One other reason for the eastward motion of the cyclones with us is that they seem to prefer damp air, and so tend to follow the North Sea and pa.s.s towards the Baltic, where they often die away.

In the British area, though the depressions move faster in winter than in summer, they have only a mean speed of about 16 miles an hour, while in the United States their mean speed is 25 miles per hour, and their effects are often disastrous except when discounted by the warnings of the Weather Bureau.

In the case of the British Isles cyclones are most frequent and best marked in winter, and they are of great importance in producing our mild and windy winters. In summer they travel further northwards, and as a rule affect our climate less. When, however, from causes still inadequately known, they are better marked in summer than usual, we have a "bad" summer, that is, one which is wet and relatively windy.

The fact that the English Channel is one of the favourite tracks of cyclones has been an important element in protecting the British Islands from foreign invasion, while we all know that it is also a factor in diminishing free intercourse with the Continent.

[Ill.u.s.tration: Fig. 11.--Diagram showing the changes in temperature, pressure and wind due to a cyclone pa.s.sing to the north of a point of observation A. The pa.s.sage of the cyclone figured occupied a period of six days. It will be noted that as it approaches A the wind is southerly and light (arrows with single barbs) and the temperature high. As it pa.s.ses the winds become violent (arrows with double barbs), and shift to the south-west, and the barometer falls rapidly. As it disappears the pressure rises, the temperature falls, and the wind veers to the north-west, while remaining violent. The fall of the wind and its shifting to a south-westerly direction mark the return to the normal condition of affairs, the influence of the cyclone being past.]

The second point of importance about our weather is the periodic occurrence at some part of our area of anticyclones, or areas of high pressure, out of which the winds stream gently in the same direction as the hands of the clock. These areas of high pressure do not display the same tendency to move as do the cyclones, and are most frequently merely displaced by advancing cyclones. For reasons into which s.p.a.ce does not permit us to go fully here, anticyclones have a very different effect in summer and in winter. In winter they may bring to us the continental cold, and make our weather abnormally severe, though often bright and fine. On the other hand, in summer they bring to us continental warmth, so that "good" summers are those in which anticyclonic conditions are most frequent, while "severe" winters are due to the same cause. Anticyclones also sometimes induce a curious form of inversion, in that places to the north of a given spot may have temporarily a higher temperature than places to the south. It is such facts which are entirely masked by "mean" figures.

We do not as yet understand the causes which make cyclones sometimes more numerous or better marked than usual, which cause them sometimes to cross our area, and at other times to travel too far north or too far south to influence our weather. It is possible that further investigation in the future may unravel this problem; it is practically certain that a freer use of wireless telegraphy, and the establishing of meteorological stations in northern seas, would give weather forecasting a definiteness and accuracy which it does not yet possess.

[Ill.u.s.tration: Fig. 12.--British weather map for Nov. 29, 1910. A cyclone lies over the south of Scandinavia, and into it the winds are sweeping strongly in a counterclockwise direction. An anticyclone lies over Iceland, and from it the winds are streaming gently in a clockwise direction.]

We cannot follow this interesting subject further here, but we have said enough to ill.u.s.trate its geographical significance. As a science or sub-science by itself it will form the subject of a special volume in this series. It may be enough to point out that the _Daily Weather Report_, published by the Meteorological Office at a cost of one penny, and reproduced in some daily newspapers, is a doc.u.ment well worth the careful study of those with any interest in geography.

CHAPTER V

THE PRINCIPLES OF PLANT GEOGRAPHY AND THE CHIEF PLANT FORMATIONS OF EUROPE AND NORTH AMERICA

We have now taken a general survey of the earth's surface, have noted its mountain heights and its ocean depths, watched the formation of hills and valleys which is due to the joint action of atmospheric agents, running water and ice, and considered briefly some of the points of interest about climate. We next pa.s.s to that most characteristic feature of the surface, its clothing of plants. Except where the surface of the ground has been artificially sterilised by man, or is rendered unproductive by ice, by lava, by a total lack of water, or by the existence of poisonous salts, it is clothed with vegetation, and it is the presence of this vegetation which is its most obvious character.

