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
