Hiyer!
I recently had a discussion with few Aroid Oriented Individuals about proper terminology for the non-root, subterranean aroid parts. Or to put it another way, do plants like Amorphophallus, Arum, Helicodiceros, Typhonium, Colocasia, Ambrosina, and the like have tubers, corms or bulbs?
The answer is that they have tubers. (Or for dear Julius’ sake, “chubas”).
A bulb is composed of thick, modified leaves, arranged in layers, for food storage. An onion is a perfect example.
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 bulbil, in the aroid sense, is just a tuber that forms on leaves or leaf axils. It is an unfortunate term as it obviously leads to confusion.
I really hope that this is helpful to the Aroid community at large, and I hope it cuts down on some of the confusion surrounding these terms. I am sure Pete, Wilbert, Tom, Julius, The Banta or someone can elucidate further, particularly as far as tuberous rhizomes or rhizomatic tubers are concerned.
Happy days,
Christopher
D. Christopher Rogers
Senior Invertebrate Ecologist/ Taxonomist
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