Here, however, as in other regions of thought, the geographical standpoint has only been reached slowly. Man's habit of a.n.a.lysis made him study gra.s.ses and trees for long generations before he got back to the forest and to the gra.s.sland as they occur in nature. Plants as individuals are the province of the botanist, but those plant groups which are the expression of the interaction of climatic factors, soil, and surface relief, are the concern of the geographer.

When we take a general survey of the face of the earth from the point of view of plant geography, we note three main conditions. In certain regions, alike in the tropics and in temperate zones, we find that plants reach their maximum size, combined with great differentiation of structure, and the formation of woody stems which offer great resistance to varying conditions of climate and weather. Such highly-organised plants form forests, which still dominate over a large part of the earth's surface.

Man's nearest allies, the anthropoid apes and the monkeys, are for the most part forest animals, and the lowest races of men are still forest dwellers. Where man is a forest dweller he seems not to reach his full size, as we see in the case of the pigmies of the Congo forest, and the negritos of the Philippines, and he suffers from a chronic insufficiency of food, which acts as a check both to his mental and physical development. There has, therefore, always been war between evolving man and the giants of the plant world, a war which has swept the forests away from many of the more civilised parts of the globe, and which still continues, though man's victory is now so complete that he can afford to be generous, and give protection to the remnants of his former foe.

But over parts of the globe the climate, and especially the amount or distribution of the rainfall, makes it difficult or impossible for forests to grow naturally. Here other types of plants, lower in stature, and evading rather than facing the problems of winter cold or summer drought, flourish and form what we call the gra.s.slands. The gra.s.slands favour man in several respects. They feed the animals upon which he depends for food, for clothing, and for the conveyance of his person or property, and they offer much less resistance than the forest to his agricultural operations. Even the large herbivorous mammals which in their wild state haunt the forests, usually leave these at night to feed upon the gra.s.slands, so that it is the gra.s.slands which have largely fed man at every stage of civilisation. The atmospheric conditions within the woodlands also, the deficient sunlight, the humidity, and so forth, seem unfavourable to human development.

Where the conditions are especially unfavourable to plant life, we find that even the gra.s.sland plants are unable to keep up the struggle, and diminish in number, losing their power of forming a complete covering for the soil, and thus the gra.s.sland pa.s.ses into desert, whether the hot waterless desert of low lat.i.tudes, or the cold frozen desert of northern ones.

In the most general sense, then, we may say that these three formations, woodland, gra.s.sland and desert, divide the surface of the land among them, and between them there is constant conflict. The gra.s.slands are for ever attempting to encroach upon the woodlands, and in this attempt they have been a.s.sisted, sometimes to too great an extent, by the operations of man. Similarly the desert is always striving to encroach upon the gra.s.sland, and in this endeavour it has been sometimes involuntarily aided by man, who has also done much voluntarily to reclaim the desert land for the gra.s.ses.

Let us note next the particular conditions which favour woodland, gra.s.sland and desert respectively. The distribution of plants over the surface of the earth at large is determined by a number of factors, by the amount of heat, by the amount and distribution of precipitation, by the nature and strength of the winds, by the characters of the soil, and so on. But forests occur under the equator and also far to the north; we have cold deserts as well as hot ones; there are extensive gra.s.slands in the Sudan as well as in the Canadian Far West. This proves that the varying amounts of heat may be neglected in considering the cause of the distribution of the three great plant formations.

Again, the soil is of minor importance, for different types of forest and of gra.s.sland occur on different types of soils. We are thus led to the conclusion that it is the precipitation and the wind which determine the distribution. To understand the reason for this we must consider the needs of different types of plants in the matter of water.

Plants can only take in the mineral const.i.tuents of their food in the form of a solution, and this solution must be weak, or it has a poisonous effect. For example, sulphate of ammonia is a valuable manure, but if a considerable amount be dissolved in water and applied to the roots of a growing plant, death may very likely take place.

