Читать книгу Aspects of plant life; with special reference to the British flora - R. Lloyd Praeger - Страница 5

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“I got up the mountain edge, and from the top saw the world stretcht out, cornlands and forest, the river winding among meadow-flats, and right off, like a hem of the sky, the moving sea.”—Maurice Hewlett: Pan and the Young Shepherd.

Travelling from Scotland by the London and North-Western Railway, as the train roars down the long incline which leads from Shap to the coastal plain of Lancashire, the eye catches, on the left-hand side, a strange grey hill of bare rock rising abruptly, the last outpost of the mountains. It is so different in appearance from the Westmorland fells which have just been traversed, that one looks at it with curiosity, and desires an opportunity of a nearer acquaintance. During the preceding half-hour we have been passing through country of the type that is familiar in the Lake District and in Wales—picturesque ridgy hills with rocky or grassy slopes, and fields and trees occupying the lower grounds. But over much of the surface of this grey hill there appear to be scarcely any plants. A dense scrub of Hazel and other small trees clings to its screes in patches, but the continuous mantle of vegetation is lacking.

The train speeds on through fertile ground with ripening crops and woods standing dense and green, and now on the right, where the low land merges with the sea, we view salt-marshes, which display yet another type of plant growth. Here trees and shrubs are absent, and the low-growing grey and green plants look fleshy and stunted.

In the last thirty miles, indeed, since the train left the summit of Shap, we have seen a number of very different types of vegetation, which appear associated with different types of landscape—the moory uplands, the naked limestone, the deep woods, the desolate salt-marsh. Let us in imagination climb the steep scarp of Farleton Fell, the grey hill of our opening sentence, and consider at leisure some aspects of this teeming plant world and its relations to the Earth on which it grows.

Clambering through a wilderness of stony screes we emerge at length on a bare grey tableland on which, in contrast to the rich country below, vegetation is strangely sparse, and bare rock is everywhere in evidence. If we let the eye sweep round the horizon, we note a similar contrast displayed on broader lines. On the one hand is the mountain-land, with its carpet of grass and heather extending to the very summits; on the other hand the broad expanses of bare sand and mud fringing Morecambe Bay, apparently devoid of any vegetation. And it occurs to us that, before we ponder over the variety and distribution of plant life on this world, we are faced at once with a more profound problem. On this breezy summit, with our minds expanded and stimulated by the sunlight and the breeze, and the broad and beautiful panorama spread around, we must for a moment try to take a wider outlook than

Him that vexed his brains, and theories built

Of gossamer upon the brittle winds,

Perplexed exceedingly why plants were found

Upon the mountain-tops, but wondering not

Why plants were found at all, more wondrous still!

I trust the paraphrase may be pardoned. Why, indeed, should there be plants at all? This great globe, with its whole land surface covered, save at the Poles and in desert regions, with green plants in ten thousand forms, is indeed something to be wondered at. One fascinating question that arises is this: How far is our “lukewarm bullet” unique in its possession of a green plant mantle? Have we any evidence for the supposition that plants exist on the Moon, or on any planets of the solar system other than the Earth?

Vegetation as we know it on our world requires certain physical and chemical conditions for its existence. For instance, a temperature which, at least during the growing season, is well above the freezing-point of water is requisite; yet the temperature must remain a long way below the boiling-point of water; neither could plants as we know them exist in the absence of an atmosphere containing oxygen, carbon dioxide, and water vapour, and incidentally, by its capacity for retaining heat, warding off violent extremes of temperature which otherwise would be a daily and nightly occurrence. What evidence is there as to the condition in these respects of those heavenly bodies which are sufficiently near to allow us to know something of them? To take first our own Moon. Astronomers are agreed that on the Moon there is neither air nor water; it is a dead mass of solid material, scorched by the Sun by day, held in the grip of appalling frost by night. The Moon was no doubt at some remote period of the Earth’s history cast off from that body, and it carried off with it a portion of the Earth’s atmosphere, or of the materials which later formed the Earth’s atmosphere. But the attraction of the Moon is so small that it was unable to retain these gases on its surface; they diffused into space, much of them returning probably to the Earth, leaving the Moon without any covering of nitrogen or oxygen or hydrogen or water vapour, and thus condemning it to permanent sterility.

As regards Mercury, the planet nearest the Sun, conditions appear equally unfavourable. Mercury has ceased to revolve round the Sun, and continually presents one side towards that luminary. On the opposite side an extraordinarily low temperature prevails, low enough to solidify and bind permanently most of the gases of any possible atmosphere; while, on the other side, the very high temperature, due to perpetual and intense sunshine, has assisted the diffusion into space of the more volatile gases, such as hydrogen, which might have remained unfrozen.

