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
> 
> 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
> http://koning.ecsu.ctstateu.edu
> 
> ----------
> 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
> parent?
>   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).
> 
> Thanks,
> Janice
> 
> ***********************************
>  Janice M. Glime, Professor  
>  Department of Biological Sciences
>  Michigan Technological University
>  Houghton, MI 49931-1295
>  jmglime at mtu.edu
>  906-487-2546
>  FAX 906-487-3167 
> ***********************************
> 
> 
> 
> 
> 
> ---
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