This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
The northern isothermal line of a mean annual temperature of 50° enters England on the eastern side in 51° 51' N. lat., and proceeding to the north-west, leaves it in about 53° N. lat., passing through the centre of Ireland, and onwards nearly in the same latitude, across the Atlantic, to about 40° W. long. From this point it forms a gradual curve southwards, until it reaches the American coast, in about 42° or 43° N. lat;; thence onwards, leaving the lake district to the north, in about 41° 50' N. lat., and then again taking a northerly direction until it reaches 50° on the western side of the continent. In the Pacific it stretches still farther to the north, reaching 55° N. lat. in 160° W. long. Returning to England, and following the line eastward, we find it touches the continent of Europe on the coast of the Netherlands, turning gradually to the south as we travel into the interior, along the north shore of the Black Sea, across the Caspian in about 45° N. lat., through Tartary, Mongolia, Mantchouria, and the Japanese Islands, in about 43° N. lat. In the centre of the American and Asiatic continents, the high summer temperature brings the mean annual of 50° much farther north in proportion to the cold of winter as compared with the climate of Britain. But although plants growing a considerable distance south of the mean annual of 50° in these countries are hardy with us, they do not ripen their seeds, and some shrubs and trees rarely flower. The two most interesting regions on the 50° line to the gardener are the western coast of America and the eastern coast of Asia in North China and Japan, for from these countries we get a large number of plants chiefly hardy in the south-west, but requiring protection more or less in other parts of the British Isles.
The course of the southern isothermal line of 50° is much more uniform, as it passes through no broad expanses of land. Roughly speaking, we may put the latitude at 45° S. It in-cludes the southern part of Patagonia in South America, and the extreme south of the middle island of New Zealand, a very small tract of country indeed, in comparison with that traversed by the northern line of the same annual temperature. And as these countries, from the same causes as our own, have a high winter mean, they offer few plants that can be successfully cultivated in the open air in Britain.
The mean of 40° for January passes through the centre of Britain, and in both Asia and America it deflects southward to 39° N. lat., or about 3° farther than the mean annual of 50° Fahr.
The rainfall varies throughout these countries proportionate to their extent in the same ratio as in the British Islands, and according to local influences; but as we have records for a few localities only, we forbear giving them.
One more observation should be made respecting the foregoing figures - that they must be treated as rough approximations, both those relating to temperature and to latitude. And we may here repeat, that the cultivator's special study should be the soil and climatal conditions of his own locality.