Reaction wood
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Reaction wood forms when part of a woody plant is subjected to mechanical stress, and helps to bring parts of the plant into an optimal position. This stress may be the result of gravity, wind exposure, snow buildup, soil movement, etc. The reaction wood is not externally visible, although asymmetric growth is a reliable indicator. The cork cambium in the affected part of the trunk is more active on one side, leading to thicker growth rings. Branches practically always have reaction wood, since they need support to maintain their horizontal or nearly horizontal position. There are two different types of reaction wood, which represent two different approaches to the same problem by these groups of plants:
- In angiosperms reaction wood is called tension wood. Tension wood forms on the side of the affected part of the plant, pulling it towards the affecting force. It is composed almost entirely of cellulose.
- In conifers it is called compression wood. Compression wood forms in the bend on the opposite side of the applied force, thereby lengthening/straightening the bend. Compression wood is rich in lignin.
As a rule, reaction wood is undesirable in any commercial application, primarily as its mechanical properties are different: it breaks the uniformity of timber. It also responds differently to changes in moisture.
