What is a succulent - was Schizobasis intricata

Jim McKenney jimmckenney@jimmckenney.com
Wed, 20 Aug 2008 11:14:41 PDT
Dylan, I'm sure you've already considered everything I'm about to say here,
but bear with me for the sake of those who might not have. 


Of course, we're trying to make up the definitions a posteriori: the events
which caused things to be as they are are long in the past.  


Good examples, Dylan. In particular, thanks for introducing the concept of
modularity into the discussion. It’s sine non qua for bulbs and corms. You
also wrote " Scouring various glossaries and botanical dictionaries it is
remarkable to find that there is little clarity or consensus on the
definition of "corm". If "corm" is only defined as being stem tissue then
there is no boundary between tuber and corm or thickened stem, etc."


How true: I can still remember when, after years of gardening, I heard
someone for the first time call the tuber of an aroid a corm. At the time,
that really irritated me (i.e. it didn't agree with my bad ingrained
habits). But for me, that usage was the beginning of understanding what a
bulb is. It was the beginning of my realizing that the nature of stem tissue
was the key to relating and understanding these concepts. 


Aren't there two main reasons why it's so hard to pin down these concepts?
For one, no matter which morphological features one studies in nature, there
are almost always intermediate forms which don't fit our neat definitions.
And if the intermediate forms don't presently exist, we invent them for
discussion's sake. For another, these various structures have evolved
independently in a bewilderingly wide range of plants. 


When you wrote about "no boundaries" you might very well have been alluding
to the sort of evolutionary paths by which these structures developed. 


It was a big step in my understanding of things when it finally dawned on me
that there are non-monocots which have "bulbs". Some of those non-monocots
have true bulbs (some Oxalis for example); some of those non-monocots have
the things gardeners call bulbs for convenience (for instance Cyclamen).  


One example I like to use is the condition seen in the gesneriaceous genus
Achimenes. Gesneriads are dicots, not closely related to the majority of the
plants we think of as bulbous. 


Just what is the structure from which the annual stems of the Achimenes
grow? Most references hedge on this: "scaly rhizomes" is one description
I've often seen.


Have you ever seen the bulb of Lilium superbum? Superficially, it looks
pretty much like the structures produced by Achimenes. Are they (in either
case) bulbs? Are they something else? The lily people sometimes call such
structures "rhizomatous bulbs". I don't like that term: after all, the bulbs
are not rhizomatous, the case is that these lilies have rhizomes which bear
bulbs. I call them bulbiferous rhizomes. 


One of the advantages of science is that if we're determined to have
answers, in many cases all we have to do is quantify things and then pick
(arbitrarily) limits for each concept. This opens the door to great
precision of usage, but it also tends to spoil the poetry. 


In the case of the Achimenes/Lilium superbum structures, we can begin by
saying "if they are bulbs, then the majority of the storage tissue must be
in the form of modified leaves". Quantification of the amount of storage
tissue in the form of modified leaves on the one hand versus the amount in
some other form (stem tissues) will provide an answer. We can then say "if
more than 50% of the storage tissue is in the form of modified leaves, then
we have a bulb."


I'll bet that most people find such finely reasoned distinctions vaguely
repellent; but they do provide a logical way of both asking and answering
the question. 


On this forum of all forums, there ought to be a lot of opinions about all
of this: I can't wait to hear more.


Thanks, Dylan. 



Jim McKenney


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

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