It is a necessary consequence of the fact that plants can only absorb weak solutions of their food salts, that their roots take in more water than is actually needed by the plant. _One_ of the functions of the leaves is therefore to get rid of surplus water, the process being called transpiration. Transpiration takes place faster in a tall plant like a tree, which grows up into dry layers of the air, than in a low plant like a gra.s.s. It takes place faster in windy weather than in calm.

Other things being equal it takes place faster in warm weather than in cold, and the larger the plant and the more numerous its leaves the more water is given off, that is, the more water is returned to the air from the soil.

The result of all this is that forest trees require far more water than gra.s.sland. It has been calculated that a beech wood aged 50 to 60 years gives off during the growing season 354 tons of water per acre, which ill.u.s.trates the drying effect of the presence of the wood. Similarly, the effect of tree-planting in the marshy regions of France and Italy, where the soil as a consequence has dried and the marshes disappeared, shows how great a demand upon ground water trees make, as compared with gra.s.ses and low growing herbs.

On the other hand, although trees take an enormous amount of water from the soil, they can draw their supplies from a large area. It is the extremities of the fine branches of the roots which take in the water, and these pa.s.s deep down into the soil, and spread out over a vast area.

In other words, trees avail themselves of the water in the deeper layers of the soil, and can tolerate relatively long periods of drought, if the surface drying of the soil does not extend to the deeper layers. In hot summer weather gra.s.slands brown and wither long before the trees show any signs of water-famine.

In consequence, we may say that as long as the total rainfall of a region is sufficient to ensure a constant supply of moisture in the subsoil during the growing season, trees can thrive, even if little or no rain falls during this season. On the other hand, drying winds are very hurtful to trees, especially if they occur at a period when the tree is unable, either because of the coldness of the subsoil, or because of its dryness, to take in fresh water to replace that which is lost. The hurtfulness of late frosts is largely due to the cold suddenly checking root absorption at a time when the growing parts, acted upon by the spring winds, are giving out water freely.

Gra.s.ses transpire less freely than trees, but their root system is much shallower and less well-developed. They depend upon the water in the upper layers of soil, and must have frequent, even if gentle, showers during their growing season, while they are quite indifferent to drought and even to cutting winds during their resting period.

A little reflection will show that it results from these facts that woodland, gra.s.sland and desert do not form a continuous series. It may quite well be that woodland pa.s.ses through scrub into desert without the intervention of gra.s.sland. Right across Europe there is (or was) a broad belt of forest. Southward towards the Mediterranean this thins out into a characteristic form of scrub, called maquis, whose characters we shall describe later, and this scrub pa.s.ses in all directions into desert land. Here no belt of gra.s.sland intervenes, for the rainless Mediterranean summer makes the growth of gra.s.s virtually impossible, except where special conditions, _e. g._ hills, introduce modifications.

Contrasted with this we have the conditions in North America where, _e. g._ in Canada, the western coast is densely forest-clad, as is also the eastern region. In journeying eastward after crossing the Rocky Mountains the forest dies away into gra.s.sland, and the same thing happens, though more slowly, in a westward journey. The reason is that in this case there is a steady diminution of precipitation on pa.s.sing to the interior, but what precipitation remains is, as we have seen, largely, though not wholly, summer rain, and is, therefore, sufficient to determine the growth of gra.s.s, though not of trees.

Again, in North Africa the forests of the Atlas Mountains pa.s.s directly, without intervening gra.s.sland, into the Sahara desert, but to the south of the desert the gra.s.sy and park-like Sudan separates the desert from the luxuriant tropical forest. In the latter case, however, it is possible that man's influence has counted for something.

On mountains, in whatever lat.i.tude, the conditions are much more uniform, partly because it is wind, a.s.sisted by temperature variations, which is the dominating factor. Moisture is usually abundant, but high up what is called physiological drought occurs; that is, the temperature is too low for the plants to be able to absorb the abundant water.

