Bulbs and Corms and Tubers, Oh My!

Judy Glattstein jgglatt@gmail.com
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

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