182 OLAF GALLGE 
rain-forest, and the lichen-covered heaths of Iceland, would appear 
when thus compared according to relative weights of produce! It 
would be extremely interesting to ascertain. 
But also the luxuriance of the individual species or growth- 
form in an association, would be able to be characterized by this 
method. It would be very interesting to see, for instance, the result 
of comparing a piece of Danish heath with an Icelandic heath, in 
respect to the lichen-vegetation. How many kilograms of lichens 
each sample-area contains up there in the North and down here 
in Denmark. 
Unfortunately I have not been able to employ such a “weight” 
method in my investigations in Iceland, nor had I at that time 
considered ‘this matter more closely. But I am convinced that we 
have here an exceedingly valuable means of description, by which 
to characterize the difference between the masses, be it of the in- 
dividual species, the growth-form or the whole association; and, as 
already mentioned, practical men have long ago used it in pursuit 
of their object. 
a. The Deserts. 
Large tracts of Iceland have a desert-vegetation, i. e. a very open 
vegetation consisting of scattered individuals. Where to draw the 
boundary line between the desert and the closed vegetations, i. e. 
vegetations which cover the ground completely, is entirely a 
matter of opinion, and the boundary can only be an artificial one. 
We may for instance decide, according to Raunkiær’s method, 
to take a large number of equally large sample-areas, and note 
down their vegetation separately. It will then be seen that many 
of the areas are entirely devoid of plants, and such areas may be 
designated 0 (nil); and then resolve that a tract of land in which 
75 °/ of the sample-areas were devoid of vegetation, should be de- 
signated ‘desert.’ But, as already mentioned, whether this “per- 
centage of voidness” is chosen, or an entirely different one, for the 
designation “desert,” it is and must be a matter of opinion. Such 
an analysis of “voidness” would be interesting for purposes of 
comparison, for instance between the Arctic cold-deserts, and the 
sub-tropical heat-deserts. But such an analysis has not yet been 
made, nor have the deserts of Iceland been, as yet, sufficiently in- 
vestigated in this respect. 
In Iceland many different kinds of deserts are found, the best- 
— 
