THE LIFE AND DEATH OF TIMBER.*
THE vertebrate monsters of geological antiquity have long since yielded to the subtle influence of natural selection ; but the giants of the vegetable world still exist, and trees are yet to be found in Australia and America that would over- top the dome of St. Paul's. Nevertheless, trees, like other plants, can offer only a passive resistance to the numerous hostile agencies they have to contend with ; and they are, on the whole, more scantily provided with protective and repro- ductive devices than the humble weeds of the roadside which spring up one year to die the next, while their great neighbours may endure through a dozen centuries. The secret of this obstinate longevity, racial and individual, is to be found in the vegetative structure of the tree rather than in the character of its organs. And the investigation is not only one of the highest scientific interest, but of immense practical value as well. For among the most precious treasures of this earth forests are to be counted ; and the various ills to which timber, as well as flesh, is heir, can neither be understood nor remedied without a knowledge of its physiology and structure .
In the pleasantly written and admirably illustrated volume before us, Professor Marshall Ward has summed up the results of recent botanical work on the vegetative life-history of timber. His book, like the other volumes of the excellent series to which it belongs, is satisfactory and instructive to the expert, and intelligible to the ordinary reader who possesses a rudimentary knowledge of the division of science with which it deals. Untechnical, rather than merely popular, interpretations of science are absolutely necessary in these days of minute, varied, and accurate research, if the general public is not to remain in entire ignorance of the results of the characteristic intellectual activity of the century. We owe, therefore, at least a warm welcome to publications like the present, which, though of modest pretensions, could only have been written by an accomplished botanist, and which, indeed, contains in piccolo the essentials of the science and art of forestry.
The main distinction between trees and herbs is the enormous development in the former of the wood and cork tissues common to all flowering and many other vegetable organisms. But the greater part of the wood and bark of a tree-trunk is physiologically dead ; only a narrow ring of tissue between the wood and the bark—the cambium—preserves the characteristic vital power of cell-division. It is to this massive cylinder of wood—a mere by-product of the wonderful chemical operations always being carried on within the sub- stance of plants—that trees owe their extraordinary success in the struggle for existence. We have said that wood-tissue is physiologically dead, but it does at least subserve one phy- siological end, the transmission or the storage of water, without which the constructive and eliminative processes of the organism would soon come to an end. As every one knows, the trunks of most trees—of almost all that grow in temperate climates—on being eut across, show a number of irregular concentric rings traversed by lines radiating from the centre to the circumference. The latter consist of cells containing more or less starchy and protoplasmic material, and therefore alive, but less so than the very narrow outermost layer of the outermost ring, the cells of which are capable of dividing, and by their division, principally at right angles to the radius, give rise to the con- centric rings. Usually, the outer layers of each ring are closer-grained and darker than the inner, the cells of the latter being formed from the cambium in the spring, and those of the former in autumn, when, owing to increased warmth and sunlight, the nutrition of the tree is at its highest. In the ordinary deal-fir, the cells are vertically elongated, nearly rectangular, hollow, closed prisms tapering off above
• Timber, cud Some of its Diseases. By H. Marshall Ward, M.A., F.11.8.,F.L.S., Professor of Botany, Cooper's Hill. With Illustrations. " Nature Series." London : Macmillan. 1889.
and below, about one three-hundredth of an inch in diameter, and several times as long, communicating with each other not directly, but through a thin elastic membrane, stretched across the bottom of sunk pores or pits in the walls. A full de- scription of these cells, and of other elements of the wood found in dicotyledonous trees (oaks, beeches, &c.), together with an account of their principal modes of arrangement and colloca- tion so as to form tissues, is given by Professor Ward, and with the help of his illustrations will be easily understood.
