coniferous trees defined in 1944 yearconiferous trees - Coniferous Trees;
coniferous trees - The coniferous or needle-leaved trees are technically known as Gymnosperms, meaning naked-seeded plants, but this distinction applies to the unfertilised seed-ovule rather than to the ripe seed. This structure is borne superficially on an open scale pr carpel, whilst in the higher plants it is enclosed within a seed- box or ovary formed of one or many closed carpels. The seed-leaves or cotyledons of Gymnosperms are usually numerous, though in some genera two or three cotyledons are the rule. Very few herbaceous plants occur within the group at the present time; they appear to have been superseded by herbaceous forms of the higher plants.
The most remarkable features of the Gymnosperms, however, are their restricted form of sap-conducting tissue, and their failure to develop broad leaf surfaces with branching veins. Their wood contains no large vessels to carry sap up their stems, and this has no doubt restricted the amount of water which their foliage can transpire, and compelled them to develop needle-like leaves. Amongst the higher plants, needle-shaped leaves are usually met with only amongst those which must restrict transpiration - such as the heath-plants growing in dry and acid soils from which moisture, even when present, is difficult to extract.
The needle-leaf, therefore, is the distinguishing feature of conifers, and the only broad-leaved tree of Britain approaching this type of leaf-form, is the Tamarisk, a rare plant of the salt sea-shore. One imported conifer, the Gingko, has small broad leaves, but the shape and venation of these mark it out from all our true broad-leaved trees. Though certain of our conifers bear succulent fruits, the woody cones of the majority are a distinctive feature, and account
for the name of cone-bearers which has been applied to the whole group. One of our common broad-leaved trees, the alder, bears false cones, but its branched habit is very different to that of any conifer, and when leafless it is readily identified by the young male catkins of the following season.
Most conifers are evergreen, and deciduous ones have either " short shoots " on their longer stems, or else a rigid habit of growth which indicates their true affinity even when leafless. Most of them appear never to have mastered the problems of growing a short-lived leaf, which justifies its production by intense activity in one summer; this factor is probably linked with their restricted sap-conducting tissue.
In their manner of growth and branching, the conifers conform to rigid, exact, and fore-ordained patterns, of which the Norway Spruce, or Christmas Tree, may be taken as an example. In its first year of seedling life, the whorl of epigeal cotyledons is succeeded by a single vertical axis, which ends in a cluster of buds. Next year, the central bud of this cluster elongates as the leading- shoot of the tree, and ends in another group of buds. The side- buds at its base likewise elongate horizontally; they are usually four in number, so the seedling grows that number of side-branches, neither more nor less, and these side branches do not themselves break up until they have accomplished a full year's growth. Then their behaviour resembles that of their upright leader, except that normally only two side-buds, to right and left of the main axis of each horizontal branch, are developed. Thus the tree endeavours to spread its foliage according to a regular geometrical pattern, regardless of the surrounding light and shade, and the? side-branches continue to branch and inter-branch until they are over-shaded by larger branches of the same tree borne higher up, when they cease to develop and gradually die away as the life-span of their needles ends. If any damage befalls the leading shoot, the tree receives a severe check, but with most species a side-branch or side-bud (or epicormic bud), usually changes its direction of growth in order to replace it, provided the damage occurs to a young and growing leader not too far from the tip.
If a conifer is damaged by the removal of a large portion of the main stem, it seldom, if ever, recovers. A side branch below the injury may prolong life for a time, but upward growth is not resumed. Pollarded conifers, therefore, are seldom seen. The power of coppicing from the felled trunk, or of growth from sucker shoots, is confined to a very few species, of which the Californian Redwood (Sequoia) is an example. Reproduction from cuttings is limited to a very few genera; cuttings from side branches are usually only capable of horizontal growth, producing the prostrate forms cultivated in rockeries. Grafting is practicable with most conifers, but is difficult to carry out owing to the resinous nature of their stems.
Although the leaves of conifers are limited in shape to the needle or scale, great diversity is found in the orientation of these needles upon the stem. In most cases, there is a distinct " juvenile " type of foliage found in seedlings and sometimes in leading shoots. Occasionally this type persists in the adult tree, producing distinctive forms which are propagated for garden use by nurserymen. Primary leaves are spirally placed, but various devices, such as a twisted or elongated leaf-stalks, frequently cause them to lie in two ranks in one horizontal plane, as is the case with most Spruces and Silver Firs. This arrangement only occurs in horizontal branches, being designed to give the best possible exposure to the light; vertical or drooping branches have their needles spirally ranged. Other genera, such as larches and pines, bear the greater part of their needles on special " short shoots" in clusters or tassels; here again the needles of seedling or leading shoots are simply and spirally placed. In the cypresses, thuya, some junipers and libocedrus, the scaly, spiral, leaves of the juvenile forms, become grouped in sets of fours which inter-lock to give either a square or a flattened form of leaf face, completely enclosing the parent stem. Here, as with other conifers, the object is apparently to attain the greatest possible exposure to sunlight, together with the lowest possible transpiration of water.
On account of their evergreen and persistent leaves, conifers are particularly sensitive to impure atmosphere, and do not thrive in smoky towns. Some species, however, have a marked ability to withstand salt-laden winds. Probably the salt, being a solid, cannot pass the stomata, or breathing mouths, of their leaves, but the gases, particularly sulphur dioxide, found wherever coal is burned, pass through into the leaves and poison them. The soil nutrition of many conifers is assisted by fungi working in mycorhiza on their roots.
