The Flower

The Flower:
The flower is the reproductive unit of some plants (angiosperms). Parts of the flower include  petals, sepals one or more  carpels (the female reproductive organs), and stamens (the male reproductive organs).

The Female Reproductive Organs:
The pistil is the collective term for the carpel(s). Each carpel includes an ovary (where the ovules are produced; ovules are the female reproductive cells, the eggs), a style (a tube on top of the ovary), and a  stigma (which receives the pollen during fertilization).

The Male Reproductive Organs:
 Stamens are the male reproductive parts of flowers. A stamen consists of an anther (which produces pollen) and a filament. The pollen consists of the male reproductive cells; they fertilize ovules.

Fertilization:
Pollen must fertilize an ovule to produce a viable seed. This process is called pollination, and is often aided by animals like bees, which fly from flower to flower collecting sweet nectar. As they visit flowers, they spread pollen around, depositing it on some stigmas. After a male's pollen grains have landed on the stigma during fertilization, pollen tubes develop within the style, burrowing down to the ovary, where the sperm fertilizes an ovum (an egg cell), in the ovule. After fertilization, the ovule develops into a seed in the ovary.

Types of Flowers:
Some flowers (called  perfect flowers) have both male and female reproductive organs; some flowers (called imperfect flowers) have only male reproductive organs or only female reproductive organs. Some plants have both male and female flowers, while other have males on one plant and females on another.  While Complete flowers have  stamens, a  pistil, petals and  sepals.  Incomplete flowers  lack one of these parts.

Pollination and flower structure

Although many angiosperms have the potential to self-fertilize, most plants cross-fertilize. To transfer pollen from flower to flower, angiosperms make use of wind dispersal and animal pollinators. Animals which pollinate flowers include insects, birds, and mammals. One of the important functions of flowers is to attract animal pollinators. Because of this, flower structure, color, and odor vary dramatically in correlation with the primary pollinators. For example, flowers which are pollinated by flies smell like rotting meat, while some flowers pollinated by wasps mimic the appearance of a female wasp (potential mate). The benefits to pollinators include nutrition from nectar and pollen.

It is often possible to determine what type of pollination/pollinator a plant uses by the structure, color, shape, and smell of the flower. You should be familiar with this table:

Flower Trait	Pollinator
Reduced sepals and petals	Wind
Large sepals or petals	Animal
Petals white	Moth or bat
Petals colored
Flower tubular
Sweet odor	Butterfly
No odor	Hummingbird
Flower not tubular	Bee or beetle

Inside the anther, microspores are produced through meiosis. These microspores give rise to themicrogametophyte – the pollen grain.

The female reproductive organ, the ovary, contains developing ovules. Each ovule produces megaspores (haploid) through meiosis. One megaspore survives in each ovule and develops into a multi-nucleated megagametophyte.

When pollen reaches the stigma, it grows a pollen tube to extend down the style. Two sperm nuclei enter the ovule: one fuses with the egg to produce the zygote, the other fuses with two polar nuclei to make the 3n endosperm. The endosperm is part of the seed and provides nutrition for the sporophyte embryo. The fusion of two sperm nuclei with megagametophyte nuclei is called double fertilization.

Following fertilization, the zygote develops into an embryo, the endosperm grows, the ovule forms a seed, and the ovary develops into a fruit.