[Re:] Thorny questions
termites at USP.BR
Tue May 16 10:54:39 CDT 2000
I didn't read Croizat's Botanica, but surely there are plants which are not
angiosperms which do have thorn-like structures. I did not understand
thoroughly the preceding thread, but it seemed that it was about the open
formation/forest debate, that is whether plants in open formation would wear
thorns as defense against huge mammalian vegetarian browsers or something like
Again, some of these plants which are not angiosperms and which have very heavy
thorn-like structures occurr in the dense rainforest.
Also, I vaguely remember that, in Germany, I learnt that there are two
different thorn-like defense structures, namely "Dornen" and "Stacheln". I
wonder whether this distinction meets any equivalent in English and what was it
about and if it has anything to do with the preceeding.
On ( Mon, 15 May 2000 16:02:17 -0500), John Grehan
<jrg13 at PSU.EDU> wrote:
>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.
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