Responses of Pea Roots to Application of Certain Growth-Regulating Substances

Pea roots with their tips cut off showed marked response to application of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in lanolin and a less marked one to tryptophane. Only slight reactions followed similar application of asparagine, indoleacetic acid, 2,3,5-triiodobenzoic acid, or filtrate from a culture of rhizobia. 2. Uncut roots enlarged more than cut roots following direct application of tryptophane or 2,4,5-T to each. Uncut roots enlarged more than cut roots on the same plant, following application of 2,4,5-T to the latter. 3. Roots of pea showed the following effects after treatment with 2,4,5-T: (a) an increase in diameter brought about largely by meristematic activity of the pericycle, accompanied by some proliferation of the endodermis; (b) active division of cortical parenchyma near the meristematic region; (c) in the enlarged region, stretched and torn cortical parenchyma cells; (d) in the mature region, meristematic activity of the pericycle limited to the vicinity of branch roots; (e) hypertrophy of epidermal cells; (f) an increase in number of root hairs, especially at the level of greatest diameter of the enlarged root; (g) failure in differentiation of vascular tissues beyond the stage reached at the time of application of the growth substance. 4. Pea roots showed the following effects from application of tryptophane: (a) an increase in diameter brought about chiefly by proliferation of xylem parenchyma and cambium; (b) some divisions in pericycle and endodermis; (c) scattered cells in the stelar region maturing as scalariform tracheids. 5. Tryptophane-treated roots resembled those with developing nodules in that there was some division of both pericyclic and endodermal cells and in that there was differentiation of scalariform tracheids in the proliferated tissues of the stele. 6. In pea roots none of the growth substances used stimulated division of the cortical parenchyma cells, in which most of the proliferation takes place in developing nodules.