Читать книгу Collins Complete Guide to British Trees: A Photographic Guide to every common species - Paul Sterry - Страница 14

Leaves are among the most conspicuous and distinctive features of any tree. They grow in a huge variety of shapes, sizes, colours and combinations and are usually the best feature for identifying the tree because of their unique structure. Leaves may vary from one species to another but they all perform the same vital function as the principal producers of food for the tree.

The first pair of leaves to emerge from the seed are simple, and bear no resemblance to the true leaves of the tree; they are derived from the seed’s food store. They are green, however, and supply the tiny seedling with its first food made from sunlight energy. Once the seedling has begun to produce leaves that are miniature versions of its true leaves, growth can begin very rapidly. Tiny seedlings are vulnerable to grazing, trampling, drought and competition, so very few survive.

Recently germinated oak seedling.

A plant’s leaves are its powerhouse, trapping energy from sunlight and converting it into basic food.

Leaves are basically thin layers of living tissue with the ability to trap light energy and use this to convert the raw materials of water and carbon dioxide into a simple sugar. This reaction, known as photosynthesis, is arguably the most important chemical reaction in the world, for it is the basis of all other food production. Animals do not have the ability to convert these simple materials into food; they have to rely on plants to do it for them. The simple sugar produced in the leaves is glucose, and this can be formed into a variety of other important materials, particularly starch, which many plants store, or pack into their seeds. A vital by-product of this reaction is oxygen, the gas essential for the respiration of all members of the animal kingdom. This explains the vital role of trees in the ecosystem: they are major consumers of carbon dioxide, one of the so-called ‘greenhouse gases’; and they are major producers of oxygen, the gas we need for our respiration. They are also major producers of food for much of the animal kingdom.

Contained within a leaf are numerous specialised cells. Some are concerned with the transport of materials in and out of the leaf, some are the vital energy-trapping cells that utilise sunlight, and others are concerned with the regulation of water movements. The cells that trap light-energy contain a light-absorbing pigment called chlorophyll, which gives leaves their green colour. Other pigments of different colours may be present in varying amounts, and it is this variety that gives leaves of different trees their own subtle shade of green. Without the green chlorophyll or other light-absorbing pigments, leaves would be unable to perform their important function, and also, if deprived of light, they would be unable to manufacture the tree’s food.

Leaves arrange themselves in such a way to absorb the maximum amount of sunlight, so spreading canopies or trees growing taller than their neighbours, are both ways in which trees maximise the light-gathering power of their leaves. Some leaves have paler patches that lack green chlorophyll; these are known as variegated leaves and certain trees, such as some cultivars of the Highclere hollies, regularly produce green-and-yellow leaves. If the leaves were completely lacking in chlorophyll they would be unable to manufacture food for the tree; the small areas of green tissue in the leaf produce all the food needed by the whole leaf.

All leaves have tiny pores in their surface (normally just the lower surface) called stomata. These allow water to evaporate into the atmosphere. To some extent the tree can regulate the opening and closing of these stomata, but during daylight hours, when the tree is trapping sunlight, they will be open, allowing water out and also allowing the circulation of the gases involved in photosynthesis. This can lead to problems for trees growing in hot, dry areas, or in well-drained soils where little ground water is available. In order to allow the essential gases to circulate, and at the same time minimising water loss, many leaves have become reduced in size, such as the needles of firs and pines, or have thick waxy upper surfaces such as the glossy green leaves of hollies and magnolias. This reduces water loss to a minimum without impeding photosynthesis.

The great variety of leaf shapes and sizes is an indication of the variety of ways in which trees can cope with environmental conditions. Some trees grow in areas where water is at a premium, so they have small leaves, to cut down on water loss through their thin skins. Some grow in shady conditions, so they may have larger leaves that can trap the maximum amount of light energy. Some trees are subject to grazing by animals, so their leaves are spiny or prickly, or protected on tough, thorny stems.

Beech

OVAL AND ENTIRE

Hornbeam

ELLIPTICAL AND TOOTHED

Small-leaved Lime

CORDATE

Red Oak

LOBED

Horse-chestnut

PALMATE

White Ash

COMPOUND

Yew

NEEDLE-LIKE

Scots Pine

WITH PAIRED NEEDLES

Leaf types.

Deciduous trees, such as Horse-chestnut, produce fresh leaves each spring, which burst forth from buds.

Autumn leaf colour is spectacular in many maple species: as chlorophyll and other pigments are withdrawn, remaining red pigments prevail.

Evergreen trees do not lose all their leaves at the end of every growing season; most leaves remain on the tree through the winter, although there is always some loss and some replacement. In many of the pines, for example, the needles will remain on the tree for about 3 years. As the shoot grows longer each year, a new set of needles grows on the tip of the lengthening shoot. The older needles, finding themselves further and further away from the tip, gradually fall off. Small leaf scars remain, and these are quite distinctive in some species and may be a useful aid to identification. Broadleaved trees such as Holly also replace their leaves gradually so there is always some leaf-fall, but plenty of green foliage remains on the tree.

Deciduous trees generally shed all their leaves at the end of the growing season, before the onset of winter. Many of them produce spectacular displays of colour before the leaves finally fall. These colour changes are the result of the gradual withdrawal back into the tree of all the useful materials in the leaf; as the various pigments are removed the leaf itself changes colour until finally a corky layer, called the abscission layer, grows at the base of the petiole or leaf stalk. This seals off the shoot and when the leaf finally falls, a scar is left through which mould spores and other harmful materials are unable to pass. The twigs of Horse-chestnut have very distinctive leaf scars that look like tiny horseshoes. If these are examined carefully through a hand-lens, the sealed-off ends of the vessels that conducted materials in and out of the leaves can clearly be seen.

There may be as many as 250,000 leaves on a mature oak tree, whilst a large spruce probably has 10 times as many, in the form of needles. The oak’s leaves will be shed at the end of the growing season, adding to the rich accumulation on the ground beneath it, whilst the spruce’s needles will be shed and replaced gradually, each individual needle remaining on the tree for about 4 years.