I don't think Dave Starrett's question got completely answered.
Flowering plants have "double fertilization" in which one sperm nucleus
joins the egg and one joins the polar nuclei to form the endosperm
nucleus. This endosperm nucleus has at least a 3n condition (in many
dicots), 5n in lily, and a variety of other n conditions depending on how
many divisions occurred before the mature embryo sac formed. Following
fertilization, this new endosperm nucleus undergoes free nuclear division,
i.e., it makes nuclei but no cell walls are formed. This permits it to
develop from the middle of expanding tissues without having to form its
own tissue until it has reached a sufficient size to permit the remaining
cells to form without crushing the previous ones. These nuclei move
outward in the ovule like chains in circles. Eventually the outermost
ones begin to form walls and become structural. This becomes the "meat"
in the coconut. The younger ones are still liquid, acellular chains of
nuclei and form the "milk." Thus the edible coconut has mature meat
endosperm and young milk endosperm.
Both monocots and dicots go through this process, but in dicots the
endosperm is quickly used by the developing cotyledons and seldom noticed
as a milky endosperm.
The coconut is a great example to help students understand the whole
double fertilization process and the free nuclear division. They are able
to visualize what it is like for a cell to divide in the middle of other
tissue and to understand that to expand there must be a provision made to
provide sufficient room. From the coconut example, then they can think of
other examples of monocots they know where the endosperm is important or
where they have seen milky endosperm. Corn is a good example because
there is enough old, starchy corn on the market for them to understand
that something happens as it ages. This introduces the change from sweet
to starchy and change from sugar to starch as the product is stored in the
seed for overwintering.
We have a foods lab with this in which the students get to eat endosperm
- coconut, popcorn, rice wrapped with nori. You can also give them
cotyledons to eat - peanuts, walnuts. But it is the coconut that really
enables them to understand the whole process.
Janice
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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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