244 OLAF GALLOE 
number is then F °/o 50, F °/o 100 and F °/o 20 respectively, all of 
which has already been known and employed for several years in 
ecology, with regard to phanerogams. 
This frequency-number serves to indicate how equally the lichens 
are distributed in an association or similar limited area. This has 
the great advantage, that even non-specialists, who have a general 
botanical training, can note down various facts with regard to the 
distribution of special groups of plants (lichens, earth-algze, mosses, 
etc.) in the associations, without knowing the name of a single 
species found. 
A specialist, when he has time at his disposal, will be able to 
go more into details, and even determine the distribution of a single 
species within a certain area. 
The determination of the mass-occurrence of lichens has 
never yet been undertaken; it has been mentioned under the treat- 
ment of the heath-lichens. For this determination it is necessary 
to reap everything that grows on each sample-area, and weigh it. 
By this means one obtains figures, which are directly useful for 
purposes of comparison, as regards the relative extent of mass- 
occurrence of the plant-association in question. This method is 
useless as regards the crustaceous lichens, but in their case it is 
possible to state, with some certainty, the size of the area covered 
by them. 
If we are to compare the abundance of the lichens of the 
various countries, according to the methods which have been briefly 
treated here, and, by means of these methods, try for instance to 
answer the general question: “Where are the lichens to be found 
in greatest abundance, in Iceland or in Denmark?” This question 
must be further detailed, in order to be answered, and cannot, 
upon the whole, be answered as yet. The Icelandic heaths can be 
compared with the Danish, the Icelandic grasslands with those of 
Denmark, etc., as has been done above, by way of experiment, in 
the special sections, with regard to frequency-number and mass- 
occurrence (in weight per unit of area). But a thorough comparison 
cannot yet be made, as it requires many more investigations in the 
field, than have hitherto been undertaken. 
It is, however, my impression, as it has been the impres- 
sion of other botanists, already in former times, that as regards fre- 
quency number and abundance the Arctic regions and Iceland appear 
to be richer than other regions, no doubt chiefly on account of the 
ee EE 
