In botany, succulent plants, also known as succulents, are plants with parts that are thickened, fleshy, and engorged, usually to retain water in arid climates or soil conditions. It is a characteristic that is not used scientifically for the definition of most families and genera of plants because it often can be used as an accurate characteristic only at the single species level. The word succulent comes from the Latin word sucus, meaning 'juice', or 'sap'. By definition, succulent plants are drought resistant plants in which the leaves, stem, or roots have become more than usually fleshy by the development of water-storing tissue. Other sources exclude roots as in the definition "a plant with thick, fleshy and swollen stems and/or leaves, adapted to dry environments". This difference affects the relationship between succulents and "geophytes" – plants that survive unfavorable seasons as a resting bud on an underground organ. These underground organs, such as bulbs, corms, and tubers, are often fleshy with water-storing tissues. Thus if roots are included in the definition, many geophytes would be classed as succulents. Plants adapted to living in dry environments such as succulents, are termed xerophytes. However, not all xerophytes are succulents, since there are other ways of adapting to a shortage of water, e.g., by developing small leaves which may roll up or having leathery rather than succulent leaves. Nor are all succulents xerophytes, since plants such as Crassula helmsii are both succulent and aquatic.