Structure of the Trunk

Structure of the Trunk
by Jamie Rainbows

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 primary vein divides the blade into two, generally equal, halves. In some woody plants, however, the halves are not identical, especially at the base, and these are termed asymmetric (elm, hackberry). In other species the leaf may have several veins branching out from the base (maples); such leaves are usually palmately lobed.

Most European broad-leaved trees are deciduous, in other words, they shed their leaves in the autumn. Only in southern, and in temperate parts of western Europe do some trees retain their leaves throughout the winter, e.g. the common holly, the laurel and the box. In the autumn the organic substances produced by the leaves are concentrated in the body of the tree and the leaves begin to change colour as a result of the decomposition of the chlorophyll and growing predominance of the red and yellow carotenoid pigments, along with the increase of anthocyanin in the cellular sap.

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.

The process is slow and gradual, and the tree sappears unchanged. Only the amount of needles on the ground beneath i serves as an indication of its shedding rate.

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