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 Raunkier’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, algæ, etc.). If lichens occur in all 
the samples, then the frequency-number of the lichens is F °/o 100 
(F °/o 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* 
