Histological Reactions of Bean Plants to Certain of the Substituted Phenoxy Compounds

Four substituted phenoxy compounds have been applied at the bases of the nearly full-sized heart-shaped leaves of the bean at 0.5% concentration, using both Carbowax 1500 and lanolin as carriers. Lanolin was ineffective as a carrier for 2-chlorophenoxyacetic acid and Carbowax was ineffective for 4-chlorophenoxyacetic acid. The gross responses of the plants to the weaker concentrations were almost identical with those reported previously (2). 2. Histological studies were made of material collected and preserved at 12-hour intervals up to a total of 480 hours. Certain marked differences in responses were observed as between the second internode on the one hand and the first internode on the other. There were also differences in the degree of responses of the plants to the four compounds, although 2,4-di- and 2,4,5-trichlorophenoxyacetic acids resulted in strikingly similar responses. 3. The second internode showed little or no histological response to 2-chlorophenoxyacetic acid. When treated with the three other compounds it became much swollen and generally failed to elongate appreciably in a high percentage of the plants. With the exception of the epidermis, possibly the pericycle, and limited portions of the outer cortex, all the other tissues responded actively; but no adventitious roots developed, even after 20 days following treatment. 4. The histological responses in the first internode and hypocotyl were strikingly similar from all four compounds, but-in contrast to the second internode-only the endodermis, cambium, phloem, and ray parenchyma were activated. Roots developed in abundance.