Chambers's Edinburgh Journal, No. 438

It becomes necessary, first of all, to study the influences—whether general or special—which affect the distribution of vegetation; to inquire into those freaks or aberrations of nature which favour in one place the production of plants that will not grow in another, under apparently similar circumstances; and why similar plants are found in places widely separated. Oranges will ripen on one side the Alps, but not on the other; grapes scarcely come to perfection out of doors in England, while on the other side of the Channel they ripen by thousands of acres; and several fruits which fail in our northern counties, are grown without difficulty in Denmark in the open air. Investigation soon shewed that temperature alone, mere heat and cold, was insufficient to account for the phenomena; but that moisture and dryness, the prevalence of certain winds, the chemical and physical conditions of soil, and the constitution of the plants themselves, would have to be considered in a proper inquiry into the subject.

Here we must notice a fact which has proved of essential service in the study of botanical geography—namely, the discovery 'that there is some law presiding over the distribution of plants which causes the appearance of particular species arbitrarily—if we may so say it—in particular places;' from which, the conclusion has been arrived at, 'that countries have become populated with plants partly by the spreading of some special kinds from centres within those countries where they were originally exclusively created; and while these have spread outward into the neighbouring regions, colonists from like centres lying in the surrounding countries have invaded and become intermingled with the indigenous inhabitants.'

Looking at the effect of climate on vegetation, we find that as we proceed from the north towards the south, the number and luxuriance of plants increase in a remarkable degree, and the same result is observable in altitude as in latitude. 'Step by step,' writes Mr Henfrey, 'as the land rises in any mountain region, the vegetation assumes, more and more, a polar character; and in the mountains of the tropics, a succession of stages has been distinguished, corresponding in the general peculiarities of the plants which clothe them, to tracts extending horizontally, in succession, on the sea-level, from the base of these mountains to the frozen regions within the arctic and antarctic circles. Increase of elevation is accompanied by an alteration of climate, bringing with it a set of conditions analogous to those prevailing at certain distances further from the sun. Ascending the Peak of Teneriffe, a series of regions are traversed, one above another, displaying with the approach to the summit a continually closer approximation in character to the polar regions, till the traveller who left the palm, the cactus, and the thousand varied forms of tropical vegetation at the foot, finds himself at last among the stunted shrubs and scaly lichens, the borderers who hold the outposts on the limits of the eternal snow.'

It might be expected that places on the same parallel of latitude would be equal in temperature; but on tracing out the distribution of heat over the globe, and laying it down in what are called isothermal lines on a map, most striking deviations are found to exist, and the contour of the lines is anything but regular. The line of greatest cold, for example, which leaves the eastern coast of Labrador at about the 54th degree of latitude, rises six degrees as it approaches Greenland, and strikes the coast of Lapland a little above the 70th degree, or sixteen degrees nearer the pole than at its starting-point—thus shewing that the northern parts of Europe have a more genial climate than those of America. The line then curves fifteen degrees to the south across Siberia, rises again on the western coast of America, and falls once more as it advances towards the east. Again, 'the isotherms of Canada pass through Iceland, across about the middle of Norway and Sweden, St Petersburg and Kamtschatka. Those of New York through the north of Ireland and England, twelve degrees further north, North and Central Germany, and the Crimea. That which leaves the United States at about 36 degrees north latitude, crosses Southern Europe from the north of Spain to the Adriatic in a tolerably straight line, some eight degrees further north, and then falls south again, where the influence of the north-east polar current is more felt, in Greece and Turkey.'

But although these are marked as lines of equal heat, it is only in the average temperature that the equality consists; and it is clear that a country with 80 degrees of summer heat and 20 of winter cold, would have a very different climate from another with 60 and 40 as the highest and lowest degrees of temperature, although the mean of the two would be the same. And herein we have an explanation of what at first sight appear to be anomalies: we know, for instance, 'that plants will flourish perennially in the British isles which are killed by the frosts of winter in places lying considerably to the south upon the continent; thus the laurel, that bears our winters steadily in Ireland and the west of England, and is only affected by very severe frosts in our eastern counties, is killed by the winters of Berlin, equally fatal to the myrtle, the fuchsia, and a host of other shrubs which attain considerable age and size in the western portions of the British isles. Again, Canada, which lies south of Paris, has the climate of Drontheim, in Norway; while at New York, lying in the latitude of Naples, the flowers open simultaneously with those of Upsala, in Sweden. Moreover, those very countries suffering so severe a winter's cold, enjoy a summer's heat far exceeding ours, since the snow lies for months on parts of Germany which yet receive sufficient heat in summer to ripen the grape and Indian corn.'