The question of life on Mars, which in many respects suggests conditions resembling those prevailing on our own globe, has long occupied the attention of men of science, among whom strong advocates of a Martian flora and fauna have not been wanting. If we may accept one of the most recent summaries[1] of the pros and cons of this question, the conditions are not hopeful. Although an atmosphere exists, it appears to be extremely thin; water vapour seems to be present in only very limited quantity; the temperature is very low, and, except in the warmer portions of the planet during the summer season, would be insufficient to support life. The evidence suggests a frigid climate, with dust-storms whirling over vast deserts and salt seas frozen solid, while near the Poles land and sea alike are buried under snow. Summer produces a slight thawing, but even then the cold, salt-saturated soil would appear to be very unfavourable for plant growth. Arrhenius suggests that the presence of a low vegetation such as snow Algæ near the Poles in summer is as much as could be hoped for under the conditions prevailing on Mars.

Of the planets whose distance from the Sun is small enough to allow heat and light to reach them in quantity sufficient to permit of vegetation such as we know it, there remains Venus, and here at last we meet with conditions suitable for life. Venus possesses an atmosphere densely charged with water vapour, and maintaining a high temperature all the year round. The conditions prevailing there recall, in fact, those believed to have existed on the Earth during the Carboniferous Period, when our great deposits of coal, composed of the remains of tropical plants, were laid down in marshes and steaming lagoons; but on Venus the conditions are still more extreme—the temperature higher, and the moisture much greater, than those of Carboniferous times. If it is allowable to assume that the prevalence of physical and chemical conditions similar to those which in bygone ages supported an abundant vegetation on our globe, would produce plant life on another world, then we may imagine a luxuriant vegetation on Venus. Whether such an assumption is reasonable is a very interesting and highly speculative question, which the present writer is not competent to discuss. But if one is inclined to indulge in speculation, it may fairly be asked, Why should one limit the possibilities of life to the strict range of conditions under which it is manifested on our Earth? May not the inhabitants of the Sun, ensconced ninety million miles away in a comfortable temperature of 6,500° Centigrade, have long since proved to their own complete satisfaction the impossibility of the existence of life under the appalling conditions of climate prevailing on the Earth? Who can say? There are more things in heaven and earth than are dreamed of in our philosophy. A quotation from one of the foremost of modern men of science helps us to put such flights of thought in their proper perspective. “One can hardly emerge from such thoughts,” writes Soddy,[2] in pointing out the remarkable adaptation of the human eye to the peculiarities of the Sun’s light, so as to make the best of that wave-length of which there is most, “without an intuition that, in spite of all, the universal Life Principle, which makes the world a teeming hive, may not be at the sport of every physical condition, may not be entirely confined to a temperature between freezing and boiling points, to an oxygen atmosphere, to the most favourably situated planet of a sun at the right degree of incandescence, as we are almost forced by our experience of life to conclude. Possibly the Great Organizer can operate, under conditions where we could not for an instant survive, to produce beings we should not, without a special education, recognize as being alive like ourselves.”

It is generally conceded that life on our globe began in the water, and thence spread to the land. Very significant in this regard is the fact that all but the highest plants require the presence of external water for the act of fertilization, as the male cell swims through water to the ovum. Only the most recently evolved groups have shaken off this ancestral trait; and as regards the whole economy of plants the water relation remains, throughout the entire vegetable kingdom, the most obvious and universally important of the different relations existing between plants and their environment. How vegetable life originated, from what inorganic forms it was evolved, is a secret which science has not yet discovered; but since those dim first beginnings it has never been absent from the Earth, so far as we know, and has increased and multiplied, and passed through a thousand changes to higher and higher forms, till it has attained to the beautiful and bountiful and varied plant world which we know, covering with a green mantle most of the land surface of the globe and filling the shallower lakes and seas; while in its minuter forms it swarms in the soils and waters of the Earth, and its germs pervade the atmosphere.

