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The following was provided by Dr. Paul Somers, Recording Secretary The 713th meeting of the New England Botanical Club, being the 940th since the original organization, met on Friday September 11, 1998, at the University of Massachusetts' Morrill Science Center, Amherst, Massachusetts, with about 40 members and guests present. The away meeting featured a presentation by Dr. Rudolf Schuster on
the topic "Age, Reproduction and Distribution of the Hepaticae:
Problems of Being a Haploid". He was introduced by Dr. Karen
Searcy who noted highlights of his career that included a Ph.D.
from the University of Minnesota in 1948, a professorship at the
University of Massachusetts from 1960 - 1983, and authorship of
six volumes entitled Hepaticae and Anthocerotae of North America,
East of the Hundredth Meridian. Karen also noted that the
University of Massachusetts herbarium holds many of Dr. Schuster's
specimens from Massachusetts and just received on indefinite loan
from Mt. Holyoke College a bryophyte collection from A.J. Grout,
author of Mosses with a Hand Lens. Another title for Dr. Schuster's talk might have
been "Hepatic Paradoxes". Threaded throughout his presentation
were observations of evolutionary, biological, and distributional
twists or peculiarities about liverworts. We learned, for instance,
that the group, while being very old, has not been very successful
evolutionarily relative to many younger plant groups. In support
of this he compared the approximately 5000-7000 species described
for the Hepaticae, which have existed since the Devonian, with
the 25,000 species of orchids, a group that originated around
the start of the Tertiary. He speculates that the primarily haploid
existence of hepatics versus the predominantly diploid life history
of higher plants has been one of the chief limiting factors for
the former. In hepatics, the diploid matures its spores while
still enveloped by haploid tissues, thus there is no direct selection
pressure on the diploid generation, he explained. Being non-vascular,
the gametophytes are limited in size. The sporophytes, being
basically parasitic on these "incompetent" haploid generation
plants, are likewise fixed in terms of potential size and growth. Another paradox of the Hepaticae is that they are
considered long term survivors yet they are frequently unsuccessful
at sexual reproduction. A major problem for many hepatics is
that all or most of their disseminated spores result in unisexual
or totally vegetative clones. Over 95% of primitive taxa, he
says, are unisexual, thus a single isolated spore can produce
only a clone capable of maintaining itself by asexual means.
An example given by Schuster is Acrobolbus ciliatus, which
exists as male populations in Japan and female populations in
the Southern Appalachians. A converse corollary among hepatics,
it might be stated, is that many species or populations that exist
as isolated unisexual clones are successful reproducers and often
persist for eons. How do they accomplish this? Mainly by a variety
of asexual forms of reproduction. For instance, among the 350-400
"good" species of Plagiochila all are unisexual
and many are known from one sex only. The Appalachian taxa reproduce
mainly by fragmenting (e.g., P. caduciloba) or caducous
leaves (e.g., P. austinii), a seemingly lousy way to get
around, Schuster says, but then adds "maybe such fragments
adhere to squirrel feet". Such fun thinking helped get us
out of our anthropocentric mind sets in thinking about successful
strategies for these small plants. Besides asexual disseminules,
including gemmae in some taxa, some liverworts have the strategy
of pure persistence, existing as hardy clones for centuries or
millenia. Plagiochila corniculata, for instance, we were
told, is waiting for sperm from male plants in Europe to make
it across the Atlantic to fertilize the female clones in America.
The genus Haplomitrium has evolved what appears to be
a better game for overcoming lives of unisexual clones lacking
any asexual means of reproducing. Its taxa reproduce only by
spores shed as diads or tetrads which guarantees that at least
some of the germinating diads and all of the tetrads will have
the potential of producing gametophytes of both sexes in close
proximity, and, therefore, at least the possibility of self-fertilization.
A distributional paradox about Hepaticae is that
while being a very old group, they have failed to spread well.
Evidently, infrequent spore production, short spore viability,
and other limitations to dispersal have restricted many taxa geographically.
Oddly, Schuster points out, hepatics are poorly represented
in geologically old areas such as the Appalachians in spite of
their ability to persist as clones. All three endemic hepatic
genera in North America, Schuster says, occur in orogenically
young westernmost areas. Likewise in South America, the geologically
old Guayana shield area has yielded only 3-4 endemic genera compared
to at least 10-12 for the northern Andes. A partial explanation
Schuster gives for this is that hepatics are basically pioneer
taxa. In orogenically unstable sites, he suggests, there is persistence
of old taxa plus opportunity for evolution to occur in newly created
raw and diverse habitats. In geologically old areas, he speculates
there has been mass extinction of species through time plus a
reduction of pioneer substrates where hepatics can easily grow
and evolve. After all this talk of survival strategies, the gathering of botanists ended the meeting thinking of their own survival by devouring food and beverage provided by the University of Massachusetts' hosts, then headed off for sleep before the following day's field trips with Dr. Schuster to the Green River Gorge and Hawley Bog in Franklin County, Massachusetts. |
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http://www.rhodora.org/Summaries/1998/Sep98sum.html -- Revised: Dec. 24, 2007 |
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