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SUMMARY The capacity of the symplast and the phloem to transport Ca 2+ and H * (OH − ) is discussed in relation to the concentrations of free and bound Ca 2+ and H * in these transport systems, and the general restrictions on the concentrations of free Ca 2+ and H * in cytoplasm. It is concluded that the low Ca 2+ and H + transport capacity of symplast and phloem are consequences of the cytoplasmic nature of these transport systems. The significance of the low transport capacity for these two ions relative to the rate at which the plant produces or consumes H + or can take up Ca 2+ and transport it in the xylem is discussed. Other transport characteristics of the phloem can also be related to the cytoplasmic nature of the transport conduit: the nature of the transported organic C and N compounds is partly dependent on the requirement that high concentrations of these solutes can be tolerated by cytoplasmic enzymes, i.e. sucrose, mannitol and amino‐acids are ‘compatible solutes'. The properties of the intracellular transport systems (symplast and phloem) are contrasted with those of extracellular transport systems such as plant xylem and animal blood.
John A. Raven (Tue,) studied this question.