LICHENOLOGY OF ICELAND 181 
the reindeer moss entangled with it. The task is simply impossible, 
the question cannot be answered. The fact cannot be emphasized 
too sharply, that the figures which have hitherto been given for 
the areas occupied, and which have been obtained by the method 
of the above-mentioned authors, do not at all poësess the numerical, 
the mathematical authority which numbers ought to have in order 
to be useful for purposes of statistical comparison. They are in 
short an illusion. 
Add to this, that even if the figures for the area occupied could 
be fixed fairly accurately, that would not give us any great knowledge 
of the abundance with which the species (or growth-form) in question 
occurs in an association. A ten-years-old beech-wood will frequently 
cover as large an area as one of a 100 years, whilst the figures for 
the area occupied would not give any idea of the enormous diffe- 
rence as regards masses in the two growths. It is true, anything 
like this need not be demanded of the figure in question, but then 
they are not very enlightening in any respect, and are therefore 
superfluous. 
In connection with the frequency percentage (F. °/o) (frequency- 
number), a far better method can be more advantageously employed, 
a method which science — as far as I know — has not employed 
very largely, but which practical men discovered decennia ago. It 
cannot be employed on excursions, with note-book and squared 
paper, or on expeditions on horse-back; it requires a sojourn on 
the spot, and some patience. It is simply this: The mass of a wood 
is determined by the forester by its timber-contents in cubicmetres 
(it may be expressed in terms of weight!); the crop of a rye-field 
may be given in weight (straw and grain); and quite similarly could 
the natural vegetation of any place be treated by a man of science: 
but then it would be necessary to reap the plants, the masses of 
which are wanted to be known. 
This method has the advantage that — of course in connection 
with other descriptive means (frequency-numbers, etc.) — it can be 
employed to characterize both the whole association, and its in- 
dividual species. Thus, it is really a valuable piece of information 
concerning an association, to know, for instance, that on a square 
metre there grow, on an average, let us say 2 kilograms of plants, 
while another association perhaps bears 200 kilograms. It must be 
admitted that this gives quite a striking impression of the plant- 
producing power in two such localities. I wonder how the tropical 