The principal modifying causes are winds and water. Islands, and countries bordering on the ocean, have a much more equable climate than those which lie in the interior of continents, and will have a greater prevalence of moist south-westerly winds. The average annual quantity of rain in the British islands is from 28 to 30 inches; on the continent, it is less; the fall in Holland is estimated at 26 inches, and in Denmark and North Germany, at 20 inches—the greatest fall occurring in summer and autumn, as in England. Then with respect to winds, we find those from the west most prevalent over what Mr Henfrey distinguishes as the north European plain, as is the case in our country. 'The west wind blows more frequently in England than in Denmark, more there than in Russia. The predominance is most marked in summer; in the winter, the easterly winds are almost as frequent as the westerly upon the continent, which is not true of the British isles.' Sometimes, however, the south-westerly winds, which bring our genial April showers, continue to arrive with their watery burden until late in the summer, to the detriment or destruction of grain-crops; and yet this same wind, losing its excessive moisture as it sweeps onward over the continent, is highly favourable to the husbandman in Southern Russia. The years 1816 and 1817 were cases in point.

The meteorology of Russia affords some striking contrasts: the yearly rain-fall in St Petersburg is 21 inches, 'and the westerly winds are most prevalent, although not to the same extent as in Western Europe; they are also predominant in Moscow and Kasan. In the southern steppes, it is stated that the average of four years has given only 6 inches fall of rain, occurring in 47 days of the year; but the irregularity is so great, that single years gave 59, 35, 39, and 53 rainy days. In 1832-3, twenty months elapsed without rain, and in some years the quantity is only one-tenth of that which falls in wet years. In the summer, there is no dew, and the ground dries up and cracks, the plants withering up: 1841, not considered as a dry year, gave only 8½ inches of rain; but in 1831, one of the wettest, the moisture interfered with agriculture more than the drought does, saturating the soil, which rests on a deep impermeable clayey formation.' In April and May, when the snows melt, the steppe is a vast sea of mud, liable to be hardened by occasional frosts, until, as the season advances, myriads of crocuses, tulips, and hyacinths, cover the soil, which perhaps a few days later will be hidden by north-east snowstorms, or drenched by gales from the north-west. No rain falls for two months after the middle of June, the luxuriant herbage withers more rapidly than it grew, and, except in a few spots near the streams, the steppe becomes a black, arid waste. Yet in some parts of these regions the vegetation is extraordinary: 'the wormwoods and thistles grow to a size unknown in the west of Europe; it is said that the thistle-bush, found where these abound, is tall enough to hide a Cossack horseman. The natives call all these rank weeds, useless for pasture, burian, and, with the dry dung of the flocks, this constitutes all the fuel they possess. One curious plant of the thistle tribe has attracted the notice of most travellers—the wind-witch, as it is called by the German colonists, or leap-the-field, as the Russian name may be translated. It forms a large globular mass of light wiry branches interlaced together, and in autumn decays off at the root, the upper part drying up. It is then at the mercy of the autumn blast, and it is said that thousands may sometimes be seen coursing over the plain, rolling, dancing, and leaping over the slight inequalities, often looking at a distance like a troop of wild horses. It is not uncommon for twenty or thirty to become entangled into a mass, and then roll away, as Mr Kohl says, "like a huge giant in his seven-league boots." Thousands of them are annually blown into the Black Sea, and here, once in contact with water, in an instant lose the fantastic grace belonging to their dry, unsubstantial texture.'

Any one who has seen the feather-like seeds of thistles and dandelions floating about in the air, will have little difficulty in comprehending the effect of winds on the distribution of vegetation. Such seeds, as Mr Henfrey observes, might readily be carried across Europe by a powerful autumn gale, blowing steadily in one direction. In physiological language, they belong to the sporadic, not to the endemic class, of which a remarkable instance is afforded in the flea-bane (Erigeron canadensis), a plant which, having found its way to this side the Atlantic only since the discovery of America, is now a common weed on the continent of Europe. Running streams and ocean currents also transport seeds from one locality to another. The gulf-stream, as is well known, carries occasionally branches of trees to the north coast of Scotland and Norway; and 'Mr Brown found that six hundred plants collected about the river Zaire, in Africa, included thirteen species, natives also of Guyana and Brazil. These species mostly occurred near the mouth of the Zaire, and were of such kind as produced fruits capable of resisting external agencies for a long time.' Then, again, the agency of birds, of quadrupeds, and of man, in the distribution of seeds and plants, is too important to be overlooked, as Sir Charles Lyell has ably shewn in his Principles of Geology; and there is 'a certain number of plants which seem to accompany man wherever he goes, and to flourish best in his vicinity. Thus, the docks, the goosefoots, the nettle, the chickweed, mallows, and many other common weeds, seem to be universal, though unwelcome companions to man—dogging his footsteps, affording by their presence, even in now deserted districts, an almost certain index of the former residence of human beings on the spot.'

