Self-supporting plant stems are slender, erect structures that remain standing while growing in highly variable mechanical environments. Such ability is not merely related to an adapted mechanical design in terms of material-specific stiffness and stem tapering. As many terrestrial standing animals do, plant stems regulate posture through active and coordinated control of motor systems and acclimate their skeletal growth to prevailing loads. This analogy probably results from mechanical challenges on standing organisms in an aerial environment with low buoyancy and high turbulence. But the continuous growth of plants submits them to a greater challenge. In response to these challenges, land plants implemented mixed skeletal and motor functions in the same anatomical elements. There are two types of kinematic design: (1) plants with localized active movement (arthrophytes) and (2) plants with continuously distributed active movements (contortionists). The control of these active supporting systems involves gravi- and mechanoperception, but little is known about their coordination at the whole plant level. This more active view of the control of plant growth and form has been insufficiently considered in the modeling of plant architecture. Progress in our understanding of plant posture and mechanical acclimation will require new biomechanical models of plant architectural development.