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Life Cycle

Angiosperm life cycle
As in all plants, apart from the liverworts and mosses, the dominant phase of the flowering plant is the sporophyte. The sporangia are associated with sporophylls in the flower. The microsporophylls are the stamens.

If we look at a section of a developing anther we find four sporangia with an outer wall of diploid cells enclosing a layer of nutritive tissue, the tapetum, and inside this the microsporocytes.

The microsporocytes undergo meoisis to produce tetrads of four spores - pollen grains.

The nucleus of the immature pollen grain divides mitotically to form a tube cell and a generative cell - the latter will divide at some stage to produce two sperm cells.

An ovule (inside an ovary) is in effect a megasporangium and consists of integument(s) around a nucellus enclosing a megasporocyte. There is an opening in the nucellus called the micropyle.

After meiosis only the distal (from the micropyle) megaspore develops into the female gametophyte which usually has 8 nuclei. These nuclei go through a rather complicated "dance" - the details vary from one species to another. Typically, first there are four nuclei at each end of the embryo sac; then one from each end moves to the center, forming the central cell. Three "antipodal" nuclei are left at the distal (from the micropyle) end and these play no further part. The three nuclei proximal to the micropyle are two "synergids" or helpers and an egg cell. The archegonium which has been a constant feature of the female gametophyte up to this point is not formed.

Pollination occurs when a pollen grain lands on the stigma. The pollen grain germinates and sends a tube down the style and into the micropyle. By this time there are two sperm nuclei and the germinated pollen grain with three nuclei (tube nucleus and two sperm) is in effect the mature male gametophyte. The sperm nuclei move down the pollen tube and enter the embryo sac by way of a synergid, which degenerates. One sperm fuses with the egg cell to form a diploid zygote; the other fuses with the two polar nuclei of the central cell which ends up as a triploid cell. This "double fertilization" is a characteristic feature of all angiosperms.

The triploid cell divides to produce the endosperm. This provides a food reserve and is well developed in many monocots. Its development in dicots is variable; often it gets absorbed by the developing embryo and the cotyledons form the main food reserve.

Lily embryo at globular stage

Wheat seed showing embryo and part of endosperm

The zygote divides, first producing a globular mass of cells; in dicots this becomes heart-shaped and then like a "torpedo". The lobes of the heart form the fins of the torpedo which grow into the cotyledons. The pointed end of the torpedo becomes the radicle. Between the cotyledons and radicle is the hypocotyl; this bends as it develops so that the cotyledons are folded round to meet it at the "walking stick" stage. The embryo and endosperm enlarge and accumulate food reserves to variable extents, until the seed begins to dry out preparatory for dispersal. The embryo contains the beginnings of the shoot and root meristems whose development we will follow later.


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