From an examination of the causes affecting distribution, Mr Henfrey passes to a survey of the characteristics of the countries of Europe, from north to south—from the peninsula of Scandinavia to those of Spain, Italy, and Greece. The remarkable contrast is pointed out between the climate and cultivation of the east and west sides of the mountains of Sweden and Norway. Barley ripens as far north as the 70th degree, in latitudes whose mean temperature is below the freezing-point; while in Switzerland, corn ceases to ripen at 9 degrees above the same point, and in the plateaux of South America, at 22½ degrees—a fact which goes to shew, 'that the growth of grain is much more dependent on the summer temperature than on the annual mean. The long summer days of the polar regions afford a very brief, but a comparatively exalted summer heat.' It is, however, only the barley which ventures so far north: the limit of rye is 67 degrees, of oats, 65 degrees, of wheat, 64 degrees, on the west side of the peninsula, and from 1 to 2 degrees less on the east. In Southern Norway, the spruce-fir ceases to grow beyond the line of 2900 feet above the sea-level; while in Switzerland, it is commonly met with at the height of 5500 feet, and in some situations, 7000; shewing that the influences which affect the growth of grain do not similarly affect that of trees—proximity of the sea decreases the summer temperature. Again: 'In Scandinavia the tree-limit is indicated by the birch; in the Alps, by firs. The two lower mountain zones of the Alps, the regions of the beech and the chestnut, do not exist in the Scandinavian mountains. Compared with the climate and tree-limits, the cultivation of corn does not go so high in the Alps as it does toward the north; for it ceases about with the beech in the Alps, and grazing is the regular pursuit in the region of firs; while in Scandinavia, the beech only goes to 59 degrees, and corn-culture to 70 degrees—that is, as far as the conifers. Corn succeeds in the latter under a mean temperature below the freezing-point, while in the Alps it ceases at 41 degrees Fahrenheit. The cause of this is the hot though short summer of the north. The Alps have maize and the vine, which will not grow around the Scandinavian mountains; the meadows are throughout richer in the Alps, and grazing is therefore much more extensively pursued.'

The peculiarities and comparisons afforded by other countries, are not less interesting than those we have selected, and we might multiply instances, if space permitted. Enough, however, have been adduced to shew that the mode of accounting for differences of vegetation is so far satisfactory, that it appears to be in perfect accordance with discoverable natural laws; and it is no longer a surprise or mystery to find plants of Southern Russia and of Asia Minor on the high table-lands of Spain; or that the effects of an unvarying temperature, as at Quito, in the table-land of Peru, are to cause the culture of wheat to cease at the mean temperature of Milan, and woods to disappear at the mean of Penzance. A few remarks respecting our own country is all that we can now find room for.

Including snow-falls, the number of rainy days in Dublin in a year is 208, in London, 178, while in Copenhagen it is not more than 134. The number of British plants indigenous or naturalised is from 1400 to 1500, comprising mostly the vegetation of Central Europe, but including specimens from Scandinavia and the Pyrenees. The highest point at which grain has been known to grow, is 1600 feet above the sea-level, at the outlet of Loch Collater, in the Highlands. In Drumochter Pass, an elevation of 1530 feet, potatoes can scarcely be raised; and from 1000 to 1200 feet is the more common limit of the cereal and the esculent. On this point a statement is made, which may be useful to cultivators in the hill districts: it is, that 'the common brake-fern (Pteris aquilina), distributed throughout Britain, is found to be limited by a line running nearly level with the limit of cultivation, and thus affords a test, when cultivation may be absent, where nature does not deny it success. In one sheltered spot in the woods of Loch-na-gar, it was observed at 1900 feet; and in another part of the same woods, at 1700 feet; but on the exposed moors it is very seldom seen beyond 1200 feet, unless in hollows, or on declivities facing the sun.'

In accounting for the varieties of plants in Britain, it is assumed that, during the glacial period, when the tops of our mountains were mere islands in a great sea, under which lay the greater part of modern Europe, they were then peopled by the arctic and alpine species, which now inhabit them. Then came an upheaval; a vast tract of land rose above the water, without any break, as at present between England and the continent; and at this period 'there appears to have been a migration of both plants and animals from east to west, the descendants of which still constitute the great body of the flora and fauna of the British lowlands.' Meantime, the elevation of the former islands into mountain summits, placed them in a temperature suited to the perpetuation of their vegetation. Then, to account for the presence of a Spanish flora in the west of Ireland, a bold hypothesis, started by Professor Edward Forbes, is put forward—'that the west of Ireland was geologically united with the north of Spain;' admitting which, there is no difficulty in supposing the plants to have travelled along the intervening land, which has subsequently disappeared, and that, owing to climatic changes, the hardier sort of plants, such as saxifrages and heaths, have alone survived.