Plant Life Cycles

David W. Kramer kramer.8 at OSU.EDU
Mon Oct 13 20:05:47 EST 1997


>        David Kramer's post brings up one of my pet peeves in plant life
>cycles.  Text books generally discuss zygotic, sporic, and gametic
>LIFECYCLES, but they are quite confusing and often not well presented. I
>think a better way to think of these terms and present them is as types of
>MEIOSIS, each related to a specific type of life cycle.
>        Phycologists, who generally work with a greater diversity of
>lifecycles than the rest of us, often use a different set of terms.
>Haplobiontic refers to plants (if algae really are plants...) that have one
>vegetative phase in the life cycle (either 1n or 2n).  Diplobiontic refers
>to those with 2 phases (=alt. of generations).
>        An even better set of terms that I learned from a phycologist as a
>grad student are haplontic, diplontic, and haplo-diplontic.
[material deleted]
>        Because of the overabundance of terminology in biology and because
>the book I use has sporic, gametic, zygotic life cycles, I teach the
>students the different life cycles without the names.  Then I tag the names
>on at the end, pointing where meiosis occurs.  This at least gives them
>something meaningful to hang the name on.

>Doug Jensen

Doug,
Of course, life cycles in my course are VERY WELL PRESENTED!!  If you don't
believe me, just ask me!!!!

I think we understand one another but it's important that we use terms and
especially explanations that are understandable to our students.  Authors
and instructors sometimes make seemingly simple mistakes that can really
make life difficult for students.  For example, in the otherwise excellent
textbook by our friends, Moore, Clark, and Vodopich (Ed 2, p. 637) they use
"sporic," "gametic," and "zygotic" to characterize the life cycles but the
first two are drawn so the stages progress clockwise while the latter is
drawn counterclockwise!  This adds an element of confusion that is totally
unnecessary.

I see problems with the names sporic, gametic, and zygotic but my students
seem to have to problems if we characterize them as follows:

zygotic:
  meiosis occurs in zygote, zygote is the only diploid cell in the life
cycle, it never divides by mitosis, the organism spends most of its life in
the haploid state

gametic:
  meiosis produces gametes, the gametes are the only haploid cells of the
life cycle, the zygote divides by mitosis to form a diploid individual
which is the way the organism spends most of its life

sporic:
  meiosis occurs in a spore mother cell to produce haploid spores, the
spores divide by mitosis (germinate) to produce haploid individuals, the
haploid individuals produce gametes so are called gametophytes, the gametes
fuse to form a diploid zygote which divides by mitosis to produce a diploid
individual (a sporophyte) which produces spore mother cells and spores.

I have used the haplontic, diplontic, and haplo-diplontic terminology but I
don't think it made it any easier.  The key here is to have enough hands-on
observations of all life cycle types so the students have a fair chance of
understanding, not just memorizing.

Dave Kramer


Dr. David W. Kramer
Department of Plant Biology
Ohio State University at Mansfield
1680 University Drive
Mansfield, OH  44906-1547
(419) 755-4344  FAX:  (419) 755-4367
e-mail:  kramer.8 at osu.edu





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