Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to overwinter and regrow the next year and as a means of asexual reproduction. Two different groups of tubers are: stem tubers, and root tubers.
Stem tubers generally start off as enlargements of the hypococtyl section of a seedling but also sometimes include the first node or two of the epicotyl and the upper section of the root. The stem tuber has a vertical orientation with one or a few vegetative buds on the top and fibrous roots produced on the bottom from a basal section, typically the stem tuber has an oblong rounded shape.
Tuberous begonia and Cyclamen are commonly grown stem tubers. Mignonette vine (Anredera cordifolia) produces aerial stem tubers on 12 to 25 foot tall vines, the tubers fall to the ground and grow. Plectranthus esculentus of the mint family Lamiaceae, produces tuberous under ground organs from the base of the stem, weighing up to 1.8 kg per tuber, forming from axillary buds producing short stolons that grow into tubers.
The tuber has all the parts of a normal stem, including nodes and internodes, the nodes are the eyes and each has a leaf scar. The nodes or eyes are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the stolon. The terminal bud is produced at the farthest point away from the stolon attachment and tuber thus shows the same apical dominance of a normal stem. Internally a tuber is filled with starch stored in enlarged parenchyma like cells; also internally the tuber has the typical cell structures of any stem, including a pith, vascular zones and a cortex.
The tuber is produced in one growing season and used to perennialize the plant and as a means of propagation. When fall comes the above ground structure of the plant dies and the tubers over winter under ground until spring, when they regenerate new shoots which use the stored food in the tuber to grow. As the main shoot develops from the tuber, the base of the shoot close to the tuber produces adventitious roots and lateral buds on the shoot, The shoot also produce stolons that are long etiolated stems. The stolon elongates during long days with the presence of auxins and high gibberellin levels that prevent root growth off of the stolon. Before new tuber formation begins the stolon must be a certain age. The hormone lipoxygenase is involved in the control of potato tuber development.
The stolons are easily recognized when potato plants are grown from seed, as the plants grow, stolons are produced around the soil surface from the nodes. The tubers form close to the soil surface and sometimes even on top of the ground. When potatoes are cultivated, the tubers are cut into pieces and planted much deeper into the soil. By planting the pieces deeper there is more area for the plants to generate the tubers and their size increases. The pieces sprout shoots that grow to the surface, these shoots are rhizome like and generate short stolons from the nodes while in the ground. When the shoots reach the soil surface they produce roots and shoots that grow into the green plant.
A tuberous root is a modified lateral root, enlarged to function as a storage organ. It is thus different in origin but similar in function and appearance to a tuber. Examples of plants with notable root tubers include the sweet potato, cassava and Dahlia. It is a structure used to perennialize the plant for survival from one year to the next.
The thickened roots are storage organs that differ from true tubers. The massive enlargement of secondary roots typically represented by Sweet Potato (Ipomoea batatas), have the internal and external cell structures of typical roots. True tubers have the cell structure of stems, In root tubers there are no nodes and internodes or reduced leaves. One end called the proximal end has crown tissue that produces buds that grow into stems and foliage. The other end called the distal end normally produces unmodified roots. In true tubers the order is reversed with the distal end producing stems. Tuberous roots are biennial in duration, the first year the parent plant produces the root tubers and in the fall the plant dies. The next year the root tubers produce a new plant and are consumed in the production of new roots and stems and flowering. The remaining tissue dies while the plants generates new root tubers for the next year. Hemerocallis fulva plus a number of Daylily hybrids have large root tubers, H. fulva spreads by underground stolons that end with a new fan that grows roots that produce thick root tubers and then send our more stolons.
Plants with root tubers are propagated in late summer to late winter by digging up the tubers and separating them, making sure that each piece has some crown tissue and replanting.