Posts Tagged ‘phloem’

We may imagine the whole tree grows upwards but this is an illusion. If we tied a yellow ribbon round an old oak tree and came back a year later it would, luckily for romantics, be in the same place and not higher off the ground. Only the apical or terminal buds of trees (see pic) will grow upwards and maintain dominance through the production of a hormone called auxin which keeps lateral buds dormant.

Apical or terminal bud

The Apical (terminal) and Lateral Buds (http://landsat.gsfc.nasa.gov/)

This is how trees maintain their traditional shape, although this has been influenced by humans through the practice of coppicing, pollarding and pruning where dominant stems are cut to enhance lateral growth and the production of new shoots.

The yellow ribbon, however, would have been stretched tighter. This is due to the thin part of the trunk that is actively growing called the cambium. The cambium produces cells which divide and specialise into the xylem and the phloem. The xylem tissue transports water and minerals from the tree’s roots upwards. The xylem forms the sapwood and as these cells age and die they turn into harder heartwood, which is found towards the centre of the tree. The phloem is the thin green layer under the bark which transports the tree’s lifeblood, the sap. The bark is made up of several layers, primarily the cork cambium which is an extremely thin layer of cells that divide to form the cork, or more commonly called bark (see pic). In this way, as trees age their diameter increases.

Tree cross-section

The Cross-Section of a Tree (Mirriam Webster)

It is thus not necessary to cut down a tree to kill it; all that is needed is to remove a ring of bark and the thin underlying tissue to prevent the transport of the sap to the roots via the phloem. This is called girdling, and sometimes practiced to remove a particular tree from an ecologically sensitive area without causing greater damage. This also shows for all their mightiness, trees are certainly vulnerable.

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