Histological Responses of Vicia faba to Indoleacetic Acid

Plants of Vicia faba were decapitated just below the fifth node and the cut end treated with 1.5 per cent indoleacetic acid in lanolin or with lanolin alone. 2. The plants treated with indoleacetic acid responded by the formation of a tumor. The area, at most not more than 5 mm. long, below the treated surface became swollen and yellowed. There was little increase in size after the first 2 weeks following treatment. 3. The anatomy of the fourth internode of V. faba is similar to that in Pisum sativum, except for the number of bundles in the central cylinder. 4. Histologically every parenchymatous tissue became active to some extent in the formation of the tumor. Epidermal and outer cortical cells enlarged radially; the inner cortical cells divided transversely, occasionally the outer cortical cells tangentially; the endodermis divided tangentially; the cambium increased its activity and an interfascicular cambium was formed and became active as the early signs of response. Later parenchyma of the xylem, phloem, pericycle, pith, and rays became active to varying degrees. Cells of the pith divided repeatedly, and those bordering on the central cavity gave rise to an internal callus, the cells of which also divided repeatedly. In later stages all the cells of the cortex became highly meristematic. In some cases a callus was formed above the cut surface, starting from active cells of the vascular region and added to eventually by all parenchymatous cells. In the oldest tumors an occasional tangential division of the epidermis may be found. 5. Vascularization of derived tissues is somewhat extensive. Particularly notable is the response of the endodermal region of the central cylinder and of the cortical bundles. This group of extravascular strands may be supplemented by an occasional one formed in the pericycle or phloem. Small groups of wound tracheids are differentiated in the pith, pith callus, cortex, and surface callus when present. Occasionally vascular strands are formed in the pith close to the protoxylem points of the regular vascular cylinder. 6. Root induction, characteristic of some other plants treated with indoleacetic acid, is uncommon in V. faba under the conditions of this experiment. Those formed are probably derived as described for other plants. Occasionally, however, large, rather unorganized masses of tissue are developed which are probably best interpreted as potential roots incompletely organized. 7. Occasional polyploid nuclei are found in the large parenchymatous cells of the cortex and pith in both treated and control stems of V. faba, although somewhat more numerously in the treated stems. All divisions appear to be regular.