Summary Zea Mays plants were raised under normal and boron-deficient conditions in a growth chamber under controlled conditions, to see how pHPPA3) is metabolized in the stems of 4-, 5-, and 6-week-old plants. The plants were allowed to absorb the labelled pHPPA and to metabolize it for 24 hours. The ethanol soluble and insoluble p-coumaric and ferulic acid derivatives were isolated and analyzed quantitatively and for radioactivity. This was also done for lignin and its oxidation product, vanillin. The incorp rates3) for the insoluble acids and the vanillin were essentially lower for the B(−) plants. The incorp rates for the soluble acids and the lignin in B(−) plants were either the same or somewhat lower than for the normal plants [B(+)]. Kinetic studies were made for the transformation and incorporation of pHPPA, tyrosine, ferulic acid, and glucose. The aromatic compounds were labelled by C14 in the C-3 position of the side chain, while glucose was uniformly labelled. The plant stems were analyzed after 10, 20, 30 or 40 hours for the incorp rates of the labelled precursors into the fractions mentioned above. The metabolism of the applied aromatic precursors followed essentially parallel curves for the B(−) and normal plants, with the incorp rates for the B(−) plants being somewhat lower. The metabolism of glucose did not follow parallel kinetic curves. The sequence of the incorp rates of glucose into p-coumaric and ferulic acids (soluble and insoluble) was reversed for the B(−) plants in comparison to the normal plants. The incorp rates into the lignin aldehydes of B(−) plants were essentially lower, and the kinetic curves did not follow the same order as for the normal plants. This indicates that the transformation of phenols into lignin is not essentially influenced by boron deficiency, but this element seems to be involved more probably in the synthesis of aromatic structures from glucose.