Role of Growth Regulators in Initiation of Secondary Xylem and Phloem Cells

Naphthaleneacetic acid in lanolin, placed on the fourth internodal stumps of decapitated bean plants, replaced the stem apex in controlling the formation of secondary xylem cells but did not affect secondary phloem cell initiation. Reputedly specific antiauxins negated the response to auxin. Gibberellic acid or BAP each could partly replace the stem apex for xylem cell formation, and GA partly replaced the stem apex for phloem cell initiation. Mixtures of auxins and GA and of auxin and BAP were less effective than auxin alone in xylogenesis, but a mixture of auxin and GA synergistically stimulated phloem cell initiation.