Histological Responses of the Kidney Bean to Aqueous Sprays of 2,4- Dichlorophenoxyacetic Acid

Aqueous-spray applications of a 0.1% solution of 2,4- dichlorophenoxyacetic acid induce a considerable degree of cell proliferation in the stem of the kidney-bean plant. 2. The histological changes induced are as follows: Epidermis.-No response. Cortex.-Little response, although cells adjacent to the endodermis frequently enlarge and divide. Endodermis.-Active; transverse and tangential divisions occur, as well as considerable radial elongation. Pericyclic fibers.-Active if embryonic, but show no response if differentiated. Phloem.-Sieve tubes inactive; phloem parenchyma very active and may enter into root formation or differentiate into isolated tracheal groups. Cambium.-Very active; derivatives remain meristematic, no phloem or xylem being regularly differentiated, although tracheal groups may be laid down; takes part in root formation. Rays.-Very active, being the primary tissue involved in root formation. Xylem.-Formation of xylary elements greatly inhibited and, when occurring, much disorganized; xylem parenchyma slightly active, but to no great extent. Pith.-No response. 3. In general, it may be stated that meristematic tissues and those capable of reverting to a meristematic condition are most readily affected by 2,4-dichlorophenoxyacetic acid. The derivatives of such tissues remain meristematic for considerable periods, and, if differentiation occurs, it is never in an orderly fashion. 4. The effect of 2,4-dichlorophenoxyacetic acid is systemic in nature, even at relatively low concentrations. In this respect it differs from other growth-regulating substances.