Developing cladodes had lower water potentials and developing fruits had higher water potentials than the underlying cladodes of the widely cultivated prickly pear cactus, Opuntia ficus-indica. The 0.06 MPa lower value in 4-week-old daughter cladodes indicated a typical water potential gradient from the underlying clad-ode along the xylem of −0.2 MPa m−1; the 0.17 MPa higher value in 4-week-old fruits, which decreased to 0.07 MPa by 10 weeks, implicated the phloem as their supplier of water. The phloem sap of the underlying cladodes had an osmotic pressure of only 0.90 to 0.98 MPa, so the phloem could supply a relatively dilute solution to the photosynthetically dependent fruits (daughter cladodes of O. ficus-indica are photosynthetically independent at 4 weeks). Although the water potentials were similar for adjacent tissues, the osmotic pressures were lower for the water-storage compared with the photosynthetic tissue; the osmotic pressures were higher for xylem sap from fruits, for which xylary flow apparently occurred toward the underlying cladodes, than for daughter cladodes. The relative capacitance (change in relative water content divided by change in tissue water potential) was approximately 0.71 MPa−1 for the water-storage tissue and the photosynthetic tissue of both daughter cladodes and fruits at 4 weeks of age. When these organs approached maturity at 10 weeks, the relative capacitance increased about 40% for their water-storage tissue, but decreased 30% for their photosynthetic tissue. As the plant water content decreases during drought, about twice as much water will thus be lost per unit volume of the water-storage tissue compared with the photosynthetic tissue of maturing fruits and cladodes.