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Variations in hydraulic conductivity (Lp) and the underlying anatomical and morphological changes were investigated for main root-lateral root junctions of Agave deserti and Ferocactus acanthodes under wet, dry, and rewetted soil conditions. During 21 d of drying, L(p) and radial conductivity (L(R)) increased threefold to fivefold at junctions of both species. The increase in L(R) was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of the junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. During 7 d of rewetting, LR decreased for junctions of A. deserti, as apoplastic water movement was not apparent, but L(R) was unchanged for F. acanthodes. Axial conductance (K(h)) decreased during drying for both species, largely because of embolism related to the degradation of unlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admit air bubbles at pressure differences of only 0.12 - 0.19 MPa. Rewetting restored K(h) for both species, but not completely, due to blockage of xylem elements by tyloses. About 40 % of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For the dicotyledon F. acanthodes, which can form new conduits in its secondary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral roots that persist during drought in the field compared with the case for the sympatric A. deserti.