The conventional view of root‐cap mucilage as an expanded blob of mucilage is characteristic only of root tips in contact with free water. In soil, the mucilage is almost always a dry coating over the tip to which soil particles adhere. The kinetics of expansion of root‐cap mucilage ofZea maysroots grown in field soil, in soil in pots, and axenically on agar, were determined when the mucilage was exposed to water. On the soil‐grown roots the increase in mucilage volume was linear with time, sometimes reaching a constant volume during the 6 h of measurement, but sometimes not. This linear expansion is interpreted as limited by the rate at which the condensed mucilage in the periplasmic and intercellular spaces of the root cap passes to the exterior of the cap, expanding as fast as it arrives outside in the water. The height of the plateau is interpreted as a measure of the amount of mucilage initially present in the interior spaces. Because of the greater availability of water in the axenic roots grown on 1% agar, the mucilage was already outside the root cap, and it expanded more rapidly. It reached a final volume about 10‐fold greater than that on the soil‐grown roots. The volume increase was curvilinear with time. An analysis of these curves suggested that this swelling on axenic roots was a diffusion of mucilage outwards from the flanks of the root cap, and the diffusivity of the mucilage was estimated as 4 × 10−8cm2s−1. The molecular radius derived from this diffusivity was 34 nm, and the estimated molecular weight w