Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to survive seasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. Of considerable interest is how this process appears to reset the aging clock. As aforementioned, plants vegetetatively propagate both artificially and naturally. Most common methods of natural vegetative reproduction involves the development of a new plant from specialized structures of a mature plant. In addition to adventitious roots, roots that arise from plant structures other than the root, such as stems or leaves, modified stems, leaves and roots play an important role in plants' ability to naturally propagate. The most common modified stems, leaves and roots that allow for vegetative propagation are:  Also known as stolons, runners are modified stems that, unlike rhizomes, grow from existing stems just below the soil surface. As they are propagated, the buds on the modified stems produce roots and stems. Those buds are more separated than the ones found on the rhizome. Examples of plants that use runners are strawberries and currants. Bulbs are inflated parts of the stem within which lie the central shoots of new plants. They are typically underground and are surrounded by plump and layered leaves that provide nutrients to the new plant. Examples of plants that use bulbs are shallots, lilies and tulips. Tubers develop from either the stem or the root. Stem tubers grow from rhizomes or runners that swell from storing nutrients while root tubers propagate from roots that are modified to store nutrients and get too large and produce a new plant. Examples of stem tubers are potatoes and yams and examples of root tubers are sweet potatoes and dahlias. Corms are solid enlarged underground stems that store nutrients in their fleshy and solid stem tissue and are surrounded by papery leaves. Corms differ from bulbs in that their centers consists of solid tissue while bulbs consist of layered leaves. Examples of plants that use corms are gladiolus and taro. Also known as root sprouts, suckers are plant stems that arise from buds on the base of parent plants stems or on roots.