Experiments on the origin of oak-bark tannin

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Abstract
In Quercus pendunclata, the tannin appears to be a waste product of metabolism, for it is localized in stembark tissue which does not participate in metabolic activities, and which is made up principally of dead elements. In young plants, tannin accumulates during the period of active photosynthesis and growth and the enhanced level is maintained until the following spring. In older trees, this progressive increase in tannin content with increase in age is balanced by increase in the dead and living bark elements, with the result that the level remains without apparent variation throughout the season. This suggests that the tannin undergoes further chemical changes in the dead elements of the outer bark. The effect of ringing experiments on the normal distribution of tannin in the stembark of mature trees shows an accumulation of tannin above the girdle, suggesting that synthesis has taken place from phenolic precursors which are normally translocated downwards from the leaves. Examination of the exudate obtained by tapping the sieve-tube system of growing trees shows the presence there of many of the phenolics found in the bark, including the pyrogallol precursors [(+) -gallocatechin and leuco-delphinidin] of oak-bark tannin. The evidence suggests that these pyrogallol phenols, which originate in the leaves, are translocated by the sieve-tube system to the cambium and undergo oxidation there, and the resulting phlobatannin is stored in the bark.
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