The Trunk

Some trees have a short thick trunk which divides into several boughs at a point six to fifteen feet above ground level. Others, particularly conifers, retain the main axis for many years as they grow upwards so that the branches arise laterally.

The internal structure of the trunk can be understood best by referring to the lower diagram on p. 7, Bark. This develops at an early stage, replacing the epidermis of the young stem. It is a continuous protective layer which is constantly replenished as the stem increases in girth. It can be thin and smooth (Beech), deeply fissured (Oak) or very thick (Redwood). The bark may be conlinually worn away or shed in large flakes (Plane),

Phloem, This is a thin layer beneath the bark containing vertical rows of special cells, sieve tubes, which conduct soluble food materials from the leaves where they are synthesized to various parts of the tree. Vascular cambium. During the active growing season the cells of this thin layer divide to produce cells both outwards and inwards. These derivatives differentiate into phloem towards the outside and xylem towards pnloem root hairf pillferouK layer pnloem root hairf pillferouK layer

undifferentiated iylem

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