Acceptable Oxalis

Jim McKenney
Wed, 29 Apr 2009 09:47:19 PDT
Dylan asked "Do any other dicots have what could be called bulbs?"

Offhand, I can think of several generally accepted examples and several
others which are more controversial - it all really depends on your
definition of bulb.

But if by "bulb" you mean something like a periodically free standing
structure made up of a central bit of apical meristem (the bud for
vegetative growth) surrounded by succulent scales which serve to protect and
nourish the bud during dormancy and which are themselves modified leaves or
leaf bases...then several examples come to mind. 

In addition to Oxalis, there are Drosera with similar structures. 

Gesneriads such as Achimines have structures which are bulb-like (they are
much like the so-called rhizomatous bulbs of some lilies).

For my own purposes, in classifying these intermediate types I look at the
degree to which the storage tissue is contained in modified leaves vs. in
some other way (for instance, in the bulk of the rhizome). If modified
leaves are the primary storage organs, then I put them in the bulb category.

The swollen bases of Florence fennel (in contrast to the non-swollen bases
of the wild form of Foeniculum vulgare) have been called bulbs by those
looking only at the morphology, but Florence fennel "bulbs" do not function
as typical bulbs. For one thing, they are always rooted and never free

Having said that, I've opened another can of worms because there seem to be
tropical plants (monocots related to plants with undisputed bulbs) with a
seemingly bulbous morphology but which never have a free-standing phase. As
an example, the structure from which Crinum grow is morphologically a bulb,
but since in some species there is never a dormant period and the "bulb" is
permanently in growth, can it be said to have a free-standing phase? And
without a free-standing phase, can it really be considered a bulb? Can
something be a bulb part of the time and not a bulb the rest of the time? 

In the same way, there are morphological similarities between typical bulbs
and plants such as rosette forming Sempervivum and Crassula, but these
typically do not function as bulbs (and the leaves in these cases are fully
functioning leaves, not leaves modified as storage organs). A week or so ago
I received some bare root Sempervivum in the mail. In that state they
reminded me of lily bulbs. 

However one draws the lines, it's obvious that dicots have not exploited
this morphology in the way monocots have. I wonder why? The bulb-like
structures in dicots generally rely on modified stem tissue as storage
structures, whereas in monocot true bulbs the role of the stem tissue is
simply to form the nexus for all the other structures.  

Sorry to have wandered on so: I’ve grown bulbs all my life and have to laugh
when I realize that I’m still not able to give a watertight definition of
the term. But then again, I’ve lived long enough to realize why there need
not be one. 

But to answer your question, Dylan, yes, there are at least a few true bulbs
found among dicots. 

Jim McKenney
Montgomery County, Maryland, USA, 39.03871º North, 77.09829º West, USDA zone
7, where a spell of July-like weather with daily highs in the 90sF has
finally ended: fried tulips, anyone? 

My Virtual Maryland Garden
Webmaster Potomac Valley Chapter, NARGS 
Editor PVC Bulletin 
Webmaster Potomac Lily Society

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