PDF - CES (IISc)

DISTRIBUTION 345
but, as yet, has been found only in the eastern parts of North America; and
Cl. pycnodada, a plant which braves the climate of Cape Horn and the
Falkland Islands, but has not travelled northward beyond temperate North
America: the southern origin of that species is thus plainly indicated. Wainio
also finds that evidence of the primitive locality of a very widely spread
species may be obtained by observing the locality of species derived from
it, which are as yet of limited distribution ; presumably these arose in the
ancestral place of origin, though this indication is not always to be relied
on. If, however, the ancestral plant has given rise to several of these rarer
related species, those of them that are most closely allied to the primitive
plant would be found near to it in the original locality.
A detailed account of species distribution according to these indications
is given by Wainio and is full of interest. No such attempt has been made
to deal with any other group, and the distribution of genera and species can
only be suggested. An exhaustive comparison of the lichens of different
regions is beyond the purpose of our study and is indeed impossible as,
except in some limited areas, or for certain species, the occurrence and distribution
are not fully known. It is in any case only tentatively that genera
or species can be described as local or rare, until diligent search has been
made for them over a wider field. The study of lichens from a floristic point
of view lags behind that of most other groups of plants. The larger lichen
forms have received more attention,' as they are more evident and more
easily collected ; but the more minute species are not easily detected, and,
as they are largely inseparable from their substratum of rocks, or trees, etc.,
on which they grow, they are often difficult to collect. They are also in
many instances so indefinite, or so alike in outward form, that they are
liable to be overlooked, only a m'icroscopic examination revealing the differ-
ences in fruit and vegetative structure.
Though much remains to be done, still is enough known to make the
of extreme interest. It will be
geographical distribution of lichens a subject
found most instructive to follow the usual lines of treatment, which give the
three great divisions : the
of the globe.
Polar, the Temperate and the Tropical regions
B. LICHENS OF POLAR REGIONS
Strictly speaking, this section should include only lichens growing within
the Polar Circles; but in practice the lichens of the whole of Greenland and
those of Iceland are included in the Arctic series, as are those of Alaska:
the latitudinal line of demarcation is not closely adhered to. With the
northern lichens may also be considered those of the Antarctic continent,
as well as those of the islands just outside the Antarctic Circle, the South