these little sample-areas, which 

 of these growth-forms occur in it. For instance, in a series of 

 samples, crustaceous lichens may be found in 50, fruticose lichens 

 in 100, and foliaceous lichens in 20 sample-areas. The frequency - 



16* 



