244 01 AF C.ALL0E 



number is then F % 50, F /o 100 and F % 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-alga;, 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 