Now, the question of the degree and manner in which these cells convey water from the roots to the leaves has been as hotly debated amongst botanists during the last decade or so as the question of Home-rule amongst politicians. The problem is no easy one, involving as it does the explanation of the rise of water to a height often of hundreds of feet in the absence of all mechanical means. The principal theories, which are stated and reviewed in the fourth chapter, turn upon the alleged capacity of the cell-walls to pass on water much as the molecules of sea-water may pass on the particles of salt, or upon transmission through the cells and a filtration through their pores, or, lastly, upon, what is probably a near approxima- tion to the truth, upon the power possessed by the living cells of the plant (cambial, radial, and others), by means of their protoplasm, to absorb and pass on, by a chemical process, the molecules of water, the ascent being perhaps aided by capil- larity acting at short intervals, by the resting-places, reservoirs, or " locks " afforded by the wood-cells, and by the looser texture of the higher portions of the trunk and branches, and the de- creasing pressure of air or gas within their substance. Professor Ward lays stress, too, upon a certain periodicity in protoplasmic changes to which biologists are more and more looking for an explanation of many of the phenomena of organic life. It is not, however, easy to see how this can bear upon the problem of water-ascent, though, possibly enough, alternations of tem- perature may have some share in producing the phenomenon.
Trees suffer from many diseases, but the most frequent and deadly attacks they have to encounter are those of the fungi which infest them and flourish at their expense. Disagreeable- looking fungous masses are commonly enough seen on the branches, trunks, or roots of trees, sometimes of a felt-like character ( Tra metes radiciperda), sometimes projecting horizon- tally in fleshy or even woody plates, sometimes, again, true mush- rooms or toadstools (e.g., Agaricus melleus). It is not, however, these external masses, lamellae, or stalked discs, that work the mischief ; they are but the fructifications bearing on the gills, or on the living membrane of the small cavities that lend a honeycomb look to the upper or lower surfaces of the lamellar outgrowths, microscopic spores that, like those of ferns, but without any intermediate sexual stage, give rise to new in- dividuals. The injury is done by the mycelium, or thin root- like threads (the so-called "spawn" of mushrooms), from which the stalked or sessile fructification springs. These threads find their way into the tree through a crack or wound, and send fine ramifications into the cambial, vegetative, and ligneous tissues, which come into close contact with the elemental cells, rob them of their contents if they have any, if not, disintegrate and destroy their substance. How they do this we do not know; the ultimate chemistry of vegetable life is for the present a profound mystery. The result of the process is the decay of the tissues, involving loss of solidity and resisting power, interruption to the nutrition, and finally, death from a sort of inanition. The fungi named above are true parasites, their mycelium attacking healthy as well as diseased tissues. Another common form is Polyporus sulphureus, extending out- wards in fleshy plates, bright yellow below where the spore- bearing pores are, orange or vermilion above. The thick white mycelium sometimes completely fills up the rays, and separates each concentric ring, giving a very characteristic appearance to the cut surface of an infested branch. The dirty yellow toadstool, Agaricus melleus, is a particularly deadly enemy to trees, its firm, black mycelial threads often burrowing many feet under ground, thus spreading the infection far and wide. Of liferuliuslacrymans, the watery, slimy fructification- masses of which are common in damp cellars, the mycelium causes dry-rot." This fungus is peculiar in that it is never found, so to speak, wild,—in the forest. Other fungi attack the cortex, the inner portions of which elaborate and transmit nutritive material, while the sapwood principally conveys water, and are equally fatal to the tree, though less injurious to the timber. The leaves, too, are affected by various smaller fungous growths—even the young seedlings are—and one of. the best chapters in the book is that devoted to the life- history of Phytophthora omnivora—aptly enough so named— which plays such havoc with the tender structures of the infant beech.
The ravages committed by these pests are incalculable. The present writer recently passed through a coniferous forest in Norway wh ere hardly a tree was free from fungous blisters called by the woodmen " larv,"—probably clusters of weidia (spore- clusters) of the fungus, Peridermium pini, fully described in the twelfth chapter of Professor Ward's book. When more shall be known of the habits of life of these low forms of vegetation —at present it is extremely difficult to induce their spores to grow artificially, as will be readily understood by all who have cultivated mushrooms—perhaps some remedy against them will be found. Hitherto they have been fought directly by stamping- out processes, and Nature as a rule is too coy to yield to any but indirect blandishments. She must be circumvented by resort to her own methods, and usually laughs at mere mechanical procedures.
Previous page
