Certainly one of the most recognized groups of flowering plants are flowering bulbs. Since most can be divided, dried, and shipped essentially rootless while dormant, it is only natural that a huge industry should develop around them. Just about anyone is familiar with flower bulb catalogs filled with tantalizing (and often over saturated) images of tulips, amaryllis, daffodils, and crocus. What big box store or local nursery doesn't carry at least a few Christmas amaryllis or Easter lilies in season?
From a more technical angle, one might ask just what makes a plant a flowering bulb, or for that matter a bulb bearing plant. Bulbs are in fact fattened stems, leaves, or leaf bases that are water and nutrient storing structures and often contain a growing point and/or an unexpanded flowering shoot within. Commonly, bulb forming plants go through an extended dormancy due to seasonal changes of their habitats - cold winters, hot and dry summers, and the like. Most bulbs grow either underground or near the surface, but a in some species they can be formed along leaf axes on exposed stems (some lilies do this) and serve as a way of distributing new plants. In general, any underground storage organ is known as a geophyte.
Not surprisingly, variety within this group is extreme. In truth many plants that are lumping into the category "bulbs" are not technically true bulbs at all. These would include stem and root tubers, fattened rhizomes, corms, taproots, caudices (fattened stems of woody plants), and even pseudobulbs (fattened above ground stems limited to orchids). If you peruse any flower bulb catalog you will find most of these represented for sale. Since all are commonly lumped in the horticultural trade, I will treat them as a group, but with the recognition of their distinct differences from both a structural and evolutionary perspective. What follows is a quick rundown of the various flowering bulb groups including examples.
True bulbs - there are two basic types, tunicate and non-tunicate bulbs. The former are the commonly known bulbs best represented by the genus Allium (onions) with outer dry scales and inner layers of fleshy ones. Common genera include Narcissus, Oxalis, Tulipa, and some Iris. Less well know genera include the lovely and odd Asian Lycoris (spider lilies) and South American Ipheion (star flowers). Non-tunicate bulbs include Lilium and Fritillaria have no dry outer scales, but rather fleshy, more loosely arrayed ones. These usually can be divided from the main bulb to propagate new plants.
Tubers - two basic types exist, stem and root tubers, but diversity is extreme within this group. They contain no scales, but rather are fattened structures designed to retain nutrients, water, and energy. Common stem tuber genera include potatoes, cyclamen, and tuberous begonias. Root tuber forming plants include Dahlia, some Ipomoea, daylilies (Hemerocallis) and a number of terrestrial orchid genera, such as Dactylorhiza, Habenaria, Pecteilis, Ponerorchis, and Pterostylis.
Rhizomes - these are fattened, mostly underground stems that can withstand at least some drying. Both vegetative growths and roots grow from these structures. Common genera include most Iris, various gingers, trilliums, canna lilies, and some orchids such as Bletilla and Spathoglottis.
Corms - these are short fatten underground stems that look very similar to tunicate bulbs and often even have outer dry scales (the tunic). They are unalike in that they are fattened stems with a different internal structure than bulbs, which are mostly composed of layers of fleshy scales. Common genera include Crocus, Crocosmia, Freesia, Gladiolus, and less well known plants such as Arisaema (jack-in-the-pulpits) and the terrestrial orchid species Calypso bulbosa.
Taproots - these are fattened, vertically downward growing, and often large, roots. Common examples would be dandelions, turnips, radishes, and carrots. While many make great food plants, few are grown for their flowers.
Caudices - these fattened, woody stemmed plants are much less well known. Desert roses (Adenium) however are sometimes considered within the realm of flowering bulbs.
Pseudobulbs - these are mostly above ground fattened stems produced by the orchid family. In some genera these can be at or just below ground level, for example, many Calanthe, Cymbidium, Liparis, Phaius, and Pleione species.