A survey is given of the present state of the theory of symplasmic transport. The symplasmic movement of organic and inorganic substances in Vallisneria leaves shows polarity. The polarity is not a stable factor of the tissue, since it can be changed in different ways. Data regarding the movement of auxin in Vallisneria leaves have been compared with data about auxin transport in coleoptiles. Attention has been given to the localisation and the function of the plasmodesmata. The conclusion is arrived at that there are two fluxes in coleoptiles, a longitudinal flux which uses the plasmodesmata in the transverse walls and a transverse flux which uses the plasmodesmata in the longitudinal walls. Movement in the symplasm is caused by concentration differences in combination with electric potential gradients and by gravity. Active transport requires the maintenance of these potential gradients by cell metabolism. The forces presumably influence the transit of the substances through the plasmodesmata, from cell to cell. At a cut surface cells are opened and substances are released to the exterior. The exit of auxin at the basal wound surface is the consequence of the continuous polar movement of auxin through the plasmodesmata in the transverse walls of the cells and the opening of the adjoining cells by cutting. The movement of endogenous and exogenous auxin are different. Endogenous auxin is moved by electric potential gradients in the tissue and by gravity, while exogenous auxin is moved by the same factors and moreover by the concentration difference which is the result of the active uptake of auxin in the symplasm.