The Flower: Where it All Begins

In flowering plants, the specialized, spore-bearing leaves called microsporophylls (male) and megasporophylls (female) are arranged in a FLOWER, a collection of highly specialized leaves.

Ready? On your mark. Get set...

Let's look at the flower from the bottom UP.

The stalk attaching the flower to the stem is known as the PEDUNCLE, and the slightly inflated "platform" on which the flower itself rests is called the RECEPTACLE. The outermost leaves of a dicot flower are usually small and greenish, and are known as the SEPALS. Collectively, they comprise the CALYX.

Internal to the sepals are the PETALS, collectively comprising the COROLLA. The calyx and corolla together make up the PERIANTH.

The Male

The male sporophylls (microsporophylls), located just inside the petals, are the STAMENS. Each stamen consists of a thin, stalklike FILAMENT and the boxlike ANTHER.

Inside the anther, special diploid cells known as microspore mother cells are dividing via meiosis to give rise to microspores. Each microspore will grow and differentiate via mitosis to become the male gametophyte, POLLEN. Each pollen grain is an individual, genetically unique plant that contains two sperm.

The Female

The female sporophylls (megasporophylls), located just inside the stamens, are the CARPELS. Each carpel consists of a terminal, sticky pad called the STIGMA (where pollen will land and germinate), a stalklike STYLE, and a rounded, ovule-containing base known as the OVULARY.

Inside the ovulary diploid megasporangia contain megaspore mother cells which undergoing meiosis to give rise to megaspores. Each megaspore will undergo a specialized series of mitotic nuclear divisions without cytokinesis, resulting in coenocytic organism, the female gametophyte. Though she's little more than a mass of cytoplasm containing 8 nuclei, she is an individual, genetically unique plant living in side her mother, the sporophyte--though she hardly looks like a plant!

A closer look at the female gametophyte:

The mature gametophyte inside the megasporangium, which will eventually become the seed, is known as the ovule. An ovule is, essentially, a seed waiting to happen.

The Fates of the Nuclei

ovum - fuses with one sperm to become the zygote

polar nuclei - both fuse with one sperm to become 3n, nutritive endosperm

synergids - disintegrate, after fertilization (help guide the pollen tube and sperm to the ovum)

antipodals - disintegrate after fertilization (function unclear)

Let's have a look at plant fertilization and ovule development.

The result is the seed:

And that's where we'll go from here.

The Embryo

Inside the seed, the zygote undergoes an initial, asymmetric, transverse division. This establishes the polarity of the embryo: which end will be the root, and which will be the shoot.

The smaller (upper) end of this two-cell stage is the chalazal pole, and the lower end is the micropylar pole.

The small apical cell at the chalazal pole will become most of the plant embryo.

The larger basal cell at the micropylar pole will become the suspensor, which anchors the embryo to the inside wall of the seed.

The apical cell divides and grows into a spherical embryo proper (EP in the diagram below) connected by a long suspensor to the seed wall.

The first leaves of the plant are known as the cotyledons. These arise via divisions along the margins of the globular embryo to form the heart shaped embryo. The cotyledons and shoot elongate to form the torpedo shaped embryo, the walking stick embryo, and finally the mature embryo.