Acceptable Oxalis

Jim McKenney
Fri, 01 May 2009 12:31:07 PDT
Yes, Dylan, it’s a fascinating and variable topic, and one which teaches us
that the categories we dream up can be very hard to apply to what is really
happening on the ground. 

In the interests of science    :  )     I’ve sacrificed and autopsied one of
my bulbs of Oxalis lasiandra to show the gross anatomy. It’s about the same
as that of a hyacinth or culinary onion  bulb. In the image shown in the
link below, you will see that the percentage of storage tissue derived from
leaf bases is pretty high – much higher than the percentage derived from
stem tissue (in the form of the basal plate aka perennial stem). 
Take a look here: this should resolve any doubts about at least one Oxalis
having a true bulb.…

Keep in mind that even in monocots there are genera some of whose members
have true bulbs, some others have true corms, some others have rhizomes. To
my eyes, Oxalis lasiandra has an undoubted bulb. But other members of the
genus have worked out other arrangements: Oxalis enneaphylla for instance
has something reminiscent of what we see in Achimines, a sort of scaly
rhizome where the perennial stem probably serves as a more important storage
organ than do the attached scaly modified leaves. Since I can barely keep
this one in growth here, I’m not going to cut one open to see exactly what’s
going on. Someone else can step in and make that generous offer. 

You cited the criterion that a corm “replaces” itself yearly. What in the
world can that possibly mean? The new corm is simply an outgrowth of the old
one, or to put it differently it is the newest, liveliest part of the old
one.  Depending on the growing conditions, crocuses will sometimes produce a
new corm on top of the shriveled remains of the old one. But so long as both
are alive, the new one and the old one are never distinct entities, they are
both part of one living entity. It’s true that the older portions typically
shrivel up and die, but it sometimes happens that they do not. In Crocosmia
one can often find chains of corms connected only by long, thin stolons. The
older ones do not necessarily sprout, but they are not dead and they have
not “replaced” themselves because they are still there.   

In some aroids, the distinction between old and new is even less clear: the
rotting and decomposition of corm tissues takes place in healthy plants in
the oldest parts of the corm; but in many aroids the corm is a lot like the
rhizome of a bearded iris. It has a primary axis of growth, and as long as
the plant is healthy, the rotting on the old end never catches up with the
growth on the new end. In those cases, I would not want the job of saying
where the old corm ends and the new one begins – or to determine what is
original and what is replacement.

You wrote “I read somewhere that indeed tulip bulbs (Calochortus, too, I
are different in that the bulb is derived from modified cataphylls or
prophylls rather than leaf bases”.  Since I was speaking broadly, I was
including cataphylls and prophylls in my concept of leaf (as the etymology
of the words suggests). But I think what you really meant to say is that the
storage tissue of tulips (not the entire bulb) is derived from cataphylls
and prophylls. The basal plate, perennial stem, “corm” of the bulb has a
separate origin.  

You also wrote “Since the stem tissue portion of a 'normal' bulb is not
replaced each season it would not qualify as a corm,” . But the stem tissue
component of a tulip bulb does “replace itself” yearly: the new bulb forms
on a new extension of the old, existing basal plate. The old portions and
the storage leaves attached to them eventually die. That origin from the
existing basal plate  is essentially what happens in a typical corm, too. It
seems to me that both typical bulbs and typical corms both “replace”
themselves the same way: by budding along the perennial stem. 

You’re right to quibble about my calling the basal plate of a tulip a corm:
of course there are common sense differences. But both are essentially stem
tissue, both are the perennial stem of the plant in question. Reread what I
wrote substituting “perennial stem” for “corm” and see if it does not become
more agreeable to you. 

For me, once I understood the significance of the concept of perennial stem
in these bulbous, cormous, tuberous, and rhizomatous plants, it all came
together and made sense. People get hung up on the distinction between a
bulb and a corm because they go into the discussion thinking that these two
things are something fundamentally different, and that good definitions will
make that difference clearer. 

I take the other approach: what we are dealing with here is not a difference
in kind, it is a difference in degree. I try to understand them in terms of
what they have in common, not as fundamentally different concepts but as
concepts which share certain similarities. After that, the “difference”
becomes irrelevant except as a difference of degree. And as the references
you consulted show, there is plenty of leeway in the degrees of differences
of opinion on what these things mean. 

Jim McKenney
Montgomery County, Maryland, USA, 39.03871º North, 77.09829º West, USDA zone
7, where little Calochortus uniflorus is blooming.
My Virtual Maryland Garden
Webmaster Potomac Valley Chapter, NARGS 
Editor PVC Bulletin 
Webmaster Potomac Lily Society

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