Bulbs and Corms and Tubers, Oh My!
Judy Glattstein (Mon, 14 Dec 2009 13:45:14 PST)

Here is the relevant handout material from back when I taught a required
bulb i.d. course for School of Professional Horticulture students at the
New York Botanical Garden:

Geophytes - definitions

common point: the ability to store food over adverse conditions.

bulb: a true bulb is composed of modified leaves called scales. They may
be loose and open (as in lilies) or tight and compact (as in narcissus).
Some bulbs have a thin papery covering, called a tunic, as in tulips.
Roots grow from a basal plate located at the bottom of the bulb. In
autumn true bulbs contain the embryo plant complete with flower buds.
True bulbs form offsets from lateral buds on the basal plate.
Examples: allium, fritillaria, galanthus, lily, narcissus, tulip, scilla

corm: a corm is a mass of undifferentiated storage tissue derived from
modified stem growth and covered by dry leaf bases. These coverings may
be reticulated (netted) or annulate (ringed). Roots grow from a basal
plate. The growing points on top may be single or multiple. A new corm
is formed each year.
Examples: colchicum, crocus, erythronium, freesia, gladiolus

tuber: a tuber is a solid mass of stem tissue, like a corm, but lacks
the tunic-like covering and basal plate. Roots and shoots arise from
growing points or eyes scattered over the tuber.
Examples: anemone, caladium, cyclamen, eranthis, ranunculus

tuberous root: similar in appearance to tubers but composed of root
tissue, not stem tissue. Fibrous roots are produced during the growing
season, and new growth buds arise at the base of the old stem.
Examples: alstromeria, begonia, dahlia, polygonatum

rhizome: composed of swollen stem tissue, growing laterally at or just
below the surface, generally freely branching. Roots develop on the
lower surface, shoots on the top.
Examples: calla, canna, convallaria, rhizomatous iris