In ascending any mountain, the following are the chief changes which occur. The lower slopes will probably be cultivated. As we ascend the precipitation increases, and forests appear. First we have probably a belt of deciduous trees, pa.s.sing above into the more resistant conifers.

This belt usually ascends higher on the south than on the north side, and higher on mountains which occur in a group than on isolated peaks.

As the wind is more and more felt, and increases the dangerous transpiration of winter the trees become more and more dwarfed to escape its force. There may be a belt of prostrate mountain pines above, marking the tree limit; in any case the trees are gradually replaced by dwarfed shrubs. Then comes the zone of Alpine plants, the gra.s.ses making a complete sward, but being accompanied by many other plants.

Gradually, as the soil becomes scantier, and the surface more rocky and exposed, the continuous sward disappears, and the conditions of a cold desert appear. A few scattered plants occur, ceasing near the snow-line, the highest being usually plants of simple structure like mosses and lichens.

As we have already indicated, in the case of the mountains of Europe there are often glacial shelves at considerable elevations, whose covering of fine debris determines the growth of peculiarly fine gra.s.s.

The economic value of this gra.s.sland has in many cases in the Alps induced man to destroy the forest in order to increase pasture land. The result has often been disastrous, for once the trees are cut down the forest soil is rapidly destroyed by weathering, especially on slopes, the courses of streams are altered by the more rapid run-off, and widespread flooding and destruction of pastures have sometimes resulted.

In North America, similarly, man's attempt to increase pasture land or arable land at the expense of woodland has often led to disastrous consequences.

We have already spoken of the special features of the Mediterranean climate, and indicated that its peculiarities are reflected in its vegetation; we must now consider this vegetation in a little more detail. The fact that the region is chiefly visited by the inhabitants of more northern climates in spring gives rise to a somewhat erroneous impression in regard to the plants. In spring the Mediterranean vegetation is at its best. The mild winters permit the plants which further north die down or cease to grow, to go on blooming. The rains so moisten the soil that the first warm days cause very rapid growth in those plants which finish their activities before the hot, dry summer begins. They must flower and seed in spring, and die down till the rains of autumn awaken them again.

In our own country we have a few plants which hurry through their activities in this way. The lesser celandine, the wood anemone and a few others strive to flower and fruit before the forest trees are thickly clad with leaves. The snowdrop, even the wild hyacinth, though it is much later, similarly limit their active life to a short period in spring. This phenomenon, only suggested in our climate, is very marked in the Mediterranean area.

That region is especially characterised by its richness in bulbous and tuberous plants. These, as all who have grown hyacinths or narcissuses know, demand relatively large amounts of water during their short growing period. In spring, therefore, the sh.o.r.es of the Mediterranean are bright with many kinds of anemones, with narcissus, asphodel, bell hyacinth, Allium, tulips, and so on, all awakened by the spring warmth and the spring rains. Accompanying them are many bright-coloured annuals, also in a hurry to race through their life-history before the terrible drought of summer. Now also the gra.s.s grows, and the autumn-sown corn becomes tall. As the weather grows hotter and drier, the plants with bulbous and tuberous roots die down to the ground, the annuals die altogether, leaving their seeds to wait till the autumn rains before they sprout. The gra.s.ses turn brown, and the peculiar parched appearance of the Mediterranean summer spreads over the land.

To a northern visitor at this season it is not luxuriance but desolation which is the prevailing note. Except on the hill slopes there are no ma.s.ses of broad-leafed foliage trees--there is not the deep bright green characteristic of the summer woods further north. The trees do not reach a great size; the leaves are usually small, and the fact that they strive to avoid the sun by arranging themselves with the edge upwards instead of the flat surface, makes them appear smaller than they are.

They are often needle-shaped, sometimes shining and coated with resin, sometimes silvery owing to a coating of hairs on the under surface. Many plants have spines or thorns, and succulent plants like agave, aloe and p.r.i.c.kly pear are common. The absence of a complete covering of vegetation causes the surface soil to dry completely, and so form clouds of dust which adds to the generally desolate appearance. Indeed, the brown powdery appearance of the soil is one of the points which especially strikes the stranger, accustomed to the darker, moister soil of the north, always covered with vegetation, except where man has interfered.

