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Tuber vs. Corm


I suggest we file this discussion somewhere, so that it doesn't
resurface all the time. My two Euro-cents:

Bulb: this is a condensed stem or stem part, INCLUDING all appendages!
Therefore a bulb shows e.g. scales, which are modified leaves, used for
storage (Lilium!), or storage scales and some outer, thin, protective
scales (Iris, Crocus). Obviously no aroid exists (....I hope...) that
shows such a device. Quite often, bulbs also contain the new season's
growth in condensed form (inflorescence etc.).

Tuber: modified stem or stem part, for storage WITHOUT appendages (or
non-modified ones). So, in a bulb, the appendages form the main storage
facility, wheras in a tuber, the stem (-part) itself swells and performs
the storage function. Therefore a tuber has no thickened appendages but
may have thin scales protecting buds or whatever. A tuber may contain
more than one growth-module (e.g. some Anchomanes species) or just one
(Amorphophallus). 

A rhizome is in my opinion nothing more than an elongate tuber (or vice
versa), often containing more nodes (some Arisaemas), more rarely only
one (some Typhoniums). .

Corm: here we get into trouble with differentiating this from tuber. One
book I have claims that a corm is annual but the rest of the definition
is identical to tuber. This would fit a tuber (consisting of one node)
that is renewed every season, which would include 97% of Amorphophallus.


In Dutch, there is no distinction between tuber and corm, they are all
called "knol", whereas a bulb is called a "bol". So much for Dutch.....
However, I fail to see a significant difference between corm and tuber,
but any colleague of mine reading this is welcome to correct me. 

This leaves one phalloid phenomenon: the "chained tubers" of Am.
arnautovii, coaetaneus and pingbianensis): here we see a new tuber built
every year BUT the old ones do not die immediately and remain,
developing a chain of tubers. This could well be an intermediate
situation between a multi-noded rhizome and a single annual tuber. No
real name for it. It is technically called a "moniliform rhizome". Go
figure.............

Now to some of the individual contributions to this discussion:

Don Burns: Amorphophallus has a tuber, with the exception of three
rhizomatous species (hayi, verticillatus and rhizomatosus) and the
aforementioned "moniliform" ones.. In the strict sense, accepting the
corm as something different, it would be corm, based on the annual
nature of it. But I use "tuber".

Dewey: the remark of your PhD guy about cells is unknown to me. I have
found no definition using this character. Anybody?

Jeanne: your supplier is wrong in using "bulb" for tuberous (what's in a
word!!) aroids. Proper pronunciation of Latin botanical words/names is a
pure fiction. All Romans who knew about it are long since dead..........
Don't make a fuss of it. Every claim made that there are internationally
accepted pronunciation rules is false.

Paul Resslar: I don't think a corm needs to have scales. I found no
definition to that effect. If this definition would be the correct one
though, then we would indeed have a more useful distinction between
corms and tubers. The "rhizome"-like Amorphophallus tubers you mention
(like as in longituberosus, albispathus, Peudodracontium species) are in
fact vertically elongate, mono-nodular tubers. They do not essentially
differ from the globose or depressed tubers common to many Amorphs.
(however, recently I have found some species with very strange looking
vertically elongate tubers which suggest to be
multi-nodular............AAAAAARGHHHHHH!!!!!!!!!!!!!).

Thomas Mottl: yup, cormus is indeed an entire plant body with
differentiated appendages, and is indeed not a synonym of "corm". Root
tubers is another matter: as long as tuber, corm etc. are based on
definitions using a reduced stem, they are not the same thing and must
be mentioned with an adjective pointing to the location of the structure
("root" in "root tuber").

Clarence Waldron: we seem to agree that corm and tuber may not be such a
useful distinction. This is underscored by the apparent lack of a
universally known distinction based on definitions. A bulbil is
something to think about, I agree. A bulbil in e.g. Amorphophallus
bulbifer is a swollen leaf rachis part and as such technically not a
direct development/modification of the stem. A definition I found says:
a small bulb of bulb-like growth, arising from a leaf axil or in place
of a flower. This would exclude the leaf bulbils just mentioned. Maybe
the word "leaf" needs to be included for that example, as in "root
tuber". 

A bigger problem may be the term for the "offsets" that many tuberous
aroids develop.......(headache coming up............)

Bulberous greetings,
Wilbert









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