Letrsquo;s take a look at the main types of life cycles of plants . Thallophyta In many lower non-vascular plants - Thallophyta - such as the

Life cycle of plants

Let’s take a look at the main types of life cycles of plants.

Thallophyta

In many lower non-vascular plants — Thallophyta — such as the majority of green algae, the first division of the zygote nucleus — is meiosis. The result is a tetrad of spores. These spores, called zoospores, are usually mobile, and further developed in the new algae.

In such plants, individual cells (gametangia) develop gametes. The gametes in the act of the sexual process coalesce to form a zygote.

Embryophyta

All higher plants — Embryophyta (mosses, ferns, horsetails, club mosses, gymnosperms and angiosperms) — and many lower plants zygote divides without reduction. Gradually, there forms a certain individual appearance and size.

Further complications of the plant life cycle relate to the fact that in each cycle, we find two types of individuals:

  • individual, forming only asexual reproduction organs (sporangia with spores)
  • individual, forming only sexual reproduction organs (antheridia, oogonia) with gametes.

Alteration of generations

Thus, there occurs an alteration of generations. The name “alteration of generations” is obviously unsuccessful. Here, in fact, there is no alteration from one generation to others, since “generations of organisms” is usually called descended from parents as a result of fertilization. In our case there is a change in the phase of development of the same organism — the sporophyte and gametophyte. However, the term “alteration of generations” from the middle of the XIX century, firmly entrenched in science and is commonly used. The plant forming the sexual reproduction organs (antheridia and oogonia), called the sexual generation, gametophyte; the plant is carrying organs of asexual reproduction — sporangia — asexual generation — sporophytes.

In higher plants, there is always a correct alteration of generations (of development phases — the gametophyte and sporophyte), and there is no phylum with their equal development. In mosses occurs prevalence of sexual generation (gametophyte dominates), but in all the other groups (ferns, horsetails, club mosses, gymnosperms and angiosperms) — asexual generation predominates (sporophyte dominates). In higher plants, goes formation of such spores, which are preceded by reduction division or meiosis; by this they differ from the lower Thallophyta plants.

Heterospory

A further complication is associated with the appearance of external differences in the forms of spores: the spores of small size, or microspores, and spores of large sizes — megaspores. This difference is called heterospory.

From microspores always develop male gametophyte that produces sperm, and from megaspores grows female gametophyte, where arise archegonium with ova. According to difference in spores, sporangia are called: microsporangia — containing microspores, and megasporangia — containing megaspores.

The leaves of Embryophyta produce sporangia that are called sporophylls. Taken from http://worldofschool.org

Spermatophyta

The next life cycle type is associated with a group of seed plants (spermatophytes). Megasporangia of spore-bearing plants by maturation lose touch with the mother plant. Development of prothallia from spores takes them outside the plant that produced them. This leads to the death of a large number of prothallia, since not all of them fall into the favorable conditions of existence. In seed plants megasporangia do not leave the mother plant until the formation of a fully formed fetus (megasporangium sheds as a seed). Germination of megaspores, formation of female prothallia, fertilization and embryo formation goes on the mother plant that supports the developing individual with nutrients. All seed plants are heterosporous. They have even stronger reduction of prothallia.

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