Here and there, however, are indications that even this parched brown earth holds wealth for man. The vines, if dusty and far less luxuriant than one expects, are loaded with ripening fruit. The gorgeous scarlet flowers of the double pomegranate gleam amid the dark foliage; the gnarled and twisted olives show on close inspection ma.s.ses of small green fruits; the oleander bushes are covered with pink flowers; there are great round b.a.l.l.s on the orange and lemon trees, and many other fruit trees are loaded with produce.

Let us sum up first what man gains from the plants of the Mediterranean, and then look at some points in regard to the wild plants. In the first place, we see that man takes advantage of the rapid growth of annuals in the early part of the year. The annuals most useful to him, here as elsewhere, are, of course, the cereals, especially wheat, which, if sown in autumn, is nourished by the winter rains, and grows rapidly with the warmth of spring to ripen in May, June or July, according to the locality.

In the second place, certain trees or shrubs, by reason of their resistance to drought, and their elaborate root system, which enables them to gather water from the deeper layers of the soil, will produce succulent fruits without needing artificial supplies of water. The most important of these, throughout the whole Mediterranean area, are the vine and the olive. The olive supplies the oil which is all the more necessary in that the absence of gra.s.s makes pastoral industries, and therefore the production of cheese and b.u.t.ter difficult or impossible except in the high grounds, while the vine supplies the wine which with bread and oil form the essential parts of the diet of Mediterranean man.

The olive tree, which is indigenous, may be regarded as one of the most characteristic trees of the area, and it is interesting to note that the novice not infrequently confuses it with another tree, almost as characteristic the evergreen or holm oak. The two are not nearly related, the olive belonging to the same family as the lilac and privet, while the evergreen oak is a true oak. Both trees, however, show similar adaptations to summer drought, and their resemblance to one another is a good example of convergence due to a similar environment. Both have small evergreen leaves; small that they may not lose too much water in summer, evergreen that they may a.s.similate even during the winter. Both have their leaves silvery beneath, which again prevents loss of water; both have gnarled trunks, branching low down, in order that the leaves may avoid the dry upper layers of the air. Adaptations of this kind are present to a greater or less degree in all the trees which are tolerant of Mediterranean conditions, and many of these trees yield useful fruits.

In addition to the cultivated plants mentioned, a great number of others are grown within the area, as we shall see later, but the point of interest is that the plants which have been of importance in the history of the region have been either annuals which ripened early, or fruit-bearing trees with special adaptations to resist drought.

Apart from the annuals and the bulbous and tuberous plants already described, the wild plants are chiefly shrubs or stunted trees with similar drought-resisting characters. During the long ages he has inhabited the Mediterranean, man has doubtless contributed largely to the destruction of the forests which are now, as we have seen, represented by the stunted scrub or maquis. But on climatic grounds we cannot suppose that the Mediterranean forests had ever the luxuriance of those further north, or of the tropical forests of the south.

Where there is sufficient rain chestnut woods occur, but this is only on the hill slopes. Above the chestnut, beech may occur, as in Sicily.

The maritime pine and the Corsican pine form open woods in the damper places, and the picturesque stone pine, with its rounded head, is very characteristic. We have already mentioned the evergreen or holm oak as common, and the cork oak occurs abundantly in some places. These trees, with the cypress, must have formed the primitive forests, and they still const.i.tute the most important forest trees of the area. The occurrence of a native palm (_Chamaerops_) is interesting as suggesting the warmth of the climate, and even on the European sh.o.r.es the date palm is extensively planted, though its true home is the margin of the African and Arabian deserts.

Of the characteristic shrubs the most striking are perhaps the many species of Cistus, with large almost rose-like flowers, and leaves which attempt to adapt themselves to the climate by many different devices.

Sometimes they are stiff and leathery, sometimes resinous, sometimes hairy. Many plants in the area have a coating of resin on their leaves.