Graviresponsiveness and the Development of Columella Tissue in Primary and Lateral Roots of Ricinus communis

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Abstract
Half-tipped primary and lateral roots of R. communis cv. Hale bend toward the side of the root on which the intact half-tip remains. The minimal graviresponsiveness of lateral roots is not due to the inability of their caps to produce growth effectors (presumably inhibitors). The columella tissues of primary (i.e., graviresponsive) roots are 4.30 times longer, 2.95 times wider, 37.4 times more voluminous and composed of 17.2 times more cells than those of lateral roots. The onset of positive gravitropism by lateral roots is positively correlated with a 2.99-fold increase in length, 2.63-fold increase in width and 20.7-fold increase in volume of their columella tissues. The minimal graviresponsiveness of lateral roots may be due to the small size of their columella tissues, which results in their caps being unable to establish a concentration gradient of the effector sufficient to induce gravicurvature and produce as much of the effector as caps of graviresponsive roots.