It is not everywhere even on our hospitable, habitable globe that conditions are suitable for plant growth. The reader will remember that the flat summit of Farleton Fell, where in fancy we still stand, was devoid to a great extent of vegetation; and that the sea-sands and mud-flats out to the westward presented a surface from which plants appeared to be absent. This question of deserts—that is, of areas of the Earth’s surface where the prevailing mantle of vegetation is wanting—is an interesting one, and may fittingly detain us for a few minutes. Deserts are produced by the failure of one or more of the conditions which are necessary for plant life. The factors in question may be briefly defined as temperature, light, water, atmosphere, and mineral salts. The majority of the higher plants have developed a complicated root-system for the purpose of collecting water (containing salts) from the soil, and of anchoring the organism firmly in its chosen abode, so a soil is also usually essential. Here on Farleton Fell soil is missing over much of the surface, which is occupied by naked limestone rock. The absence of soil is due to the fact that the material—carbonate of lime—of which the rock is composed is soluble in water, unlike, for instance, the materials of which slate or sandstone rocks are composed; the rains slowly dissolve it, and it passes in solution down through crevices in the strata, leaving behind only a small insoluble residue. This residue, where not also washed away, collects in every little hollow, and lowly plants such as Algæ and Mosses soon discover it and colonize it. Their decayed remains add nutritive material to the little pocket, and help to retain water, and thus prepare the way by degrees for higher forms of life; till at length the crevices become filled with a luxuriant vegetation which, as we shall see later, is of a rather peculiar type. It should be noted that even the bare rock is not so inhospitable as completely to exclude plant life. If we examine it with a lens we shall see that it is colonized by minute Lichens, many of which have the power of dissolving the limestone, producing tiny burrows in which they live securely.

Fig. 1.—A Burrowing Lichen (Verrucaria calciseda), living in limestone.

a, Natural size; b, greatly enlarged; c, section, greatly enlarged.

On the sands and mud-flats a semi-desert exists, due in great measure to the shifting nature of the material and the difficulty which plants find in securing an anchorage in it. But in the upper parts, near high-water mark, a few land plants—notably the Glasswort (Salicornia europæa, Fig. 2), a fleshy little annual—colonize the dreary flats with tiny forests of dark green branches, and lower down many small Seaweeds flourish. Some of these, ramifying through the surface layers, help to bind together the shifting sand, and by entangling in their branches fresh particles, and by continued growth, tend to raise and consolidate the surface, to render it suitable for the immigration of land plants such as the Glasswort, and thus eventually to reclaim it from the sea.

Fig. 2.—Glasswort (Salicornia europæa).

a, Plant, ²/₃; b, male flower; c, female flower, both enlarged.

It is in the depths of the ocean, however, that the greatest deserts of our globe are to be found. The luxuriant Seaweed gardens that decorate the shallower waters of the sea, especially where a rocky bottom provides secure foothold, dwindle rapidly as the depth increases, owing to the diminution of light, and when the coastal fringe is left they cease. In the inky darkness of the ocean depths, amid absolute stillness and a temperature little above freezing, plant life of any sort is unknown. Only the flinty skeletons of diatoms and other minute forms of vegetable life which inhabit the surface layers, raining slowly down throughout the ages, tell that plant life exists in the sea at all.

On land, the larger deserts are found in the coldest and in the hottest regions. Around the North and South Poles lie great areas where the perennial lowness of temperature and the consequent almost continuous covering of snow and ice render plant life impossible. But just as the Eskimo live under conditions which would be wellnigh prohibitive to inhabitants of more temperate regions, so many of the higher as well as the lower plants creep northward far beyond the Arctic Circle, where, awakening from a nine months’ winter sleep, they break from the still half-frozen ground to brighten the brief summer with their leaves and flowers and fruit. The flora of Greenland, for instance, which we generally think of as an ice-bound and inhospitable land, numbers some 400 species of Seed Plants. These live mostly on the cliffs and steep ground that fringe the coast, where they are clear of the great icefields which bury the interior of the country, and in many places descend as broad glaciers into the sea. But the life of these high northern plants is slow and difficult, as is evidenced by their paucity and their stunted stature. Later on we shall have to consider how they adapt themselves to the adverse conditions under which they exist (Chapter VIII.); and we shall find their life problems are reproduced in many respects by those of the interesting alpine plants which may be found nestling in the rock crevices of the higher mountains of our own country.

But the more familiar deserts of the world, those to which the mind turns when we use the term, are mainly due, not to absence of light as in the ocean depths, nor to want of heat as in the polar regions, but to failure of the water-supply. A vast desert region of this kind stretches across Northern Africa from west to east, and onward through Arabia, Southern Persia, and Baluchistan. Another, almost continuous with it, extends from the Caspian Sea across great plains into Central Asia, and on over vast mountain areas into Western China. Other similar deserts, familiar to us in word and picture, are situated in the south-western United States, Mexico, and South Africa. In all these tracts, with their diverse characters and diverse sparse floras, the scarcity of rain is the primary cause of their peculiar features. The dryness prevents a protecting covering of vegetation, and allows heat and cold—both sharply accentuated by the scarcity of the moderating influence of water in either soil or air—to pursue their work of disintegrating the surface, reducing the rocks to sand and dust, which the winds sweep hither and thither. In such circumstances plants exist under