above are also found, making classification difficult. As an example of an intermediate, the tuber of Cyclamen 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. As an example of a combination, juno irises have both bulbs and storage roots. Underground storage organs used for food may be generically called root vegetables, although this phrase should not be taken to imply that the class only includes true roots. Other storage organs Succulents are plants which are adapted to withstand periods of drought by their ability to store moisture in specialized storage organs. 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 of cacti 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 (family Euphorbiaceae), Stapelia (family Apocynaceae)). Notes and references Plant morphology Energy storage Biology terminology