Not critical, but a Calla is an aroid. There are no bulb forms represented in the family Araceae, on tubers. The explanation is not simple but I'll do my best.
A bulb is an underground storage structure that is a condensed stem usually with a basal plate and fleshy storage leaves surrounding the bud that will form the next plant. This type of stem occurs in many plant families but not in the Araceae (aroids).
The stem of a plant is the central axis or base of the plant and not the stalk that supports any single leaf. A stem is normally divided into nodes and internodes. The nodes often produce a complete leaf with a petiole as well as producing roots and buds which may grow into shoots of various forms. The stem's roots anchor the plant either to the ground, a tree or to a rock. As you can see from this definition the word "stem" is not applicable to the part of a leaf correctly known as the petiole. However, species such as Amorphophallus produce a divided leaf. It may look like a bunch of leaves, but there is only one leaf.
A bulbil is a name used for reproductive structures such as those formed in the junction of veins in some Amorphophallus species. It is a useful word that can be used without defining it much more precisely.
A bulblet or tubercle forms at the juncture of the petiole and the stem in only a very few aroids including the ZZ plant, Zamioculcus zamiifolia. That little bublet is just a very tiny stem which allows a leaf that falls to the ground to form an entirely new plant.
A corm is an underground stem to which the above-ground parts of the plant may die back in the dormant season. It often stores starch and when it regrows, foliage will come from the top, and roots from the base like a typical stem. There are no true examples of a corm in the aroid family.
A tuber is an underground structure which is almost entirely a starch storage organ. The buds for future growth and the roots all develop at the apex (“top”) when the tuber forms as the tip of a stolon which it often but not always does. In Amorphophallus, Arum and Typhonium the stem tissue is encased in the small bud at the top of the tuber full of stored food.. That bud grows upward into a leaf or two and outward into roots with the tuber beneath.
Confused yet? I hope this helps to clean up the mess:
The tiny offsets you sometimes see growing in a pot with Alocasia and other similar species are just baby tubers. Tubers are corm-like but a tuber is different than a corm.
While working on my article my friend and scientist Christopher Rogers wrote the following, "a corm is composed entirely of stem tissue. It is literally just an underground stem. It has an epidermal layer, a vascular cylinder with phloem and xylem and central pith. A corm can also be a starch storage organ, but it still has true stem tissue. This is why a corm has the new foliage growth coming from the top and the roots coming from the base. Corm examples are Crocus, Cyclamen and Gladiolus. A cormel is just a diminutive corm.
A tuber is just parenchyma (with some vascular tissue). It has an epidermal layer with some subdermal vascular tissue, and all the rest is parenchyma. It is almost entirely a starch storage organ. This is why the foliage and the roots all come from the top. Most plants with tubers have them borne on stolons, but that is not necessary. In Amorphophallus, Arum and Typhonium for example, the stem tissue is all encased in the small bud at the top of the tuber. That bud grows upward into a leaf or two, and outward into roots, with the tuber beneath. Other tuber examples are potatoes and Sinningia."
A bulb is composed of thick modified leaves arranged in layers that are used for food storage. Slice open an onion! An onion is a perfect example of a bulb. As I hope you can understand an onion is not like the tubers that grow from an aroid. The term bulb is used far more commonly in horticulture but never in relationship to an aroid, at least by a scientist.
Although many authors use bulb, corm and tuber interchangeably the only term that is truly applicable to an aroid is tuber.
I realize some of Christopher's terms are technical but I'm going to do to all of you what Dr. Croat does to me all the time. He makes me go look up the terms! I would strongly suggest all of you buy a copy of the Oxford Dictionary of Plant Sciences so you can learn what terms like these really mean. It is cheap on Amazon.com
I'll give you a bit of help and tell you the xylem is just a network of hollow cells found in a plant's vascular system that transports water and soluble nutrients collected by the roots. Look up the other words and you'll start learning a bunch more about the plants you love!
Now, here's some additional info published on Aroid l just this week by the top Amorphophallus expert in the world, Dr. Wilbert Hetterscheid . Please understand Wilbert is from the Netherlands and English is not his first language. Even though some of his writing may be considered "broken" I have elected not to edit what he wrote, " Tubers in Araceae are condensed stems with food-storage function. Such stems can also be elongate and creeping and then we call then rhizomes.
In Araceae rhizome and tuber are two parts of a "continuum". One the one extreme are long creeping rhizomes with numerous long internodes. Condensation of the internodes give us shorter rhizomes. Reduction of the number of internodes gives us shorter rhizomes (meaning mostly that the decay of older internodes is fast). We may find very short rhizomes (one or two internodes), which because they may be thicker than long, look "tuberous" (Typhonium). Also internodes may be way shorter than their width, which also gives the rhizome a kind of tuberous look (e.g. Arum). And then finally there's rhizomes that produce one internode in a season and at the same time devour the previous internode and they may also decide to grow vertically. That's what we see in e.g. most Amorphophallus and e.g. Sauromatum, several Arisaema. In these genera oft en few or more species are in fact fully rhizomatous, which goes to show how easily one state changes or reverts to another. In Amorphs there is even a fully genus-exclusive extra: the one-nodal-rhizome (we call tuber) may elongate vertically, not by creating extra nodes but by elongation of the one node present (A. longituberosus and like).
So, a majority of Amorphs have an upright, subterranean, one-nodal rhizome and we call that a tuber. The best illustration of the "ancient" condition is in Am. coaetaneus, where a chain of swollen, "tuber like" nodes is present. This species has decided not to devour old nodes but keep them intact, so a chain of "tubers" develops (see IAS website under this species) and this chain is in fact again a full scale rhizome. But in the same genus we can also find "normal" rhizomes with many nodes and equally thick all over (A. rhizomatous, A. hayi). The conditions are therefore evolutionarily interchangeable because they are several sides of the same medal."
Dr. Croat who I consider my mentor followed up with this after reading Christopher's distinctions, "That is an excellent distinction that you made for the difference between tuber and corm. I have always assumed that the corm was non existent in Araceae since most storage organs called stems are just a big bag of starch." Based on info from Dr. Croat and other scientists tuber is the most applicable term for an aroid but bublet does apply in some genera. Tuber would therefore be the correct term for what we often like to call a "bulb". All of you can meet Tom at the next meeting of the International Aroid Society MidAmerica Chapter in St. Louis on April 24. I will give a short presentation about my atrium as will other aroiders on subjects that are not terribly technical.
Tom will then lead us on a tour of the research facilities as well as the aroid greenhouse which contains the largest aroid collection in the United States. That is something you won't ever get a chance to do again since it is closed to the public! You can find all the details on the IAS website http://www.Aroid.org
Still confused? Read all of it several times! I hope it all sinks in but if not ask and I'll see if I can figure it out before I open my mouth again!
I'm sure there is nothing terribly wrong with using the term "bulb" for stem forms that are not truly bulbs but at least the explanation is here for those that want to know.
Last edited by ExoticRainforest on Tue Feb 16, 2010 9:02 am, edited 1 time in total.