Scutellum in corn not coconuts
Mark D. Spiro
spiro at BUCKNELL.EDU
Fri Aug 13 09:51:06 EST 1999
>I DO know the answer as to where corn's cotyledon is. It is a small
>structure called the scutellum. It projects into the starchy/liquidy
>endosperm. Dicots convert most of their endosperm into cotyledon material.
> In corn, the cotyledon never gains much "weight" but rather transfers the
>nutrients directly to the developing embryo proper. This is true of many
>of the grains familiar to us. I make a big deal in botany about endosperm
>vs cotyledon, one is embryonic, the other not, one triploid, one diploid,
>etc. By the way, the little slivers that come out of a corn kernal when we
>bite into it are the embryo, with most all the sliver being the cotyledon.
>One of my earlier coconut queries was whether coconut has a scutellum like
>structure as well, something suggested on a recent posting.
My understanding about the difference between monocots and dicots
is that dicots absorb the endosperm during embryo formation/maturation to
form large cotyledons and monocots absorb the endosperm via their single
cotyledon during seed germination. The scutellum is the term for the
specialized absorbtive cotyledon found in grasses and is not used for other
Simpson and Ogorzaly (Economic Botany-Plants in our World, 2nd
Edition, pages 136-138) state "The [coconut] fruit is formed from a flower
with 3 carpels (each represented by an 'eye' of the coconut), only one of
which develops. The mature fruit contains one seed, the largest known. The
embryo itself is small and located near the stem end. Initially the
endosperm is a liquid containing free nuclei. This liquid, called coconut
water or coconut milk, is drunk from green coconuts in many tropical
countries. As the endosperm matures, cell walls form around the nuclei,
and the endosperm solidifies into an oil-rich layer of coconut "meat"
inside the seed coat. It is the solidified endosperm that we eat...
If mature coconuts are not used or harmed, the embryo can germinate
within the coconut since there is no dormancy period. The germinating
seedling eventually extends the tip of the cotyledon through one of the
eyes. The base of the embryo swells into an absorbing organ that
eventually fills the entire cavity of the coconut as it digests the
endosperm. The swollen organ, called a coconut apple, can also be eaten. "
I am not sure that Simpson and Ogorzaly have correctly identified
the coconut apple as being part of the embryo proper. It may actually be
the cotyledon which swells during germination and begins to absorb the
Bowes (A color atlas of plant structure, page 148) states "In palms
the cotyledon often enlarges greatly and becomes haustorial..."
In either case the cotyledon of coconuts is very small until after
seed germination. I hope that this is useful.
Mark D. Spiro
Department of Biology
Lewisburg, PA 17837
spiro at bucknell.edu
phone: (570) 577-3486
fax: (570) 577-3537
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