In a state of nature the conifers are confined to cool temperate and mountainous sub-tropical regions, with a few obscure outliers which play a subordinate role in true tropical vegetation. This state of affairs probably results from the intense competition of the better-developed broad-leaved trees. Certain species, even certain genera, of conifers are restricted in range to narrow limits, and when discovered by modern botanists were on the verge of extinction. A few forms are known only in a state of Cultivation.
The culture of conifers as ornamental trees became a great vogue amongst British gardeners during the nineteenth century, a time when many parks and large gardens were being laid out. Consequently, a large proportion of the species suited to our climate have been grown here, and the study of the world's conifers is probably more advanced in Britain than elsewhere. Although these specimen trees were planted mainly with a view to decorative effect, they have afforded valuable evidence as to the hardihood or otherwise of the trees in question, and of their suitability for forest planting in Britain.
The forester's interest in the conifers arises from the rapidity of their growth on poor soils and exposed sites, and from their regular shape. The latter factor results from their absence of pliability in mode of growth; they are obliged to develop on set patterns, and cannot branch freely in varied directions as the broad-leaved trees do. The resulting straight stem, with small side branches, is a most desirable timber form. The success of conifers on the poorer soils is less easy to account for, but probably their method of symbiotic nutrition by fungal mycorhiza is largely responsible. Not all the genera are equally at home on poor soil, but the pines will usually colonise the barest of sands and gravels, provided no adverse factors restrict the development of their mycorhizas. Most conifers enrich the soil by the humus of their fallen needles and branches, preparing the way for the ingress of broad-leaved trees which tend to supplant them.
The dependence of conifers upon mycorhizal fungi must not be lost sight of in nursery practice. Seedlings can seldom be raised in new nurseries until the soil has become innoculated with the requisite fungi; this is usually effected by using the ground for transplanted trees first. Even so, batches of imported seed may fail to give satisfactory seedlings for a period, because their requisite fungus is lacking.
Most conifers develop resin, probably as a defence against insect or fungal attack, or possibly as a mere by-product for which no other outlet can be found. In consequence their stems and dead foliage are highly inflammable, and a great fire-risk arises wherever large plantations are made. This is combated by breaking up the area with fire-lines, and by cutting away the side branches of young trees (brashing) which limits the risk of a fire climbing from the ground to the crowns of the trees. Brashing also eliminates dead knots from the timber, resulting in cleaner, more valuable wood.
The classification of the conifers presents many problems; each separate genus follows its own line of development, and connecting links are rare, probably because they have become extinct. In the following pages, each genus is treated separately, as little would be gained by grouping genera into families or sub-families of varying importance and distinctness. The nomenclature of conifers is complex. Nearly all have been called, by one writer or another, " pine " or " fir " or both. Most have been assigned to at least two different genera by different authorities. The everyday names are sometimes distinctive, sometimes confusing. But the names selected herein are in accordance with the latest botanical knowledge, though no forester can rest assured that the botanists of a later period will not indulge in further alterations. For this reason the writer has entered a plea for the retention of the old English name of fir for a tree which is familiar to everyone, and has always been known by that name. A vernacular name is something permanent, enshrined in literature and kept alive by the speech of a people. It should not be lightly altered to accord with the findings of a botanist, which may be upset overnight by his own or another man's further researches.
The timber of coniferous trees is known by the general name of Softwood, being softer and also lighter than that of most, but by no means all, broad-leaved trees or " hardwoods ". For most commercial purposes this softness is a decided advantage, implying easier working with saws, planes, and other cutting tools. It also means that nails are easier to drive, and once driven, are held better. Softwoods are therefore preferred for most constructional work and for box-making, and the annual world consumption is enormous. They are also important sources of fibre for synthetic boards, cardboards, paper, and plastic materials, and of veneers for plywoods. The pole thinnings from plantations, and the top-lengths of mature trees, are in demand for transmission poles, pit-props, and fencing material; very little is wasted, as even the branches are used for firewood or kindling.
The chemical applications of softwoods are numerous. They are an important source of cellulose and of natural resins. By dry distillation in retorts, they may be made to yield a valuable series of by-products, including tars and complex organic acids, besides the charcoal left behind in the retorts. Owing to the limited supply of raw material, these industries are but little developed in Britain, but are important both in North America and Northern Europe, where the natural coniferous forests cover vast areas.
Economically, then, the coniferous trees constitute a most important group of plants, and their study is of great interest on that ground alone. They also exhibit a wonderful variety of structure and contribute markedly to the beauty and character of" the landscape, particularly in mountainous regions. A little observation soon reveals a definite character in each genus, aiding in its cultivation as well as its identification. In the following descriptions, attention will be focused upon those innate characteristics which govern the mode of life of each group of conifers on its own peculiar plan.
The Scarcity of native conifers in Britain, where only three species are found wild to-day, is due to the course of geological history and climatic changes which have exterminated many genera now found here only in fossil form. By contrast, Japan, with a similar area and situation, claims over thirty species. The re-introduction of the conifers' to Britain by human agency within the past few hundred years, provides an interesting study, as important economically to forestry as the introduction of potatoes and other root-crops has been to British agriculture. A wide range of genera can be profitably grown here, but in every case it is important to secure a species and strain adapted to its new climate.
pictures for coniferous trees
near coniferous trees in Knolik
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