The present study tries to clarify the anatomical basis of the hydraulic properties of the internodes and nodes of one-year-old twigs ofVitis viniferaL. andPopulus deltoidesBartr‥ In particular, the higher resistance to flow as exhibited by the nodal regions of both species, appears to be due to a lower number of large-diameter vessels in the nodes as compared with that in the internodes. InPopulus, beside this aspect, the nodal constriction is also due to a larger number of narrow-diameter vessels.