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broadleaved trees

Значение термина broadleaved trees в knolik

broadleaved trees - Broadleaved Trees
broadleaved trees - The name of Broadleaves is given to those trees which belong to the class of plants called Dicotyledons. Such plants are technically distinguished by their enclosed seed-ovules, together with two cotyledons. Monocotyledons, which do not form trees in Britain, have a single cotyledon, whilst conifers have their seed-ovules more or less exposed. In most trees, these cotyledons, which are primitive leaves within the seed, appear above ground on germination, and act as leaves. This is " epigeal germination.

In others, such as the oak, the leaves remain below ground, acting as food-stores only, and such germination is termed " hypogeal

The most important feature of the Broadleaves, from the forest point of view, is their capacity for branching in a wide variety of ways from most points of their stems. This flexibility in growth extends right down to the veins of the leaves, making possible the formation of the broad leaves typical of the group. It is aided by, and combined with, a highly developed conductive system, with large vessels of a type not found in the stems of Coniferous trees.

Broadleaved trees, therefore, grow in the manner best suited to their immediate environment. They are not restricted, as are the conifers, to the development of branches along a fixed and fairly rigid pattern. If damaged by wind or cut back, they can develop anew from old stems or shoots. Their adaptability in growth extends to time as well as space; most conifers have a short-growing season each summer, followed by a period of rest, but broadleaves may make growth at any time when they are in leaf and climatic conditions are favourable.

Broadleaved trees are not confined to any particular pattern or mode of growth or flower formation. Many are in an active state of evolution and numerous cases occur where a cut-and-dried description of a species cannot be made because the tree concerned is unstable and prone to variation. For example, the varieties of elms and willows are so prone to further variation that the definition of species is impracticable; the leaves of the White Poplar may be lobed or entire; the flowers of ash may be male, female, or have organs of both sexes.

The term "hardwoods" is sometimes applied to the Broadleaves, but is of no value as the wood of many is soft and perishable. They are also sometimes termed " deciduous ", but this is even more misleading as most of the broadleaves are evergreen trees of the tropical and subtropical zones; leaf fall is only marked in the temperate regions, and even there is not universal, as witness the holly, privet, and strawberry tree.

In general, the broadleaved trees are more highly evolved than the conifers. Thus, they are frequently insect-pollinated - as against the invariable wind-pollination of conifers, and it is more usual for their seeds to be spread by animal/agencies. In other respects they fell behind the conifers, particularly in symbiosis with fungi. This method of nutrition enables the conifers to thrive on poor soils in the adverse climate and short summer of the far North. Similarly, few broadleaves produce resin-like substances as protection against insect attack - therefore they are less inflammable than the conifers.

In general, however, the broadleaves have dominated the vegetable life of the tropics and the better soils of the temperate regions. Only in the north temperate zone, and in the mountains of the tropics and the Southern Hemisphere, have the conifers been able to withstand their rapid and complex advance.

The classification of dicotyledons is highly complex; it is based on flower structure, but other factors have to be brought into the reckoning, and no one factor can outweigh the others. The ash, for example, is classed with plants having united petals, for although it develops no petals, it is otherwise so similar that the suppression of petals is of minor importance. The classification of broad- leaved trees is interwoven with that of the plants in general, and can only be studied by bringing the herbs into account.

The simple identification of British broadleaved trees is not an easy matter, as they represent a wide range of natural orders or families, many of which only occur here in the form of trees, and consequently are unfamiliar to the general botanist. To assist the student, a series of artificial keys has been added to this book.

Two main groups of British trees, based not on natural relationships, but on a factor in life history, may readily be distinguished.

group 1. - wind-pollinated trees

This group contains the Alder, Beech, Birch, Hazel, Hornbeam, Oak, Poplar, Chestnut, Walnut and Plane with catkin-like flowers, and the Ash and Elm with flowers in clusters; the Willows are on the fringe of the group.

Wind pollination is usually regarded as less advanced than pollination by insects, but in the case of deciduous trees it may be more efficient. By flowering before they come into leaf, these wind trees have the spread of their pollen unrestricted by foliage, and they have a long season ahead in which to mature their fruits. They must produce more pollen, perhaps, but they do not have to elaborate petals or bear nectar. Their flowers are normally of one sex only, which lessens the risk of self-fertilisation.

Broadly speaking, the wind trees form the true broadleaved forest of the North Temperate Zone. They are widely spread there, becoming scarcer towards the tropics. Nearly all our principal timber trees, especially those of native origin, belong to this group. Certain of them, such as the poplar, do not appear to have developed conspicuous petal flowers at any stage in their past history.

Others, such as ash and plane, have evolved to their present state by reduction of non-essential flower parts. Wind pollination is rare in our herbaceous flora, and the peculiar catkins and inflorescences of the commoner trees contrast strongly with the so-called " perfect " or " complete " flowers of garden and meadow. But they are well adapted to their function and the intricacy of their structure indicates long evolution along their own specialised line.

group 2. - insect-pollinated trees

These include the Maples, Horse-Chestnut, trees of the Rose Tribe, Locust, Lime, and many lesser trees such as Holly, Buckthorns, Spindle, Viburnums, Privet, Elder, Box, Laburnum, and Strawberry tree. The Willows, which produce nectar but lack petals, are sometimes insect-pollinated.

These insect trees have more conspicuous flowers, with petals, sepals to protect them, nectar, and usually a sweet scent. Organs of both sexes usually occur in the same flower.

As pollination cannot be effected until the summer is warm enough for insects to be active, the fruits of such trees must be matured quickly before winter sets in, usually by drawing on reserves stored up in the previous summer.

Insect pollination is subject to much the same degree of risk as is wind pollination. The insects - chiefly bees - may not be plentiful enough, and their range of flight is limited, whereas the winds blow far and wide in the northern springtime.

In general, therefore, these insect trees are either of foreign introduction from more southern latitudes, where the evergreen foliage and greater insect activity make their method of pollination more essential and successful, or are natives on the extreme northern edge of their range.

Many are the sole representatives here of natural orders important in the tropics, such as the lime, of the natural order Tiliaceae, to which Baobabs, Durians, Cocoa trees, Cola-nuts, certain Mangroves and other tropical timbers are allied.

In the main then, the British broadleaved timber trees are distinguished from both the British herbaceous plants and the timber trees of warmer latitudes, by their wind-pollinated flowers, which usually assume the form of catkins, and are commonly borne before the leaves appear. Although their growing season is restricted, and they can only thrive on the better soils, where conditions suit them, their adaptability and pliability make them vigorous competitors both with coniferous trees and herbaceous plants. Their growth in such cases is rapid, and the volume of timber produced on a short rotation may be large. Moreover, if cut back they will coppice, producing smaller timber, often of high utility.

Their origins and ways of growth, and consequently the resulting timbers, vary widely. This fact was for long obscured by the classification of the bulk of them in an unnatural order called the Amentaceae or catkin-bearers, which was convenient but confusing.

The forester readily recognises that the principal trees of the woodland, oak, beech, alder, birch, and the others, have each their own peculiar character, and that each must be made the subject of special and individual study.

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