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Bulbs, Corms & Rhizomes
A bulb is a short stem with fleshy leaves or leaf bases. The leaves often function as food storage organs during dormancy.
A bulb's leaf bases generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. The leaf bases may resemble scales, or they may overlap and surround the center of the bulb as with the onion. A modified stem forms the base of the bulb, and plant growth occurs from this basal plate. Roots emerge from the underside of the base, and new stems and leaves from the upper side. (Wikipedia)
A corm (or bulbo-tuber, bulbotuber) is a short, vertical, swollen underground plant stem that serves as a storage organ used by some plants to survive winter or other adverse conditions such as summer drought and heat (estivation). A corm consists of one or more internodes with at least one growing point, with protective leaves modified into skins or tunics. The thin tunic leaves are dry papery, dead petiole sheaths, formed from the leaves produced the year before, which act as a covering that protects the corm from insects and water loss. Internally a corm is mostly made of starch-containing parenchyma cells above a circular basal node that grows roots. (Wikipedia)
A rhizome (from Ancient Greek: rhízōma "mass of roots", from rhizóō "cause to strike root") is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes. Rhizomes may also be referred to as creeping rootstalks or rootstocks. Some plants have rhizomes that grow above ground or that lie at the soil surface, including some Iris species, and ferns, whose spreading stems are rhizomes. (Wikipedia)
A stem tuber forms from thickened rhizomes or stolons. The tops or sides of the tuber produce shoots that grow into typical stems and leaves and the under sides produce roots. They tend to form at the sides of the parent plant and are most often located near the soil surface. The below-ground stem tuber is normally a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The offspring or new tubers, are attached to a parent tuber or form at the end of a hypogeogenous rhizome. In the fall the plant dies except for the new offspring stem tubers which have one dominant bud, which in spring regrows a new shoot producing stems and leaves, in summer the tubers decay and new tubers begin to grow. Some plants also form smaller tubers and/or tubercles which act like seeds, producing small plants that resemble (in morphology and size) seedlings. Some stem tubers are long lived such as those of tuberous begonia but many tuberous plants have tubers that survive only until the plants have fully leafed out, at which point the tuber is reduced to a shriveled up husk.