Thorny questions

John Grehan jrg13 at PSU.EDU
Mon May 15 16:02:17 CDT 2000

I missed out on the intial discussion on plant thorns, but I can offer
some addtional comment to complement those of Pierre Deleporte
and Richard Brown on the problematic nature of using adaptation to
understand thorns. I have not read "In search of deep time" questioning
 adaptationism, but I can suggest that any effort to understand the
functionality and "adaptive" significance of plant or animal form
also include a deep appreciation (structural analysis) of the structures
in question. This is often not a subject of concern when dealing with
adapations, yet predicted adapation senarios depend greatly on what
a structure is thought to be in the first place. Thus, for the thorn there
is a general question of the thorn as a structure.

Croizat's 1961 Principia
Botanica gave extensive consideration to many plant structures where
homology was widely assumed, but a more critical or detailed examination
suggested alternative possiblities. As it has been a long time since I read
on the thorn in the Principia I cannot comment other than my recollection
that the thorn is the end result of a process involving the hemming in
of primordia formely responsible for the growth of various plant
structures - in some cases possibly pre-angiospermous in origin.
The impact of thorns upon grazing may be more a consequence than a factor
in their origin.

In New Zealand there is a popular interpretation of divaricating growth
in various shrubs being defensive adapations in response to Moa
browsing. This explanation is a good example of how the growth
form itself is sidestepped so that there is no greater understanding
of divarication, or of its biogeographic as well as morphological
distribution among plant taxa.

John Grehan

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