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

Abstract

The most noticeable effect produced by 2,4-dichlorophenoxyacetic acid, aside from its herbicidal influence, is the stimulation of meristematic and parenchy-matous tissues to a very rapid rate of cell division, beginning about 36 hrs. after application. Division is preceded by nuclear enlargement and an intense nuclear staining. The greatest degree of cell proliferation occurs in the undifferen-tiated portion of the growing tip, and in cambium, phloem, and ray cells of the differentiated stem. Some stimulation to division is evident in the endodermal, cortical, and xylem parenchymatous cells, but none in the pith. The pith, however, tends to remain solid instead of becoming hollow. Sieve tubes, tracheids, and phloem fibers, when once differentiated, are unaffected. In transverse section, the proliferated area is a band of cells 30-40 wide in the phloem-cambial zone, with many root initials arising from the region of the rays. In longitudinal section, these cells are very short because of many transverse divisions. Externally, the proliferation of tissues may be seen in the form of galls, stem thickenings, split cortex, and emerging aerial roots. Cell proliferation occurs throughout the young kidney bean plant even though the application of the herbicide may have been made locally.