horsetails

[doug jensen]

The group that the horsetails belong to has a very long and distinguished fossil 
record.  Some members of the lineage (eg. Sphenophyllum) had 'sphenophylls', 
dichotomously branched leaves which were very similar to Ginkgo leaves 
morphologically.  Other members (e.g. Annularia) had elongate leaves with a 
midrib, but no lateral veins, as far as I know.  Whether to call these 
megaphylls or microphylls, I believe, should lie in their evolutionary history.  
If they evolved via the enation theory (of Bower), then they are microphylls, 
regardless of how many veins (remember the Selaginella with branched veins); if 
they evolved via reduction of a telome truss (Zimmerman, etc), then they are 
megaphylls, regardless of how many veins they have (I think pines have only one 
vein, yet we don't call them microphylls).
    The origin of sphenopsid leaves from telome trusses is quite well-supported 
through fossils--Cladoxyls are considered the progenitors.  By the way, the 
fossils also have some intermediate stages regarding the origin of the 
sporangiophore. 
    The problem I have is a lack of knowledge in the specific direction of 
evolution with regards to Equisetum leaves.  Were they reduced from megaphylls?  
or did they not evolve further into leaves in the first place?
    My short answer to the question is that I have no problem calling them 
megaphylls, but I don't know if they are reduced or unexpanded (pre-expanded??) 
megaphylls.  My guess would be reduced, but then so much for theory-free 
morphological observations......

Doug Jensen
Berea College

_______________________________________________________________________________
Subject: Re: horestails
From:    Koning at ecsuc.ctstateu.edu (Ross Koning) at Berlink
Date:    11/12/96  9:49 PM

I'd love someone in evolutionary plant anatomy to tackle this
question from Scott.  I have always taught microphyll, and
maybe they are that way now, but perhaps they arrived there
by reduction from a megaphyll?  This is a great question and
deserves a good answer by someone qualified to comment on the
evolution of fern fronds.

ross

At  5:10 AM 11/12/96 -0800, Scott Shumway wrote:
>How would you describe the leaves of horsetails?  Raven et al's textbook
>describe them as "reduced megaphylls", but Stern's Intro to Plant Biology
>calls them microphylls.  Which one is correct?
>
>Scott Shumway
>Dept. of Biology
>Wheaton College
>Norton, MA 02766
>508-286-3945
>"Scott_Shumway at WheatonMa.edu"




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