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13.

The northwards advance of the ice during the last and greatest glaciation in Greenland extinguished large parts of the early Pleistocene flora, but the present peculiarly discontinuous distribution of many species shows, that refuges were left in certain places.

Sørensen, who certainly is one of the very few botanists with a complete knowledge of the Greenland flora, is of the opinion, that this is the only theory, or should we rather say hypothesis, which satisfactorily explains the origin of the Greenland flora and its two main elements, separated in history and ecology: the northern old element and the younger southern element. Some of the species of the latter element are still advancing, while the high-arctic plants seem to be unable to advance southwards, at least at present, probably because they cannot survive a certain amount of summer heat and less light. The old and high-arctic element is remarkably stable all over the high-arctic regions and consists of about 200 species met with in refuges all over the area in the far north, while the younger and southern element is still changing and much more local in the real Arctic and with evident connections to southern regions even in the lowlands.

After these lengthy details which may have confused you at the same time as it may have helped you to see how little is really known about the history of the arctic flora, we may mention two other kinds of approaches which have given very similar results as the plantgeographical studies. The first of these is the study of the frequency of polyploids, the second one involves studies of the so-called life-forms.

The chromosomes are the bearers of almost all the genetical substance of the living beings, and it has been known for more than half a century that their number is constant within each species in the strict meaning of that term.