Let us now take a look at the structure of the tree trunk. In the centre, there is a narrow column of pith and around it a continuous shaft of wood, called the xylem. This is made up of concentric circles known as the annual rings. Then comes the thin layer of phloem and, on the outside, the bark, which in older trees may be split and furrowed. Between the xylem and the phloem, there is a thin layer, called the cambium, consisting of several layers of thin-walled cells which, during the growth period, rapidly divide to form new wood on the inside and new phloem on the outside. The annual accretion of wood in our trees is clearly evidenced by the annual rings formed by the varying rate of growth of the cambium, which differs according to the season of the year. In spring, during the period of intensive growth when the trees come into leaf, the cambium produces broad, thin-walled cells called spring wood, whereas the cells produced in the summer are narrow and thick-walled and are called summer wood; during the winter there is no growth whatsoever. The strip of compact summer wood is easily distinguished from the strip of the following year’s spring wood so that on a stump we can clearly see the yearly growth, in the form of annual rings, and thus easily determine the age of the felled tree. The width of the annual ring is proportional to the quantity of manufactured food substances, and corresponds roughly to the amount of rainfall and warmth in a given year, I.e. in a favourable year it will be broad, whereas in an unfavourable year it will be narrow. A narrow ring is, therefore, an indication of the unfavourable influence of dry weather and in trees sensitive to the cold of severe frosts, etc. This correlation today forms the basis of a new study known as dendrochronology. By examining the rings in trees hundreds and thousands of years old scientists can determine long-lasting changes in the weather and pinpoint alternating periods of dry and wet years in times about which we have no meteorological data. In trees growing in tropical regions where growth is continuous throughout the year the annual rings are not usually so clearly discernible.
In older trees the outer portion only of the wood is active. The central, internal section of the trunk consists of dead cells and is known as hcartwood. The outer live part is called sapwood and this serves as the tree’s pipeline for conducting water and other important substances up to the crown. In broad-leaved trees this function is performed by the broad tubular cells, called tracheae, visible in the cross-section as small pores. In conifers these tubular cells are narrower and shorter and are called tracheids.
In the cells and cell walls of heartwood various organic and inorganic substances are stored, e.g. tannins, resins, silicon dioxide, etc. In some trees such as the yew, larch, pine and oak the heartwood is further distinguished from the sapwood by a darker coloration. Heartwood is generally much more durable and of higher quality than sapwood and. In the case of some tropical trees in which the wood is subject to rapid decay and damage by pests, the soft sapwood is hacked off on the spot and only the heartwood is shipped for further processing.
The xylem is surrounded by a thin layer of phloem which conducts the organic substances manufactured by the leaves down to the trunk and roots. It consists of long tubular cells with perforated partitions placed end to end. The surface of the tree trunk is covered with bark which protects it from excessive evaporation, sudden changes in temperature and mechanical damage. Protection against undue evaporation is very important, for great quantities of water are conducted through the outer woody layers.
On the bark we can often see small round or slit-like patches that are slightly raised, and different in colour, from the surrounding bark. These are called lcnticels and serve as a path for the exchange of gases between the atmosphere and the living cells inside the trunk and branches.
The bark thickens every year by the addition of a very thin layer of corky tissue. The thickness differs in various trees. Trees growing in the shade usually have thin bark, whereas those exposed: to the sun often have thick bark which serves as protection against heat. Old, surface bark layers cannot adapt to the continuous thickening of the trunk and split — usually in furrows or scales. In some trees the bark remains permanently on the trunk (oak, elm), in others it peels off in scales (plane, sycamore), in lengthwise strips (cypress, eastern arbor-vitae) or crosswise strips (birch, cherry).