EFFECT OF 3-INDOLE ACETIC ACID ON CELL WALLS OF STEM AND ROOT

THE EVIDENCE accumulated on the growth hormone shows that it accelerates growth in the stem. The accelerating effect has been explained by Heyn (1930, 1931, 1934a, 1934b) as due to an increase in the plastic extensibility of the wall caused by the growth hormone. Heyn found the bending of oat coleoptiles under the influence of a weight to be greater w'hen the coleoptiles were treated with growth substance before the experiment. Plasticity of coleoptiles which were not growing was increased by treatment with growth substance. Similar results were secured with lupine hypocotyls (Heyn, 1931, 1934a). Heyn (1931) found the elasticity extensibility of oat coleoptiles to be increased by the growth hormone and S6ding (1931) secured similar results with flower stalks. Although the growth hormone accelerates the growth of the stem, it inhibits that of the root; and its inhibiting effect upon the growth of the root has not been explained.In the" translation and revision by Avery and Burkholder of Boysen-Jensen's text (1936) the statement is made (p. 115), "How the same compound can produce the opposite effects in stems and roots has not been explained satisfactorily "; and (p. 212), "It remains to be shown how the same growth hormone can retard growth in roots bringing about downward curvature and promote the rate of growth in shoots causing upward bending." If the growth hormone is effective in stems by increasing the extensibility of the cell walls, might it not produce an opposite effect in the root by decreasing the extensibility of root walls? The writers have performed some experiments on the effect of 3-indole acetic acid on the extensibility of stem walls and root walls. Under the experimental conditions used, the extensibility of cell wall material from stems is increased by 3-indole acetic acid but that of roots is decreased.