The analysis of plant growth as a physical process is briefly reviewed. Growth requires the coordinated uptake of water and the irreversible expansion of the cell wall. Any agent that affects the growth rate must act on one or more of the parameters governing water absorption (e.g. the hydraulic conductivity or the difference in osmotic pressure of the cell contents and the water source) or cell wall expansion (e.g. wall extensibility or the yield threshold). When the hydraulic conductivity of the pathway for water transport is small enough to impede the rate of cell enlargement, a substantial gradient in water potential within the growing tissue will develop to sustain the absorption of water. In such a case, the analysis shows that turgor pressure is a key indicator for determining whether an agent acts predominantly on the osmotic properties of the tissue or on the cell wall properties. Furthermore, the dynamic response to a slight perturbation from steady-state conditions is shown to be a function of parameters for both the water relations and cell wall expansion of the tissue. Blue irradiation of etiolated seedlings causes a large inhibition of stem elongation with lag times as short as 30 s and half-times as short as 20 to 25 s. The biophysical mechanism of blue-light suppression of growth was studied in cucumber and sunflower seedlings by means of direct and indirect measurements of turgor pressure. The results indicate that (a) blue light suppresses growth by influencing the cell wall properties of the growing tissue, and (b) the hydraulic conductivity of the growing tissue is large enough for it not to limit the rate of cell enlargement.