In common parlance, underground storage organs may be generically called roots, tubers, or bulbs, but to the botanist these are specific, technical terms, which apply only more narrowly:

In some plants the storage organ are short-lived and serve as regenerative organ bearing a bud or buds (e.g. a tuber); in that case the plant dies back in the resting season, except for the underground storage organ with the buds, later utilised for regrowth; afterwards old tubers decay and new ones are formed. In addition some plants produce smaller reproductive storage organs (bulbils, small tubers, etc); plants growing from them resemble in morphology and size seedlings.

This cells, with small and few chloroplasts, have often undulating flexible, scarcely lignified, and elastic wall,  that permit the cell volume to change significantly as water is absorbed or released.

The selective advantage of storing water is not just that it keeps water-storage cells alive but rather that the water can be made available to  photosynthetic cells, to the apical and axillary buds, any flower buds or developing fruits and so on. Consequently, water-storage cells should have thin, flexible walls that can contract or shrink readily such that the cell’s volume diminishes as water is transferred out. On the other hand, the cells that need the water should be more resistant to shrinkage: if all cell walls were equally flexible, all parts of the plant would suffer water stress equally, but that is not adaptive. Instead, water-storage tissues should give up water so easily that the more active cells do not experience water stress unless drought is extremely prolonged. In all cacti, cell walls of the inner cortex are especially thin and flexible, but in many cacti there is an additional modification: the walls are folded or undulate, even when young and recently produced by the shoot apical meristem. Because the walls already have folds in them, the cells are presumably set to shrink very easily. These cells have been called collapsible cortex cells. In contrast, wall of palisade cortex cells – the photosynthetic cells – are thicker and lack folds, so these cannot shrink as easily. Consequently, as the volume of water within a cactus shoot diminishes, cells of the inner cortex give up water preferentially, transferring it to other cells, minimizing water stress and thus allowing photosynthesis to continue.