Re: what is double fertilization in flowering plants? and why does it occur?

Double fertilization occurs in flowering plants (angiosperms) and a few 
gymnosperms (genus Ephedra and genus Gnetum). The pollen tube contains two 
haploid (1n) sperm. In angiosperms, one sperm fuses with or fertilizes an egg 
to produce a diploid (2n) zygote, which then divides repeatedly by mitosis and 
develops into the embryo of the seed. The second sperm fuses with two, usually 
haploid, polar nuclei to form a triploid (3n) nucleus termed the primary 
endosperm nucleus. Both sperm fuse or "fertilize" so it is called double 
fertilization. In some species the endosperm nucleus may be 2n, 5n, 9n or 15n. 
For example, in lily, one of the polar nuclei is 3n so the endosperm is 5n.

The primary endosperm nucleus divides repeatedly and becomes the endosperm, a 
nutritive tissue for the angiosperm embryo. The endosperm is usually cellular 
but is noncellular in early stages of some species. Coconut "milk" represents 
a noncellular endosperm. In some seeds, termed exalbuminous, (e.g. bean, 
peanut, buckeye, chestnut, walnut, oak) the endosperm is absorbed by the 
cotyledons so there is no endosperm in the mature seed. Other seeds, termed 
albuminous, (e.g. corn, wheat, rice, onion, coconut, pawpaw, redbud, dogwood, 
magnolia) have substantial endosperm in the mature seed. A large percentage of 
the calories in the human diet come from endosperm (corn, wheat, rice, oats 
and other grains). In a minority of species, notably the Orchid Family, the 
endosperm does not develop or degenerates early in development. 

Most gymnosperm seeds (e.g. pine, spruce, fir, ginkgo) have a haploid (1n) 
nutritive tissue derived from the female gametophyte. Sometimes the gymnosperm 
nutritive tissue is also called endosperm. However, it is preferred to reserve 
the term endosperm for angiosperm seeds. Double fertilization in the 
gymnosperm genera Gnetum and Ephedra differs from the angiosperm pattern. Two 
diploid embryos are produced by double fertilization in Gnetum and Ephedra.

We are not positive why double fertilization occurs. Arthur Cronquist (1971) 
considered double fertilization a "mere evolutionary happenstance." One 
possible advantage of double fertilization is that the plant does not invest 
energy in seed nutritive tissue until after an egg has been fertilized. 
Another possible advantage is that the endosperm nucleus is very active and 
divides rapidly. It forms the nutritive tissue very quickly. Rapid seed 
development has obvious advantages. 

Gymnosperms lacking double fertilization dominate large areas of the Earth, 
e.g. the Taiga or Northern Coniferous Forest. Therefore, double fertilization 
is not a requirement for success in seed plants.

There are many webpages that discuss double fertilization, which is still a 
hot area for research. Locate them with www.google.com

References

Cronquist, A. 1971. Introductory Botany. New York: Harper and Row.

Foster, A.S. and Gifford, E.M. Jr. 1974. Comparative Morphology of Vascular 
Plants. San Francisco, Freeman.

USDA Forest Service. 1974. Seeds of Woody Plants in the United States. 
Washington, D.C.: Govt. Printing Office.

Raven, P.H., Evert, R.F. and Eichhorn, S.E. 1999. Biology Of Plants. NY: 
Freeman.


Double Fertilization


Some reflections on double fertilization, from its discovery to the present