The Botany of Iceland - 01.12.1930, Blaðsíða 169

STUDIES ON THIÍ VEGETATION OF ICELAND
159
of the Icelandic species, partly for the question of the cultivation
of the various Icelandic types of vegetation.
Other questions of decisive importance in studies on the di-
stribution of the species in the scales of external factors are partly
the question of equidistance in division, and partly the question of
the determination of the number of external factors bearing on plant
distribution.
As far as the first question is concerned, in formations with
low density of species, the line between two formations is most
naturally drawm at the physiognomic boundary line between the
two formations, and the areas selected for examination sliould as
far as possible be laid in the middle of the formation. It is pos-
sible that the distance between the various localities examined will
not in this way become an exact expression of physical equidistance
between the localities, or tlie formations, but merely of ecological
equidistance; but since the invesligation is primarily ecological, it
will suffice if the requirement of ecological equidistance is satisfied,
even tliough physical equidistance would have been desirable.
Where we are concerned with the investigation of formations
'vith many species, the requirement of ecological equidistance be-
tween the localities examined will be considerably more difficult
to satisfy. The present treatise deals principally with formations
of this kind, and the examination of them was made in the follow-
ing way. On a gently sloping surface the investigator passed so
far up and down from one locality that the vegetation had changed
appreciably; the second locality was then examined here, whereupon
the third locality was chosen and examined in the same way.
It is possible that the distances between the localities examined
ni'e unequal botli physically and ecologically; so inuch is certain,
however, that the sequence of the localities examined expresses a
constantly increasing change of environment. If this is the case,
however, we have in the proportion of the species points
°í species occurring principally above, and species oc-
curring principally below, the formation in question an
aid in determining the question of ecological equidi-
stance between the for.mations.
Another question of equal importance is the question of the
determination of the number of plant-distributing factors. Tliis
question, however, is only topical in a plant covering rich in species
and of uniform physiognomy. If such a plant covering is examined