Plant Life Cycles

David W. Kramer kramer.8 at OSU.EDU
Mon Oct 13 10:43:34 EST 1997

>Has anyone, anywhere, figured out a way to convince students
>that THERE IS INDEED a gametophyte generation in the Plant
>Kingdom? They block it out, and want it to go away!
>-NHarrison, Santa Rosa Jr College (frustrated as Exam 2 approaches)

(Sorry for the delay in this response.)  I find it difficult, myself, to
understand the life cycles of higher plants unless one begins with life
cycles in the algae and follows the evolution of life cycles into and
through the Plant Kingdom.

In our non-majors introductory botany course (two quarters) we place heavy
emphasis on evolution.  We begin the second course with a unit on genetics
(to show how info is transferred from one generation to another,
DNA-RNA-protein synthesis, mutations, etc.) then we do a unit on plant
diversity in which we focus on a comparison of sexual reproduction in the
various groups of organisms, emphasizing adaptaion.  After those two units,
the students have a foundation for studying evolution.

In the diversity unit we focus on the three kinds of life cycles (gametic,
zygotic, and sporic) as demonstrated in the algae then compare the sporic
life cycles of moss, fern, pine, and flowering plant.  We note the
evolutionary shift from heterotrophic to autotrophic sporophytes, the
reverse trend in gametophytes, the change from dependence on water for
fertilization to the "closed" sperm-delivery systems of gymnosperms and
angiosperms, and, of course, the development of the seed as protection for
the embryo... all of this aimed at better and better adaptation to dry
habitats, i.e., the shift from aquatic to terrestrial environments.

In the diversity unit we have a lab on plant fossils (so they see the kind
of concrete evidence biologists use to support their hypotheses) and
actually extract plant fossils from coal ball material then examine it
under the microscope.  By the time we get to the evolution unit where we
discuss the mechanisms of evolution, the students are already quite
familiar with inheritance, mutation, adaptation, fossil evidence,
comparative morphology, .... the evolution unit almost teaches itself!  By
the way, we chose the textbook by Kingsley Stern (WmCBrown Publ.) partly
because of its excellent treatment of the life cycles through the use of
easily understood lc diagrams drawn on uniform formats.

The other responses you received are on the mark... it's very instructive
for the students to grow gametophytes.  We grow fern prothallia from
Polypodium aureus but many other fern genera/species will work as well.  We
grow P. aureus in the classroom and there is always one or more leaves with
sori.  Rather than using harvested spores in envelopes, we want the
students to use the actual fern leaves as a source of spores.  The pinnae
of P. aureus are about 3-4 inches long.  We sow the spores on "Moss Minimal
Media" in deep, disposable petri dishes by opening the lid, laying one
pinna (excised from the leaf, spore side towards medium!) across the dish
(not touching the medium), closing the lid to "pinch" the leaf in position,
and waiting about 30 seconds.  Remove the pinna, replace the lid, tape it
in position, and put it under the 24-hr fluorescent light banks we use for
Fast Plants.  In the 30 seconds of exposure, the fern will disperse spores
on the medium, with greater density of spores directly below the sori.  The
prothallia will be thick in some areas (revealing the pattern of sori!) but
thin enough in some parts of the plate to easily observe individuals (which
can be lifted from the plate and observed as water mounts).  The pattern of
spore distribution leads naturally to discussions of dispersal and the
adaptations for spore and seed dispersal in plants.  When the prothallia
are mature, we flood the plates with water and wait a few weeks for the
young sporophytes to appear.

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

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