Janice:
Got that. Had that before. *wink* The question that remains unanswered
still:
Where's the cotyledon in a monocotyledonous plant?
If the seed is all germ and endosperm....
Steve
Janice M. Glime wrote:
> 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
> ***********************************
> 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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