The pressure-bomb technique as developed by Scholander and colleagues is reviewed. A theoretical analysis of the equilibrium water-relations of individual cells of a twig is derived taking due account of the fact that each cell has a unique solute concentration, fluid volume, shape, and unique mechanical constraint by virtue of its cell-wall structure and attachment to nearest neighbours. These equations combine to give a complete description of the whole twig in response to mechanical (air pressure) stress. Our theoretical analysis suggests that the ‘pressure-volume curve’ can be related quantitatively to meaningful bulk parameters of water relations: viz. the total osmolar content of the symplast Ns, the original volume of the symplast Vo, the volume expressed from the symplast Ve, the gas-pressure of the bomb P, and the volume-averaged turgor pressure (the sum of the products of the relative volume and turgor pressure of each cell). An empirical relation for the volume-averaged turgor pressure of twigs is found which fits all species examined; it also fits the turgor pressure relation for single (Nitella) cells.