Summary The in vivo tissue isolation technique presented here allows localization of tissue tension within contractile roots, and thus, localization of a potential to shorten. Tissue strips of the stele, inner cortex, and outer root parts no longer have shortening potential after isolation. However, defined tissue strips of middle cortical cells shorten immediately after isolation by 10–30%. This starts when root swelling appears (zone II/I), and continues up to root zone (III). The ability to contract seems to be limited to the expanded cells, which finally collapse. Thus, changes in the longitudinal cell dimension of these expanded cells does not occur in fixed roots. However, pressure to shorten affects the inner and outer root tissues, which shorten passively. Innermost cell layers of the root cortex show partial dissolution of middle lamellar material, and thus, cell wall bubbles appear. In older roots (after contraction), cells of the inner cortex often become disconnected from each other and the inner cortex separates from the outer root parts. Quantitative comparison of cells before and after root contraction shows that the inner cortical cells become shorter by half. On the basis of ‘shortening potential’, an alternative model of the mechanism of root contraction is presented. In this ‘pneumodel’, the active, middle cortical cells function like expanded ‘elastic bands’, and a small drop in cell pressure causes the expanded cell walls to lose their elastic tension, resulting in them shortening. This shortening creates a pulling force, which eventually effects underground movement.