Rit (Vísindafélag Íslendinga) - 01.06.1939, Page 27

27.
representative of the Th life-form here is Stellaria media
(L.) Cyrill.
It will therefore be characteristic of the moss-covered
plant-formations in the Westmann Islands if the plant-
formations are divided with regard to the bird-colony into
two groups, 1. the plant-formations outside the bird-colony
with a low A % figure, as low as 17,8 %, and a higher
E 1 % figure, up to 24.2 % ; 2. the plant-formations re-
lated to the bird-colony with a very low A % figure, as
low as 0 %, and a higher Th % figure, up to 23.3 %.
I cannot refrain from mentioning the proportion a/b
in table 9, where a = the distribution of the vegetation
and b = the number of species. The figure a gives a little
indication of the density of vegetation as a rule, other-
wise it is impossible to judge the density of the vegetation
if a only is known; a may vary from 1 up to 10 and yet
the vegetation have the same density. The figure b, on
the other hand, only tells us the number of species in each
area, whether they are many or few, and obviously many
species indicate a wide distribution of the vegetation but
give no indication of the density or intensity of the vege-
tation. Likewise few species point to the small distribution
of the vegetation but show nothing more.
a is the distribution of the vegetation as a whole and
usually it stands in direct ratio with the number of species
b. The proportion a/b therefore indicates the distribution
of the species or the volume of the vegetation = the index
of the vegetation.
This general index of the vegetation can, at its most,
be 1—0 or 100 %.
Similarly, in agreement with this general index we have
a special index of the vegetation with regard to species or
life-forms.
In the sand the general index of the vegetation is low,
but as regards Therophytes it is proportionately high
when we consider the circumstances; indeed it so happens
that Solanum tuberosum L. grows nowhere better than in