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Re: Tubers, corms and bulbs, oh my!

  • Subject: Re: Tubers, corms and bulbs, oh my!
  • From: Hannon <othonna@gmail.com>
  • Date: Sat, 13 Feb 2010 02:31:30 -0800

Thanks to Wilbert for a very good synopsis of 'compact rootstocks' or
tubers in Araceae. As a first time Aroid-Ler and longtime aroider I
would like to offer a few comments.

I think what Wilbert elaborates here makes perfect sense. My only
quibble is that if we make no use of existing descriptive terminology
then we use more words than we need to to describe things, or we use
words that have imprecise or vague meanings. If all these structures
are rhizomes or (apparently) derived evolutionarily from rhizomes,
that is very useful as a concept. Yet within this scheme there are
structures that can be differentiated, if arbitrarily, into more
narrowly defined notions such as tubers, rhizomes and corms.

Some of us are weary of any confining definitions altogether. In the
case of rootstocks especially it is difficult to imagine any taxonomy
that could capture and discretely name the different types found in
nature. In fact it has not been done. Still, it seems to me that a
corm is something quite distinct, even if its origins are betrayed by
peculiar congeners, as in Wilbert's Amorphophallus example. Any
modular unit that may resemble a tuber or bulb and _replaces itself
each growing season_ can rightfully be called a corm, regardless of
the number of nodes/internodes involved or features of tunics vs.
naked flesh. Annual replacement is the key trait of a corm.

Other families exhibiting "true" corms that are replaced annually
(sources in the botanical literature give inconsistent definitions)
are the iris family (Gladiolus, Crocus, etc.), Themidaceae (Brodiaea,
Milla, etc.) and some Cyanastraceae/Tecophilaeaceae. "Chains of
tubers" can be found in Dierama and probably others and perhaps
similar ontogenic processes have been at work in these families. As
far as I know the rootstocks of these plants have traditionally, in
botany, been labeled "corm" and not tuber or bulb. This concise
designation continues to serve a useful purpose for the scientist and
layman alike. As Wilbert indicates, we have "traditionally" called the
tuber-like structures in aroids, well, tubers.

If we say there is only a morphological continuum, or that all the
rootstock types in a group are basically rhizomes, then clarification
and elucidation are sacrificed.

Dylan Hannon

On 11/02/2010, Wilbert Hetterscheid <hetter@xs4all.nl> wrote:
> My 2 cents of superfluous wisdom in this:
>  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. Condensaton 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).
>  My opinion is that "tuber" in Aroids (and in many other families) is more of an "appearance" term, than a truely reliable systematic term. Aroids have stems, which may be creeping, food storing [but not always] and semi- of fully subterranean and then are called rhizome. The shape and structure of a rhizome may be so that it looks like a tuber (and my guess is that "tuber" is an ancient relict word for a food-bringing underground plant part, and as such a human-usage driven term).
>  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. rhizomatosus, A. hayi). The conditions are therefore evolutionarily interchangeable because they are several sides of the same medal (strange medal THAT is.........).
>  Cheers,
>  Wilbert
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