Unlike other groups of woody plants such as trees and shrubs, climbing plants do not have self-supporting trunks or stems and therefore use various modifications to lift themselves up toward the light. Climbers are divided into four broad groups according to the mechanism they use to climb: tendril climbers, twiners, scramblers, and self-clinging climbers. Tendril = thin, curling tendrils that coil around their supports, whether it's wire, netting, or trellis. Twiners = climbers that twine their new shoots entwine around a support. Scramblers often climb and support themselves by means of thorns that usually curve downwards, acting as hooks. Self-clinging climbers = not as common. They produce short roots from their stems, known as aerial roots, that attach themselves firmly to supports. This way they can attach themselves to completely flat surfaces like walls.