Tubers are enlarged structures in some plant species used as storage organs for nutrients. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing season, and as a means of ****ual reproduction. Stem tubers form from thickened rhizomes (underground stems) or stolons (horizontal connections between organisms). Common plant species with stem tubers include potato and yam. Some sources also treat modified lateral roots (root tubers) under the definition; these are encountered in sweet potato, cassava, and dahlia.