Corms & Tubers

[McClements, Jim]

Martyn et al

As a non-botanist, I have gone back and forth over the past few years trying
to decide whether an arisaema grows from a corm or a tuber. Following
Tuesday's posting, I made an effort to research the question in my own
(admittedly limited) library, starting by going through my folder of
literature on the genus.

The following authors, or sources, use "tuber": Jenn-Che Wang, Engler, Flora
of China (translated), Pradhan, Judy Glattstein, Chatterjee, and Roger
Hammond.

"Corm" is favored by the following: Grey-Wilson, Bierzychudek, Gusman, and
Yadav.

Mayo, in his 1982 article says that arisaemas grow from "corms, often called
tubers by aroid botanists". In a subsequent article I find him using "tuber",
so apparently he became converted!

Ohwi, in Flora of Japan describes arisaemas as "perennial herbs with a
depressed globose tuber or corm".

So, what about glossaries? (One gets the feeling that the wording in these is
repeated one after another).

Hortus III defines "corm" as a "solid, swollen part of a stem"; a tuber is a
"short thick stem or branch bearing buds or eyes, serving as a storage organ,
as a potato"

Griffiths follows the initial part of the above description of a corm but
then adds that it is "annual, next year's corm developing from terminal bud
or one of lateral buds".  His definition of a tuber includes that it arises
from a stem, branch or root and is used for storage.

I then turned to a book which I have found helpful in such matters, namely,
"Botany for Gardeners" by Brian Capon. The following descriptions are found:

Corms:  "are short, swollen underground stems, surrounded by the remains of
the previous year's leaf bases; regular roots and contractile roots emerge
from the underside of the stem. Parallel lines seen on the defoliated corm
are nodes, separated by broader internodes. Growth of leaves and flowers
occurs at the expense of the corm's total food reserves. Following flower
formation, one or more new corms develop above the old organ, which
eventually withers."

Tubers: ---"a stem tuber is the swollen tip of a rhizome, not of the fleshy
type. Like rhizomes, stem tubers display the characteristics of the organ
from which they are adapted: axillary buds (the "eyes") are located at nodes;
the areas between the eyes are internodes. When the buds develop into shoots,
adventitious roots and rhizomes form beneath them; the rhizomes ultimately
bear more tubers. The food reserves of a tuber are completely exhausted by
such growth processes."
He goes on to say that "rhizomes, and root and stem tubers are used
extensively by horticulturalists as propagative organs, since they can be cut
into pieces and planted."

I think it is fair to say that arisaemas do not fit well into either of these
definitions. If they are "corms", they certainly are not "annual", the total
food reserves are not used up and the "old organ" does not eventually wither.

If they are "tubers" or "stem tubers", again they don't exhaust their
reserves, can't be cut into pieces and planted (at least not with success),
and have little resemblance to a potato. The latter factor is what probably
got me started calling them corms!

I think we could all probably agree that an arisaema grows from an
underground swelling of the stem, which stores starch and enlarges year after
year. It forms ANNUAL roots each year, often from the top of the structure.
Offsets can be formed, and in some species, stolons, which can be removed
from the parent structure and grown on, but the structure itself can normally
NOT be successfully divided for propagation.

A majority of writers and botanists seem to prefer "tuber" to "corm" for this
structure, but I'm not sure that anyone can flatly declare that
"corm-preferrers" are dead wrong. Arisaemas are certainly dissimilar to the
classic "corms", crocus and gladiolus, but look even less like a potato.

Does anyone know another genus with a similar structure, and, if so, does the
same question arise? (Martyn mentioned Cyclamen; is it really the same?)
Perhaps arisaemas should have a name of their own? Torm or cuber doesn't
seem to do it.

Jim McClements