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Part of the cortex in several genera of cacti consists of cells that have highly undulate walls; the cells are collapsed and shrunken. The outermost, photosynthetic cortical cells and the innermost cortical cells, those immediately adjacent to the stele, are not collapsed but instead are large, turgid, isodiametric cells with smooth walls. Cortical bundles that run through the collapsible region are contorted, probably having been distorted by shrinkage of the surrounding cells. The region of collapsible parenchyma cells apparently is a specialized water-storage tissue: by having flexible walls, they lose their water most easily, thus facilitating transfer of water to other cells whose walls are more rigid. Also, they may be the last to refill when water is available, the water going first to cells with more inflexible walls. This tissue was found in Bolivicereus, Borzicactus, Cleislocactus, Espostoa, Gymnocalycium, Haageocereus, Loxanthocereus (all considered closely related), and Jasminocereus (not considered closely related to the others).