corm; was RE: Late fall in Maryland

J.E. Shields
Mon, 14 Dec 2009 08:13:12 PST
I looked both terms up in the glossary of John Bryan's "Bulbs" revised 
edition.  I don't always blindly trust John, but he did have help putting 
that volume together.

Corm -- underground storage organ, a swollen part of an underground stem

Tuber -- underground root modified as a storage organ.

The modifier "underground" seems to eliminate pseudobulbs and other 
above-ground structures.

I myself have a hard time distinguishing some tubers from corms.  I tend to 
think of corms as annual or at most biennial structures and tubers and 
bulbs as perennial structures.  Is that reasonable?

Our emphasis on geophytes also weakens the inclusion of most but not all 
orchids from treatment here.  I think my Cymbidiums are over the line into 
"Off Topic" plants.

Jim Shields
in Westfield, Indiana
USA, where the sun is trying to shine and the temperatures are a balmy 40°F 
(+4 C).

At 10:50 AM 12/14/2009 -0500, you wrote:
>Both Jane McGary and, in a private response to my Begonia boliviensis
>"Fireworks' post of a few weeks ago, another list member, seem to be gently
>taking me to task for calling the underground parts of the Chilean
>Tropaeolum and Begonia corms instead of calling them tubers.
>I would argue that corm is a more inclusive and appropriate term. The
>etymology of the word corm (it's derived from the Classical Greek word for
>tree trunk or stump) calls attention to the fact that the structure in
>question is a stem of some sort. And corms are the true perennial stems of
>the plants in question.
>In another recent post, Jim Shields mentioned pseudobulbs. This term in my
>experience is used exclusively for orchids, yet the structures in question
>seem to be corms. The structure from which Pleione grows is little different
>than the structure from which Colchicum grows.
>The way I think about it, the corm is the basic structure which these plants
>have in common: corms are the modular perennial stems of these plants.  All
>true bulbs have at their core a corm (in the sense that the basal plate is
>the perennial stem of the plant, in effect a corm). Many corms seem to have
>evolved from ever shortening rhizomes, something easily seen in the
>Iridaceae. But I don't think our concept of corm should be limited to what
>we see in the Iridaceae.
>That we have several terms for what seem to be essentially comparable
>structures is not surprising: the plants which have evolved with these
>structures are not necessarily closely related. Most of the geophytes we
>grow are monocots, but a few are dicots (Begonia, Cyclamen, Dahlia, Oxalis,
>Tropaeolum and many gesneriads are familiar non-monocot examples). These
>non-monocot plants have not shared a common ancestor with the monocot plants
>since the early Cretaceous (i.e. perhaps as much as 145 million years ago),
>and there is no reason to think that corms are inherited from a common
>ancestor - rather it makes sense to assume that they developed independently
>over and over in different groups,  so we should not be too surprised that
>the morphology of corms varies.
>The next time you are about to call something a tuber, give some
>consideration to the fact that the structure in question might be the
>modular perennial stem of the plant in question, in other words a corm.
>Jim McKenney
>Montgomery County, Maryland, USA, 39.03871º North, 77.09829º West, USDA zone

Jim Shields             USDA Zone 5             Shields Gardens, Ltd.
P.O. Box 92              WWW:
Westfield, Indiana 46074, USA
Tel. ++1-317-867-3344     or      toll-free 1-866-449-3344 in USA

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