Re: Tubers, corms and bulbs, oh my!
Title: Re: [Aroid-l] Tubers, corms and bulbs, oh my!
While I would agree that the recent discourse on tubers vs. corms certainly makes clear an excellent distinction, the difficulty I have with it is that this is not what many sources suggest. Several examples:
From Swartz, D. 1971. Collegiate Dictionary of Botany:
Tuber – a relatively short thickened rhizome with numerous buds as in the potato; a subterranean stem which is shorter and thicker than the root stock
Corm – a solid fleshy underground base of a stem, usually somewhat spherical in shape, covered with thin membranes. It serves for storage of reserve food materials and resembles a bulb in appearance but not in structure
From Radford, A.E., Ahles,H.E., & Bell, C. R.. 1964. Manual of the Vascular Flora of the Carolinas:
Tuber – a fleshy enlarged portion of rhizome or stolon with only vestigial scales; true tubers found in the Solanaceae
Corm – a bulb-like structure in which the fleshy portion is predominantly stem tissue covered by membranous scales
And within the text, from Family 31. Araceae (The family description begins) Perennial herbs from rhizomes or corms...
From Radford, A.E., Dickison, W.C., Massey, J.R., Bell, C.R. 1974. Vascular Plant Systematics:
Tuber – a thick storage stem, usually not upright
Corm - the enlarged, solid, fleshy base of a stem with scales; an upright underground storage stem
And from Stearn, W.T. 1983 (3rd Edition, revised) Botanical Latin:
Tuber – (in part) tuber large globose or irregular, with flesh whitish bitter
Corm - literally ‘a trunk’; - for the solid bulb-like stem-base of monocotyledons, the older authors, e.g. Ker-Gawler, used bulbo-tuber; cormus magnus sub-globosus 5 cm. crassus tunicis brunneis membranaceis, corm large almost globose 5 cm. thick, with tunics brown membranous.
There is no debate regarding bulbs – a bulb is an underground storage structure comprised of thickened foreshortened leaves all attached to a basal stem disc. But with the real debate at hand, whether what we are looking at, definitely a modified stem of some sort (and the sort is not uniform across all the genera mentioned, nor even always within the same genus) is corm or tuber is I would say still up for debate, depending on who’s definition of the terms one wants or decides to choose.
As for rhizomatic(?) (rhizomatous) tubers or tuberous rhizomes, I’m sure others may have a more serious vested interest in this debate, but to me they’re both one type of stem serving as an adjective for another type of stem, and which one chooses to put first is more a matter of semantics, or else simply which aspect of the stems growth one notes as being more significant, or maybe simply which was noted first. This of course is not the case with the example of a sweet potato, which is a tuberous root, i.e a root structure that closely resembles a tuber. Though by no means the only difference, a major part of the difference here is how the vascular tissue is arranged. Between stem type ‘a’ and stem type ‘b’ the challenge may be a bit more difficult because in both of them the vascular tissue conforms to the stem type arrangement. This vascular tissue is amply present in both, perhaps even more so in the tuber since tubers typically have more than one eye, i.e a new stem starting point. And although I am sure that there are examples within the aroid genera mentioned, since the Sinningia genus (Gesneriaceae) was also cited for tubers and I am even more well acquainted with these I would point out that roots grow out from many different areas of the tubers, definitely not just from the top.
Not to purposely remuddy the waters, but there it is. Happy days, and good growing.
On 2/4/10 1:02 PM, "Christopher Rogers" <email@example.com> wrote:
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.
D. Christopher Rogers
Senior Invertebrate Ecologist/ Taxonomist
P.O. Box 4098
Davis, CA 95616
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