Читать книгу The Oak: A Popular Introduction to Forest-botany - Harry Marshall Ward - Страница 3
ОглавлениеCHAPTER I.
introduction.
Famous in poetry and prose alike, the oak must always be for Englishmen a subject of interest, around which historical associations of the most varied character are grouped; but although what may be termed the sentimental aspect of the "British oak" is not likely to disappear even in these days of iron-clads and veneering, it must be allowed that the popular admiration for the sturdy tree is to-day a very different feeling from the veneration with which it was regarded in ancient times; and that, with the calmer and more thoughtful ways of looking at this and other objects of superstition, a certain air of romance seems to have disappeared which to so many would still present a tempting charm. It is not to these latter alone that our few existing ancient oaks are so attractive, however, and a slight acquaintance with the oaken roofs and carvings of some of our historical edifices affords ample proof that the indefinable charm exercised on us by what has proved so lasting, is a real one and deep-seated in the Saxon nature. In fact, everything about the oak is suggestive of durability and sturdy hardiness, and, like so many objects of human worship in the earlier days of man's emergence from a savage state, the oak instinctively attracts us. The attraction is no doubt complex, taking its origin in the value of its acorns and timber to our early forefathers, not unaffected by the artistic beauty of the foliage and habit of the tree, and the forest life of our ancestors, to say nothing of the more modern sentiment aroused when ships of war were built almost entirely of heart of oak; for the Aryan race seems to have used and valued both the fruit and the wood from very early times, and both Celt and Saxon preserved the traditional regard for them. Memories of our Anglo-Saxon ancestors are still found in the English and German names for the tree and its fruit, as seen by comparing the Anglo-Saxon āc or œc, the name of the oak, with the English word, and with the German Eiche on the one hand, and with acorn (Eichel) on the other.
In early days, moreover, there were vast oak forests in our island and on the Continent, and, although these have been almost cleared away so far as England is concerned, there are still ancient oaks in this country, some of which must date from Saxon times or thereabouts; and the oak is still one of the commonest trees in France, parts of Germany, and some other districts in Europe.
This is not the place to go further into what may be called the folk-lore of the oak—a subject which would supply material for a large volume—but it may be remarked that giant or veteran oaks are still to be found (or were until quite recently) in Gloucestershire, Yorkshire, and on Dartmoor and other places, and a very fair idea of what an old oak forest must have been like may be gathered from a visit to the New Forest in Hampshire, or even to some parts of Windsor Forest.
As so often happens in the study of science, we have in the oak a subject for investigation which presents features of intense interest at every turn; and however much the new mode of looking at the tree may at first sight appear to be opposed to the older one, it will be found that the story of the oak as an object of biological study is at least not less fascinating than its folk-lore. With this idea in view, I propose to set before the reader in the following chapters a short account of what is most worth attention in the anatomy and physiology of the oak, as a forest tree which has been so thoroughly investigated that we may confidently accept it as a type.
In carrying out this idea there are several possible modes of procedure, but perhaps the following will recommend itself as that best adapted to the requirements of a popular book, and as a natural way of tracing the various events in the life-history of a plant so complex as is the tree.
First, the acorn will be described as an object with a certain structure and composition, and capable of behaving in a definite manner when placed in the ground, and under certain circumstances, in virtue of its physiological properties and of the action of the environment upon its structure. The germinated acorn gives rise to the seedling or young oak, and we shall proceed to regard this, again, as a subject for botanical study. It consists of certain definite parts or organs, each with its peculiar structure, tissues, etc., and each capable of behaving in a given manner under proper conditions. The study of the seedling leads naturally to that of the sapling and the tree, and the at first comparatively simple root-system, stem, and leaves, now become complex and large, and each demands careful attention in order that we may trace the steps by which the tree is evolved from the plantlet. A section will therefore be devoted to the root-system of the tree, its disposition, structure, functions, and accessories; another section will be occupied in describing the trunk, branches, buds, and leaves, and their co-relations and functions; the inflorescence and flowers will demand the space of another chapter, and then it will be necessary to treat of various matters of importance in separate chapters as follows: The timber must be considered with respect to its composition, structure, uses, and functions; then the cortex and bark have to be described and their origin and development explained. These subjects naturally lead to that of the growth in thickness of the tree—a matter of some complexity, and not to be understood without the foregoing knowledge of structure. Following what has been said concerning the normal structure and life-processes of the tree, we may turn to the investigation of its cultivation and the diseases which attack it, concluding with a necessarily brief chapter on the systematic position of the British oak and its immediate allies, and some remarks on its geographical distribution at the present time. Of course, many points which will turn up in the course of the exposition will have to be shortly dealt with, as the object of the book is to touch things with a light hand; but it is hoped that, this notwithstanding, the reader may obtain a useful glimpse into the domain of modern botanical science and the problems with which forest botany is concerned, and with which every properly trained forester ought to be thoroughly acquainted.
