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Sexual Reproduction in Angiosperms


Sexual reproduction in this group included the following phenomena: sporogenesis, gametogenesis, pollination, fertilization, and seed and fruit development.

Sporogenesis and Gametogenesis

From the Pteridophytes, the sporophytic phase in the life cycle of plants becomes the dominant or lasting, represented by the individual itself. In angiosperms, flower production represents the final state in sporophyte maturation.

During the process of microsporogenesis, occurs inside the anthers, that is, in the pollen bags (microsporangia), the formation of pollen grains or microspores, from meiotic divisions of the microsporocytes. Mature pollen grains, surrounded by a non-continuous exine wall, have a vegetative nucleus it is a germ core. When deposited on the receptive stigma of the flower, this pollen grain will germinate, forming the pollen tube, which corresponds to the microgametophyte, where it will occur. gametogenesis. The germinal nucleus splits into the sperm nuclei (= gametes).

THE megasporogenesis It is an ephemeral process that occurs at the beginning of the formation of the egg, which is filled with a tissue called nucella. It is from this tissue that differentiates the mother cell from the embryonic sac or megasporocyte. By meiotic divisions 4 cells are formed, 3 of which degenerate, the remaining form the megaspore which soon becomes gametophytic phase by mitotic divisions of its nucleus, giving rise to the embryonic bag, inside a now mature egg. The embryonic sac is made up of 7 cells, antipodes (3), synergides (2), 2 polar nuclei in a large central cell and the the sphere (= gamete).

Fertilization

It is the intimate union between two sex cells, gametes, until the fusion of their nuclei. This process results in the formation of seed and fruit in angiosperms.

After the pollen is deposited on the receptive stigma, it germinates, producing the pollen tube, which grows through the stylet, penetrating the ovary and through the micropyle, the egg. Upon reaching the embryonic sac, the tube ruptures releasing the two sperm nuclei, one of which will fertilize the oosphere, giving rise to a zygote and the other will join the 2 polar nuclei, giving rise to a reserve tissue, the endosperm (3n). This process is called double fertilization and is a unique character of angiosperms.

Double fertilization in the embryonic sac triggers a series of changes in the egg and gyno, and even in the flower as a whole, resulting in fruit and seed.

Fruits and Seeds

What are flowers for?

After pollination and fertilization, the flower undergoes an extraordinary change. Of all the components that were seen before, only the peduncle and the ovary remain. Everything else degenerates. The ovary undergoes a major change, develops and now we say it has become fruit. Inside, the eggs became seeds.

Thus, the great novelty of angiosperms in terms of reproduction is the presence of fruits.

The fruits will be the protection and the vehicle of dispersal of the mature seed, carrying the embryo of a new individual, closing the life cycle of angiosperms.

The seed and the future plant

The seed is the modified and developed egg. Every seed has a more or less rigid wrap, an inactive embryo of the future plant, and a food reserve material called an endosperm or albumen. Under favorable environmental conditions, especially humidity, seed hydration occurs and germination can be initiated.

The cotyledons

Every embryo contained in an angiosperm seed is an axis formed by two ends:

  • The radicle, which is the first structure to emerge when the embryo germinates; and
  • O stem, responsible for the formation of the first embryonic leaves.

An embryonic "leaf" deserves special attention. It is the cotyledon. Some angiosperms have two cotyledons they are called eudicotyledons and plants that have a cotyledon are called monocotyledonias.