monocot cotyledon

Ross Koning koning at ECSUC.CTSTATEU.EDU
Sun Aug 29 12:31:14 EST 1999


At 8:21 AM -0500 8/28/99, PROFDHW at aol.com wrote:
>The probable reason for so much confusion about coconuts, aside from
>ignorance (meant in the sense of "uninformed") about plant structure in
>general, is that coconuts are monocots (mono- = one, -cot = cotyledon) and
>most discussions of cotyledons are broached from the dicot (di- = two)
>perspective. This probably relates to the fact that many (Most?) dicots have
>emergent cotyledons while few (if any; Ross? David H.?) monocots use their
>single seed leaf (the other name for a cotyledon) for anything other than as
>an absorptive interface between the endosperm and the growing part of the
>embryo. Emergent, photosynthetic cotyledons are much easier to point to in
>lab.
>
>Even in dicots with hypogean (underground) cotyledons which remain inside the
>seed coat, such as the garden pea (_Pisum_ _sativum_), the cotyledons are
>easy to demonstrate. Try this with the monocot corn (_Zea_ _mays_).

HI Dave!

Indeed you can demonstrate the corn cotyledon quite
nicely. Popcorn serves well in this regard. After
soaking for two days to soften the kernel (fruit!),
you can examine and dissect it more easily. Orville
Redenbacker works as well as any (thanks to the jar
keeping the moisture level and seed viability high!).

On one side of the kernel you'll see a white shield-shaped
area while the rest of the kernel is typically yellow
(yes, there is white popcorn too). This shield-shaped
area is the cotyledon (in grasses called the scutellum).
The yellow volume encloses the endosperm.

If the soaking has gone on for a day or two longer,
the embryonic axis (radicle and coleoptile) will swell
and form a linear shape on the top of the scutellum.
If you keep soaking longer, the seed-fruit coat will
crack open over the embryo and the coleoptile and radicle
will emerge...but the scutellum will still be present
"behind" the growing axis. Indeed corn germination is
hypogeous, so this scutellum can be found on young seedlings
with a little digging and rinsing.  It is the last vestige
of tissue remaining on the seedling after the endosperm is
completely gone.

Another way to view the cotyledon is to make a section
through the kernel after a few days of soaking. This
section should be made by laying the kernel with the
shield-shaped area facing up. The blade is aligned with
the long-dimension of the swollen embryonic axis to
cut through the kernel. The two halves can be examined
both before and after staining with I2KI (iodine) stain.
Generally the endosperm area will turn very dark (blue-black)
and the cotyledon and the embryonic axis will appear a
faint brown. Some rinsing of excess stain can improve
the contrast. If you stain a freshly cut soaked kernel
(same section as above) with tetrazolium, the cotyledon
and the embryonic axis will stain a nice pink color while
the endosperm remains white.

This is a standard examination in my Botany (majors) class.

I haven't really tried to find an epigeous monocot that
would serve as well as corn does. The one commonly-known
epigeous monocot is onion...but its seeds are small and
the seed coat is very dark (unlike in common popcorn).
On the other hand, the onion cotyledon does emerge above
the soil, elongate extensively, and turn green (as in
epigeous bean). I'd love to hear about an easily-obtained,
large epigeous monocot seed with a clear seed coat. Maybe
someone out there knows of one?

ross

________________________________________________________________
Ross Koning                 | koning at ecsu.ctstateu.edu
Biology Department          | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA   | fax: 860-465-4479
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