evolution of monocots and dicots

Scott T. Meissner smeissne at PRAIRIENET.ORG
Fri Nov 15 18:44:46 EST 1996

In responce to the question by Jim Bader for ideas about the
forces that promoted the evolution of the angiosperms from 
the bryophytes there are several that come to mind:

Lots of bryophytes compete for limited space in moist conditions.
	This promotes selection for those that can exist in 
	dryer conditions.
The development of vaculature helps some plants move further
	from their competition by going further up onto dry 
	areas, as does the formation of a waxy cuticle.
The development of heterospory (see Selaginella) helps even 
	more, especially when the growth of the gametophyte
	is endosporic.  This allows the gametophytic 
	generation some protection from dry conditions some
	of the time.  Thus new areas can be exploited on land
	that were not available before, and plants that do this
	can avoid competition with existing plants.
Seed formation frees fertilization from the need for free water,
	and this allows exploitation of even dryer areas.

At each step along the way plants move into harsher 
environments, but they are pushed out by the competition 
from established species already in exploited areas.  So you
can "pitch" this story (and that is all I have given here)
as a long example of natural selection.  

Beyond that I like to remind students that not every 
structure must have a specific and necessary function.  
After all, everything that does not kill you is allowed, and 
diversity is also of value when the next totally random 
selective event occurs.  

Hope some of this helps,

Scott T. Meissner, Division of Science and Mathematics
	McKendree College, 701 College Rd, Lebanon, IL  62254
	Tel: (618) 537-6934 
	E-mail: smeissne at a1.mckendree.edu

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