Tubers are energy-saving structures for plants much in the same way an egg yolk is saved and sustains the new chick. Tubers hold a lot of plant sugars and reproductive structures. There are two kinds of tubers: stem and root. Stem tubers spring from underground stems like rhizomes. Potatoes and begonias are stem tubers. Root tubers form when the root expands and forms a bulbed mass (examples include dahlias and cassavas).
Stem tubers are just modified stems. They produce the bulk of the tuber foods humans consume. Since they are storage systems, stem tubers are rich in plant starch and nutrition. Stem tubers form from rhizomes or stolons. Rhizomes are roots that grow underground horizontally and are part of the original stem. Stolons are also vertical but are sprouts off the original stem. The stem tuber has nodes running off it that can produce a plant. Potatoes are classic examples of a stem tuber with nodes. Each eye on a potato is a potential plant and would begin its nutrition by consuming the original potato and its plant starches for food. The stem tuber is often at the top of the soil and has a vertical orientation with buds on top and fibrous root structure on the bottom. When the parent plant dies the tubers sprout new plants and die.
Root tubers are storage organs. They are modified lateral roots that have swollen to accept excess plant starches. The root functions the same as a stem tuber, enabling the plant to survive. There are two ends in a root tuber: the proximal end and the distal end. The proximal end has crown tissue that creates the stems and leaves in the next-generation plant. The distal end produces unmodified standard roots. Root tubers are biennial. In the first generation the plant produces the tubers and then dies in the fall. In the spring the tubers create a new plant that consumes the tubers as food. The cycle repeats. Root tubers can be divided to make new plants as long as each piece of tuber has crowning tissue.
The interior of a tuber is much like an unfertilized egg or seed. It has a nucleus, ovary, egg and micropyle at the very core of the tuber. This branches up to the style, which is just a conduit. The interior contains a pith, vascular zones and cortex just as any stem would. As the style stretches up like a stem it contains the sperm, pollen grains and, at the top, the stigma. Where there are not organs there is plant starch in the parenchyma-like cells, making up the bulk of the tuber. It also has a vascular system for transferring and receiving sugars to store. On the exterior there are the nodes and internodes and a leaf scar, which will be the beginnings of the new plant.
Edible Tuber Crops
The term "root and tuber" contains all the edible storage organs. There are 38 root, 23 tuber, 14 rhizome, 11 corm and 10 bulb edibles. The most widely cultivated of these is the beet, which is a root. The most widely grown food tuber is the potato. Wild tubers are not cultivated, but are still eaten and harvested widely. They are not included in the numbers above, but were a critical factor in mankind's early development. Tubers made up a large part of the early diet when meat was not available and were a good source of fiber and calories.
Not all tubers are edible. Just because something looks a bit like a potato does not mean it is to be eaten. It is best to check with an extension website or even a gourmet or culinary site. Some flower tubers like dahlias can be eaten. The tubers formed in potato vines are edible as well. Others carry high toxicity and could make one extremely sick. This goes for any wild tubers that are dug up. Caution should be taken before you eat what you find.