The tundra passes by imperceptible gradations into the moor, bog and heath of warmer climates.
Those of warmer countries cannot be cultivated in British gardens without protection from the rigours of winter; still less are they able to hold their own unaided in an unfavourable climate.
It is certain that they originally existed under warmer conditions of climate than now obtain, and that progressive refrigeration.
At the close of the Pliocene the European flora was apparently little different from that now existing, though some warmer types such as the waterchestnut (Trapa natans) had a more northern extension.
