Re: Evolution of Rhizomes, Bulbs?

If one looks at the Iridaceae as a whole, bulbs are rarely found.  Most
commonly, the rootstock is a rhizome or a corm (a compressed rhizome
modifed for food storage).  A true bulb consists of swollen appressed leaf
bases which are used for dormant food storage, and also includes a basal
plate (stem) and one or more buds for the next season's growth.

A rhizome is less specialized than a bulb, because it more closely
resembles an erect stem.  In fact, the rootstocks of Pardanthopsis and
Belamcanda are more or less erect, shortened stems.  To make them rhizomes
simply requires having them lie down.  These are the two genera that are
most closely related to Iris.

Based on this, I would say that the bulb is more evolved and developed
independently in one or more sections of the genus Iris.  A further
interesting question is the relationship among the bulbous irises.  It
seems there is a failry close relationship between Junos and Reticulatas,
but how are these related to Xiphiums, if at all?

I think such questions will eventually be answered by cladistic work (such
as that reported on at the St. Louis species symposium) combined with
molecular evidence.

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@hsc.edu>
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