The lateral movement of auxin has been sttudied with bioassays and also by application of '4C-IAA. Both techniques have demonstrated that the upper half of a horizontally positioned stem or root contains at least 30( to 45 % of the total auxin present in the tissue (2). However, such an auxin gradient is difficult to reconcile with the Cholodny-Weit theory of root geotropism because it is too small to account for the growth inhibition which occurs in the underside of a root experiencing a geotropic curvatture (2). We shall present evidence that the auxin differential actulally is considerably larger than these experiments indicate, and discuss several problems which have obscured this fact and hindered the measurement of lateral auixin transport. Previously we reported (3) that in 8 experiments, each using 20 sections and 1 ,um 14C-IAA, the specific activity of the upper half of horizontally positioned etiolated pea stem segments was 37.5 + 2.7 % that of the lower half. In an equal number oif experiments the specific activities of the upper quarter, middle half, and lower quarter relative to that of the intact section were 0.58 ? 0.07, 0.99 + 0.07, and 1.46 + 0.10 respectively (3). From these values the distribution of 14C-IAA across the diameter of the stem has been calculated (fig 1). The data show the specific activity to be fairly uniform throughout the middle of the stem, only rising or falling markedly at the ouiter suirfaces. Apparently each cell passes the auxin it receives to the next lower cell so that IAA is displaced from the upper to the lower surface with little change in concentration in the center of the section where most of the tissuie mass resides. Consequently when sections are halved the large differential between the upper and lower suirfaces is obscured by the lack of a gradient in the central builk of tisstue included in each sample. The magnitude of the gradient between the surfaces can be es-timated by extrapolating the curves (dotted lines, fig 1) and may be as large as 10:1. The problem of the central tissue mass can be circumvented by utilizing hollow cylinders such as Avena an(l corn coleoptiles, but this approach introduces new difficulties. It is well established that auxin does not spread laterally in these tissues exceept under the influence of gravity; otherwise the Avena and corn -curvature tests would not work. Consequently when coleoptiles are positioned horizontally, IAA should only migrate at the flanks from the upper to the lower half of the coleoptile (see fig 2). In addition a net movement of IAA from U1 to U2 and L1 to Lo would be expected, giving rise to the final distribution illtistrated in fi'gure 2. If this interpretation is correct the lower half of a split coleoptile must contain a mass of flank cells in which there is no gradient as well as a zone (,1) partially depleted of auxin, whereas the upper half shotuld contain an auxin rich zone (U,). Therefore, we believe that auxin gradients U1 :U9 an( I1 :L2 are greater than the differential measured between the upper and lower halves of sulit coleoD-