The oak, as is well known, is a slow-growing, dicotyledonous tree of peculiar spreading habit, and very intolerant of shade (Plate I). It may reach a great age—certainly a thousand years—and still remain sound and capable of putting forth leafy shoots.
The root-system consists normally of a deep principal or tap root and spreading lateral roots, which become very thick and woody and retain a remarkably strong hold on the soil when the latter is a suitable deep, tenacious loam with rocks in it. They are intolerant of anything like stagnant water, however, and will succeed better in sandy loam and more open soils than in richer ones improperly drained.
The shoot-system consists of the stem and all that it supports. The stem or trunk is usually irregular when young, but becomes more symmetrical later, and after fifty years or so it normally consists of a nearly straight and cylindrical shaft with a broad base and spreading branches. The main branches come out at a wide angle, and spread irregularly, with a zigzag course, due to the short annual growths of the terminal shoots and the few axillary buds behind, and also to the fact that many of the axillary lateral buds develop more slowly than their parent shoot, and are cut off in the autumn. Another phenomenon which co-operates in producing the very irregular spreading habit of the branches is the almost total suppression of some of the closely-crowded buds; these may remain dormant for many years, and then, under changed circumstances, put forth accessory shoots. Such shoots are very commonly seen on the stems and main branches of large oaks to which an increased accession of light is given by the thinning out of surrounding trees.
The short ovoid buds develop into shoots so short that they are commonly referred to as tufts of leaves, though longer summer shoots often arise later. The latter are also called Lammas shoots. The crown of foliage is thus very dense, and the bright green of the leaves in early summer is very characteristic, especially in connection with the horizontal, zigzag spreading of the shoots.
While still young the tree is apt to keep its dead leaves on the branches through the winter, or at least
Plate II.
The Oak in Winter.
until a severe frost followed by a thaw brings them down. The buds, leaves, and flowers are all much attacked by gall-forming insects, many different kinds being found on one and the same tree.
It is not until the oak is from sixty to a hundred years old that good seeds are obtained from it. Oaks will bear acorns earlier than this, but they are apt to be barren. A curious fact is the tendency to produce large numbers of acorns in a given favorable autumn, and then to bear none, or very few, for three or four years or even longer. The twisted, "gnarled" character of old oaks is well known, and the remarkably crooked branches are very conspicuous in advanced age and in winter (Plate II). The bark is also very rugged in the case of ancient trees, the natural inequalities due to fissures, etc., being often supplemented by the formation of "burrs."
A not inconsiderable tendency to variation is shown by the oak, and foresters distinguish two sub-species and several varieties of what we regard (adopting the opinion of English systematic botanists) as the single species Quercus robur.
Besides forms with less spreading crowns, the species is frequently broken up into two—Q. pedunculata, with the female flowers in rather more lax spikes, and the acorns on short stalks, the leaves sessile or nearly so, and not hairy when young; and Q. sessiliflora, with more crowded sessile female flowers, and leaves on short petioles and apt to be hairy. Other minute characters have also been described, but it is admitted that the forms vary much, and it is very generally conceded that these two geographical race-forms may be united with even less marked varieties into the one species Quercus robur.
The amount of timber produced by a sound old oak is very large, although the annual increment is so remarkably small. This increment goes on increasing slightly during the first hundred years or so, and then falls off; but considerable modifications in both the habit of the tree and in the amount of timber produced annually, result from different conditions. Trees grown in closely-planted preserves, for instance, shoot up to great heights, and develop tall, straight trunks with few or no branches; and considerable skill in the forester's art is practiced in removing the proper number of trees at the proper time, to let in the light and air necessary to cause the maximum production of straight timber.
Oaks growing in the open air are much shorter, more branched and spreading, and form the peculiar dense, twisted timber once so valuable for ship-building purposes. Such exposed trees, other things being equal, develop fruit and fertile seeds thirty or forty years sooner than those growing in closed plantations. The timber itself is remarkable for combining so many valuable properties. It is not that oak timber is the heaviest, the toughest, the most beautiful, etc., of known woods, but it is because it combines a good proportion of weight, toughness, durability, and other qualities that it is so valuable for so many purposes. The richness of the cortex in tannin warranted the growing of young oaks at one time for the bark alone, and the value of the acorns for feeding swine has been immense in some districts.
