Water and Wood part 3

To examine the rings of a tree without felling it, cores can be taken by boring into the wood with a hollow cylinder. By counting them inwards each ring can be accurately dated, the age of the tree can be determined and past climatic conditions can be inferred. For accuracy a number of trees is sampled and the ring widths compared. If they coincide, as they normally do, then narrow rings will indicate a poor growing season, probably one with a spring and early summer drought, while wide rings will indicate good growing conditions of plentiful moisture and warmth. The width of ring also depends on the quantity of leaves the tree carries in one year, and rings on an individual tree which do not follow the same pattern as others in the same area show that it may well have suffered a setback such as pollarding, or gained an advantage caused by the felling of a tree which formerly over-shadowed it.

Arguably, the most fascinating aspect of tree rings is their use in dating the past. This science, known as dendrochronology, enables timbers from ancient buildings to be dated by comparing their rowth rings with those of trees or timbers of known date. This science came to the fore recently with the discovery in North America of trees called Pinus longaeva, closely related to the Bristle- cone Pine (P arislala), which are almost 5,000 years old. Study of their rings has allowed the accurate dating of early American cultures.

Although all woody plants form their strengthened tissue from division of the cambium cells, the structure varies in different groups. In conifers the xylem is made up almost entirely of tracheids, vessels being absent. These tracheids, which are single cells, can be up to 25 mm long and have markedly thickened walls except for a large number of small circular areas which lack any thickening. These are known as bordered pits. They are also present in the tracheids and vessels of broad-leaved trees. The size and shape of cells also varies considerably between different genera and species, both in length and diameter as well as in the amount of thickening present, and in the proportion of the tracheids, vessels etc. The leaf traces also have their characteristics, and an expert can identify most woods from a section under a microscope. Some woods have patterns easily recognizable by the naked eye. The vessels are the most conspicuous feature and a regular scattering over the wood is typical of maples (Ater), beeches (Fagus), walnuts (Juglans), planes (Plalanus) and poplars (Populus) while in ashes (Fraxinus), locust trees (Robinia), elms (Ulmus) and Indian bean trees (Calalpa) those formed in spring are noticeably larger. The size and spacing of the rays is another feature which assists identification.

Knowledge of the internal structure of wood can be very important to those who work with it. For carpenters it is necessary to know which woods have a tendency to split or warp, and which is the best way to cut a piece for strength. Some woods such as walnut have highly decorative patterns and have long been used as veneers, while others are stronger but less attractive and are used for less visible parts of furniture. Beech has always been popular for everyday furniture though today its place is largely taken by tropical woods. Pine and spruce woods, being softer, are used for packing cases, and above all, in the manufacture of paper. Identification of wood types also has its value in archeology and even in criminology.

The annual increase in the girth of a tree through the activity of the cambium is known as secondary thickening. As monocotyledons have no cambium layer within their vascular bundles they are unable to increase their girth in the same way and rarely make trees. There are, however, a few exceptions, notably all the true palms, the New Zealand cabbage palms (Cordyline) and the dragon trees (Dracaena). Yuccas too have strongly thickened stems. In these examples there is a layer of cambium below the epidermis and this produces new cells, some of which develop into extra vascular bundles. In this form of thickening there are no annual rings but the trunks can still reach great girth.

As the trunk and branches of a tree increase in diameter so the epidermis must expand too, otherwise it would become overstretched and split away leaving the inner cells unprotected. The tree avoids this by the development of an extra layer of meristematic cells called phellogen, either just below the epidermis or sometimes a little deeper in the outer layer of parenchyma which is known as the cortex. The phellogen produces cork, a type of tissue not previously present in the tree. The new cells are packed closely together with no intercellular spaces, and their walls contain a fatty substance which makes them completely impervious to water and air. In fact, they take over the function of the epidermis.

In a few species the one layer of phellogen or cork cambium functions throughout the whole life of the tree. This is only possible when the phellogen layer is just within the epidermis. As the stem thickens so more of the divisive tissue is formed and the corky layer can also grow at the same speed. The smooth bark of the White or Silver Birch (Belula pendula) develops in this way, the outer layers peeling off in papery sheets. Other ornamental birches and the Paper Bark Maple (Acer griseum) show the same feature. Commercial cork, the outer layer of the Cork Oak (Quercus suber), is also produced from a continuous cork cambium. In this case, however, the growth of corky layer is very rapid and it can be stripped off every nine or ten years with no damage to the tree.

In most trees, the cork cambium layer functions for a few years only, for a new layer develops deeper in the trunk which begins to produce further corky layers. This new layer of cork effectively cuts off all the outer tissue from a supply of water and nutrients, then it dies. These dead tissues form the relatively thick layer of bark which is so distinctive a feature of many trees. Its irregularity stems from the fact that the phellogen layer is not always continuous, or even if it is, not all the cells develop the strong corky walls. Trees with markedly irregular development of this sort have bark which separates into scales: Spruces have small scales, those of most pines are larger, notably the Maritime Pine (Pinus pinasler), while the planes have very large ones which give the familiar patchy appearance to their trunks.

Possibly related posts: (automatically generated)
Water and Wood part 3

• How to Choose your Conservatory Furnitures
• Sitemap
• Scots Pine
• Five types of mower
• Seeding and Garden Shed
• Caring for Orchids in the Home continue...
• Flank Simple Highlights
• Seating Sense
• Adding a sense ofAdding a sense of period period
• White Willow