Astorage organ is a part of a plant specifically modified for storage of energy (generally in the form ofcarbohydrates) or water.^[1] Storage organs often grow underground, where they are better protected from attack byherbivores. Plants that have an underground storage organ are calledgeophytes in theRaunkiær plant life-form classification system.^[2][3] Storage organs often, but not always, act asperennating organs which enable plants to survive adverse conditions (such as cold, excessive heat, lack of light or drought).

Storage organs may act asperennating organs ('perennating' as inperennial, meaning "through the year", used in the sense of continuing beyond the year and in due course lasting for multiple years). These are used by plants to survive adverse periods in the plant's life-cycle (e.g. caused by cold, excessive heat, lack of light or drought). During these periods, parts of the plant die and then when conditions become favourable again, re-growth occurs frombuds in the perennating organs. For example, geophytes growing in woodland under deciduous trees (e.g.bluebells,trilliums) die back to underground storage organs during summer when tree leaf cover restricts light and water is less available.^{[citation needed]}

However, perennating organs need not be storage organs. After losing their leaves, deciduous trees grow them again from 'resting buds', which are the perennating organs of phanerophytes in theRaunkiær classification, but which do not specifically act as storage organs. Equally, storage organs need not be perennating organs. Manysucculents have leaves adapted for water storage, which they retain in adverse conditions.

In common parlance, underground storage organs may be generically called roots,tubers, or bulbs, but to thebotanist there is more specific technicalnomenclature:

• Trueroots:
  • Storagetaproot – e.g.carrot
  • Tuberous root or root tuber – e.g.Dahlia^[4]
• Modifiedstems:
  • Bulb (a short stem that produces fleshy scale leaves or modified leaf bases) – e.g.Lilium,Narcissus, onion
  • Caudex – e.g.Adenium (desert-rose)
  • Corm – e.g.Crocus
  • Pseudobulb – e.g.Pleione (windowsill orchid)
  • Rhizome – e.g.Iris pseudacorus (yellow flag iris)
  • Stem tuber – e.g.Zantedeschia (arum lily),potato
  • Trophopod (the persistentpetiole base of several fern genera)^[5] – e.g.Diplazium,Onoclea sensibilis
• Others:
  • Storagehypocotyl (the stem of a seedling) – sometimes called atuber, as inCyclamen

Some of the above, particularly pseudobulbs and caudices, may occur wholly or partially above ground. Intermediates and combinations of the above are also found, making classification difficult. As an example of an intermediate, the tuber ofCyclamen arises from the stem of the seedling, which forms the junction of the roots and stem of the mature plant. In some species (e.g.Cyclamen coum) roots come from the bottom of the tuber, suggesting that it is a stem tuber; in others (e.g.Cyclamen hederifolium) roots come largely from the top of the tuber, suggesting that it is a root tuber.^[6] As an example of a combination, juno irises have both bulbs and storage roots.^[7]

Underground storage organs used for food may be generically calledroot vegetables, although this phrase should not be taken to imply that the class only includes true roots.

Succulents are plants which are adapted to withstand periods of drought by their ability to store moisture in specialized storage organs.^[8]

• Leaf succulents store water in their leaves, which are thus thickened, fleshy and typically covered with a waxy coating or fine hairs to reduce evaporation. They may also contain mucilaginous compounds. Some leaf succulents have leaves which are distributed along the stem in a similar fashion to non-succulent species (e.g.Crassula,Kalanchoe); their stems may also be succulent. In others, the leaves are more compact, forming a rosette (e.g.Echeveria,Aloe). Pebble-plants or living stones (e.g.Lithops,Conophytum) have reduced their leaves to just two, forming a fleshy body, only the top of which may be visible above ground.

• Stem succulents are generally either leafless or have leaves which can be quickly shed in the event of drought. Photosynthesis is then taken over by the stems. As with leaf succulents, stems may be covered with a waxy coating or fine hairs to reduce evaporation. The ribbed bodies ofcacti may be an adaption to allow shrinkage and expansion with the amount of water stored. Plants of the same general form as cacti are found in other families (e.g.Euphorbia canariensis (familyEuphorbiaceae),Stapelia (familyApocynaceae)).

• Plant morphology
• Energy storage
• Biology terminology

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