A root vegetable is so-called because the part that is commonly eaten is the geophyte, which grows under the surface, although technically speaking it is often not the root of the plant. There are two main types of plant root systems:

  • Taproot systems
  • Fibrous root systems
Taproot systems consist of one large thick root with few side branches, whereas fibrous-rooted plants have many finer roots branching out in a big subterranean bad hairstyle.
"Root vegetables" of are of the former type, having one large edible root, for instance carrots. If you pull up a dandelion by the roots, you will see that it looks much like a scrawny, wiry carrot or parsnip (though I would imagine they taste disgusting).

Other root vegetables are actually such entities as bulbs and tubers. The bulbs are mainly (solely?) members of the onion family, and the large majority of root vegetables are tubers. A tuber is a swollen part of the stem, and is where the plant stores its energy. The stem of the plant protrudes from the top of the tuber, and the root protrudes from the bottom. Hence the tuber is not actually the root. Examples of tuber vegetables are turnips, and of course, potatoes.

Such plants as carrots, with taproot systems, may be said to have tuberous roots, the word tuber coming from the word tumere, to swell.