FW: Ginkgo seed question
"KONING,ROSS E. Biology
KONING at easternct.edu
Fri Dec 8 10:31:07 EST 2000
> Hi Janice,
> You asked several questions...
> The ovule becomes a seed...but this transition is
> NOT a categorical shift but rather a developmental
> process. Clearly the structure is an ovule when it
> contains a megagametophyte with an egg...and it
> has been reported that most of the ovules drop from
> the tree at this stage. So what hits the ground in
> most cases is an ovule. However once the pollen
> has shed the sperm and it unites with the egg, the
> ovule is becoming a seed. This seed would be
> "immature" as the zygote becomes an embryo...
> but once the embryo is fully formed and (usually)
> dormant, then we have a "mature" seed. For most
> individual ovules, this transition to seed occurs on
> the ground. However, I have seen ovules still on
> the branches in early winter and I'd bet that they
> drop as a mature seed rather than as an ovule.
> As for size, the ovule is full size when it drops and
> it does not grow during the maturation of the seed.
> The storage reserves in the megagametophyte are
> used for the development of the zygote into an embryo,
> so in a certain way, part of it is used up (at least
> converted). The aril (outer integument layer) is full
> size at the time of the drop and as it "ripens" and
> softens and smells, it is using up its reserves too.
> The flesh ferments and ultimately is lost under
> natural conditions. So the size on the ground will
> be DEcreasing.
> I don't know of other plants in which fertilization
> occurs after shedding...but I suspect that it may
> happen in cycads:
> Our Zamia furfuracea (taxon may be faulty) is
> female and produces many ovules with orange
> arils each year...these shed as expected...
> but we don't have a male for pollination, so what
> is shed is indeed a very mature megagametophyte
> in an ovule. As you might guess, I have students
> dissecting them in class...the gametophyte is large
> and the egg cell is huge...distinguishable in simple
> hand longisections under a dissection microscope.
> Ross E. Koning, PhD
> Professor of Biology - Goddard Hall
> Eastern Connecticut State University
> Willimantic, CT 06226 USA
> Pager: (860)-744-2705 (leave return number at beep)
> Office: (860)-465-5327
> Home: (860)-423-9724
> Email: koning at easternct.edu
> Home: koningr at snet.net
> From: jmglime at mtu.edu
> Sent: Wednesday, December 6, 2000 3:08 PM
> To: plant-ed at hgmp.mrc.ac.uk
> Subject: Ginkgo seed question
> I don't want to belabor the original Ginkgo question, but recent posts
> on the seed have raised my curiosity about the life cycle.
> Scott Shumway has asked what to call the "thing" (my word, not his) that
> falls from the Ginkgo tree and that ultimately contains the embryo. It
> was an ovule on the tree, containing a single megaspore, so it is still an
> ovule on the ground, soon to become a seed after it finally gets
> fertilized. But this raises a new question in my mind. Does the ovule
> continue to increase in size after it falls? If so, how large is it when
> it falls? At what point do we start calling it a seed? Are ovules with
> fertilized eggs considered seeds as soon as they are fertilized, or not
> until they are mature and, in most cases, ready to leave the fruit or
> My Biology Dictionary (Harper Collins) defines a seed as "the structure
> IN an ovule that ...," then closes with including the integuments as part
> of a seed. Am I missing something? How can it be "in" an ovule?
> Raven, Eichert, and Eichorn in Biology of Plants define seed as "a
> structure formed by the maturation of the ovule of seed plants following
> fertilization." That was always the concept I had understood, but in
> Ginkgo, how does one know when that has occurred?
> Are there any other plants that are fertilized after the seed leaves the
> parent plant (as a normal occurrence).
> Janice M. Glime, Professor
> Department of Biological Sciences
> Michigan Technological University
> Houghton, MI 49931-1295
> jmglime at mtu.edu
> FAX 906-487-3167
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