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

jimmckenney@jimmckenney.com

Montgomery County, Maryland, USA, 39.03871º North, 77.09829º West, USDA zone
7