Schematic representation of the two principles (turgor pressure and wall tensile stress) regulating plant cell growth. The top part of the figure illustrates the CWL (Chemical Wall Loosening) mechanism by using a spring, hooks and a weight, while the lower part schematizes the LOS model. In the CWL model, the spring (which represents the cell wall), under the action of the weight (the internal turgor pressure), is in tensile stress and the potential energy of the system increases; when the transducer (wall loosening enzymes) releases the weight by unfastening the hooks holding the spring (which represent the inter/intra-chain bonds of wall polysaccharides), the energy accumulated is transformed into kinetic energy (growth). Therefore, the action of wall loosening enzymes is required for growth, according to the CWL model. According to the LOS model, the compression of the spring (i.e., the cell wall) by an increase in turgor pressure (here represented as a hand pushing the weight), proceeds until a threshold is reached (wall yield threshold). Beyond this threshold, the wall yields and expansion occurs. Therefore, according to the LOS model, growth depends on the intrinsic properties of the wall.