The Leaf Specific Conductivity (LSC) of nodal and internodal segments of 1-year-old twigs ofVitis viniferaL. andPopulus deltoidesBartr. was measured. It turned out to be greater in the internodes than in the nodes in both species, the differences being much bigger inVitisthan inPopulus. The nodes can be considered as « hydraulic constriction zones » of the stem. Their resistance to flow increases acropetally in both species, causing a characteristic hyperbolic behaviour of the water potential gradients along the twigs inVitis, while inPopulusit is not so marked. Some hypotheses are advanced about the possible functional significance of the nodes.