Tubers, corms and bulbs, oh my!

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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