what is considered bulbous

Jim McKenney jimmckenney@jimmckenney.com
Fri, 25 May 2007 04:52:09 PDT
We've had several postings on the topic of "what is a bulb" lately, and this
most recent one by David Ryle prompts me to jump into the fray.

I'm by no means rigid about this: fitting an a-priory concept such as "bulb"
to the happenings in the real world always seems to involve a bit of

David Ryle wrote: "All plants that grow from what would commonly be thought
of as a bulb etc; 
are known botanically as geophytes. By definition a geophyte is a plant that

grows from a modified leaf that has evolved into a storage organ."

David gives us a definition; definitions by their nature are neither right
nor wrong, they're just definitions. To the extent that they work, we use

But the one he gives does not work for me. 

To say that "a geophyte is a plant that grows from a modified leaf that has
evolved into a storage organ" is to approximate the traditional,
conservative definition of "bulb", but it does so to the exclusion of many
other geophytes such as those which grow from corms or other stem tissues. 

It is the bulb (in the narrow sense) which has as a primary storage tissue
modified leaves. The scales of a lily, for instance, are modified leaves.
The major food storage parts of a tulip bulb are modified leaves.

Crocuses are geophytes, but they do not have modified leaves as their major
storage tissue; their major storage tissue is stem tissue. 

Since the one thing all geophytes that I know have is stem tissue in one
form or another, I think it makes sense to set up a definition where the
role of stem tissue is central. 

A bulb, in a sense, is a corm supplemented with storage tissue in the form
of modified leaves. I think of bulbs such as lily bulbs or tulip bulbs as a
corm (the so-called basal plate, which I prefer to call the perennial stem)
surrounded by modified leaves which serve as storage tissue. 

Since the plants which form bulbs, corms and so on are not always related
(the majority are monocots, but some are dicots only very remotely related
to the monocot bulbs), it should not surprise us that the observed
structures do not fall into neat categories.

For instance, what should we call the structure from which Achimenes grow? I
think of it as an elongated basal plate surrounded by modified storage
leaves, superficially similar to the underground parts of some native North
American lilies. I'm pretty much alone in calling the Achimenes structure a
bulb, but even those who insist that that structure cannot be a bulb will
call the similar structure in the lilies a bulb. 

One might say that if the primary storage tissue is leaf-derived, then it is
a bulb. If the primary storage tissue is stem derived, then it is something
else. I've never had the patience to remove all the little modified leaves
from an Achimenes bulb to weigh them and then compare that weight to that of
the structure to which they were attached. But you get the idea. Since
Achimenes are gesneriads, we should not be surprised that it is difficult to
fit them into a scheme originally devised to describe the storage structures
of liliaceous plants.

When I look at plants like Echeveria, Sempervivum and such I can't help
seeing the gross similarity to bulbs like lily bulbs. There is, in fact, a
purportedly epiphytic lily species named Lilium sempervivoides. 

Another intermediate condition is seen in Florence fennel, a cultivated form
of fennel in which the base of the petiole is swollen to form, with the
petioles of other leaves,  a bulb-like structure. This "bulb" grows
partially buried, partially exposed. Is it a geophyte? Does a bulb have to
be a geophyte? Can an epiphyte such as the lily mentioned above be a

It seems to me that we can't be too strict about any of this: there are too
many intermediate forms to allow definitions which are both tight and
workable. Also, in the discussion above the emphasis is on structure. But
surely life cycle should enter into any definition of geophyte. The
structure of a Sempervivum might suggest a bulb to some, but the life cycle
of a Sempervivum certainly doesn't. Florence fennel does not have an
underground dormant phase - bulb or not, it's hardly a conventional

I think we need to keep some wiggle room in our concepts. 

Jim McKenney
Montgomery County, Maryland, USA, USDA zone 7, where I've been hand
pollinating - apparently successfully -  the big pink Eremurus to get seed.

My Virtual Maryland Garden http://www.jimmckenney.com/
Webmaster Potomac Valley Chapter, NARGS 
Editor PVC Bulletin http://www.pvcnargs.org/ 
Webmaster Potomac Lily Society http://www.potomaclilysociety.org/

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