SUMMARY Current views on membrane construction are presented as a basis for discussing certain modifications of structure that may render membranes leaky, allowing solutes to diffuse from them. Biophysical experiments on dehydrated membranes indicate that, when their water content is reduced below 20%, they no longer adopt the lipid bilayer conformation. When dry seeds, spores or lichens are placed in water, soluble cell constituents leak from them for a few seconds or minutes until membrane integrity is re‐established; the same may apply also to the liberation of solutes from pollen grains. Moreover, there are indications that ‘dry’ in this context means having less than about 20% water. Some plants are sensitive to chilling, their tissues becoming leaky when exposed to temperatures between 0 and 10°C. This chilling‐induced leakage, attributed to a phase change in all the phospholipids of which the membrane is composed occurs at a slightly lower temperature than the change in the activation energy of membrane‐bound enzymes. At senescence, leaf tissues become leaky and may eventually dry out. So much phospholipid disappears from senescing cotyledons of cucumber that there is no longer sufficient to fabricate complete and intact membranes around the cells. Treating cucumber cotyledons with iodoacetate also causes a loss of phospholipids, allowing solutes to leak out of the cells. It is possible that exposing seeds or mature plant tissues to oxygen at pressures of 1 or more atm renders them leaky because of lipid peroxidation.

When pea embryos are placed in water solutes leak from them at a rate which declines rapidly at first and then more slowly. Embryos can be dried down over calcium chloride and will then leak as before when returned to water. Similar results were obtained with liicinus seeds after removal of the testa. Pea embryos that have first been allowed to imbibe some water through a small part of their surface (by placing them on damp filter-paper) leak relatively slowly when subsequently immersed in water; the greater the initial imbibition the slower the subsequent leakage. Likewise, embryos taken from peas that were harvested when succulent and tender show only slow leakage. It is proposed that as seeds dry out in the course of development cell membranes lose their integrity. When such dry seeds are allowed to imbibe water there is a short period, before membrane integrity is re-established, during which solutes can leak out of the cells.

SUMMARY 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.

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