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Gamete formation: the flower

The flower is the characteristic reproductive structure of phanerogamous or spermatophytic plants, formed by modified leaves.  These plants are more evolved than plants without flowers or cryptogamous (mosses and ferns), with a less visible form of reproduction (phanero means “apparent, perceptible ”, while crypto refers to “something hidden”).
Interactive activity: Phanerogam or cryptogam?.

Angiosperm flower

The flower of an angiosperm is a modified short stem, with leaves modified to perform the functions of gamete production. The flower is attached to the stem by a peduncle, and is made up of highly specialized leaves, which form different groups orwhorls. If they distinguish four types of whorls :

  • Chalice. It is the outermost envelope, formed by the sepals, green and consistent leaves that protect the flower before it opens.
  • Corolla. It is the next envelope, formed by the petals, larger leaves, generally colored and striking in order to attract the insects involved in pollination.
  • Androecium. It is the male reproductive system. It is formed by a set of stamens. Each stamen is made up of a filament and anthers, enlarged areas at the end of the filament where pollen grains originate and are stored, which contain the male gametes.
  • Gynoecium or pistil. It is the female reproductive organ. Each carpel is bottle-shaped in which three parts are distinguished:
    • Ovary: it is the wide part and contains the ovules.
    • Style: Elongated part
    • Stigma: final part, responsible for receiving the pollen grain.

According to this, a flower can be:

  • Hermaphrodite, if it has stamens and carpels in the same flower.
  • Unisex. 
    • Male, if it only has stamens.
    • Female, if it only has carpels.

Flowers often appear next to each other forming structures called inflorescences , such as clusters, the ears and umbels.

Mature flower diagram-es.svgDe Mariana Ruiz LadyofHats, translation by Serg!o - traducción de Image:Mature flower diagram.svg, Dominio público,

Gymnosperm flower

The flower of gymnosperms or conifers is much simpler than that of angiosperms since it has neither petals nor sepals and is unisexual, that is, it is either male or female, and they are arranged in groups of flowers called inflorescences.
  • Male flowers. They are smaller than the female ones, and are formed by scale-shaped stamens with two pollen sacs in which the pollen is formed, grouped around an axis forming small spike-shaped cones , and are located at the ends of the branches. lower. Pollen has structures that help it float and disperse in the wind.
  • female flowers. They are grouped together forming greenish conical structures, which later become brownish woody, called pineapples, which when opened release the piñones, the seeds of the pine trees. Each flower is formed by a scale that, on its inner face, originates two ovules. The pollen grain arrives, helped by the wind, to the female flowers, where the pollen tube develops that reaches the ovule and fertilization occurs and the zygote originates.

Interactive activity: Point out which image is a gymnosperm whose flower is a pineapple.

Game: Flower parts puzzle.

Pollination and fertilization

Pollination is the process of pollen transfer from the time it leaves the stamens until it reaches the stigma of flowers in angiosperms. There, in the pistil, the ovules that will give rise to the seeds and fruits are fertilized.

Pollination can be:

  • Anemophilous pollination. Pollen grains are carried by the wind to other flowers. The flowers that have this type of pollination are not showy and produce a large amount of pollen so that at least a few get to fertilize other flowers.
  • Hydrophilic pollination. When the water that transports the pollen.
  • zoophilic pollination. Plants attract animals (and insects in entomophilous pollination) because of their showy flowers and because they produce nectar, sugary substances that feed them, and cause them to carry pollen to other flowers.

When the pollen grain reaches the stigma (female part) of the flower, a pollen tube is formed that grows through the style until it reaches the ovary.

The male gamete travels down the pollen tube to the ovary, where it joins the female gamete and the zygote is produced.

Fertilization is the union of the male and female gamete.

The seed and the fruit

Angiosperms are plants that are characterized by producing seeds that are contained within a fruit.

The zygotes (fertilized ovules) transform into seeds, in which three parts are distinguished:

  • The embryo of the new plant.
  • The albumen or endosperm, a tissue with the necessary food reserves for the future plant.
  • Protective covers that surround the seed.

The ovary of the flower also develops and forms the fruit, which will protect the seeds and facilitate their dissemination. When the fruits ripen, the seeds are released and disperse.

The dispersion of the fruit or the seed can be carried out by the action of the wind, other animals, etc., and allows the plant to colonize other places. For this reason, the fruits usually have characteristics that facilitate their dispersion.

  • Attractive colors for animals, which eat them and whose undigested seeds are deposited with the rest of the feces at a distance from the plant that originated it.
  • Some fruits have structures that facilitate their dispersal, both to move with the wind (elm, maple, etc.) and by adhering to animals.

When it falls to the ground, if the conditions are right (time of year, temperature, humidity, etc.), the seed will begin to germinate. The seed absorbs water and swells until the protective cover is broken. The embryo begins to grow, appearing a small root that penetrates the soil, and one or two leaves (cotyledons) that will continue to develop until forming a new plant.

The seeds can wait for germination for many years, until they find the right conditions for their development.