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This device measures bark distance to calculate growth, eliminating transpiration effects.
A screw hook screwed into the wood of the tree is the stationary point from which the distance to the bark is measured with a micrometer, difference between periodic measurements giving the growth between measurements. The effect of variation in diameter due to transpiration pull is eliminated. By appropriate design, the point of measurement is placed outside the zone of stimulated growth resulting from penetration of the cambium by the screw hook.
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L. H. Reineke (1932) studied this question.
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