As the symptoms of calcium deficiency develop in plants, there is often a stage in which the tissues are water-soaked and one involving cell breakdown with loss of turgor (as in internal breakdown of apples). Eventually the tissue may become desiccated yielding a dry, more or less extensive area of necrosis. Two mechanisms are proposed. There is evidence that calcium deficiency renders membranes permeable which would account for a loss of turgor and permit cell fluids to invade intercellular spaces. An alternative situation may develop in soft, succulent fruits, the cells of which burst under hypotonic conditions in vitro. It is suggested that exogenous water may enter a fruit from the atmosphere or (in apple) through the phloem. Such exogenous water in the intercellular spaces of the fruit may cause cells to swell, so cracking the fruit or it may result in a bursting of the cells. A plea is made for further light microscope studies of the development of symptoms of calcium deficiency.