Reproduction

ReproductionLife cycle of Spirogyra

Sexual reproduction occurs when two filaments come together and undergo conjugation. Normally, one filament consists of male cells; the contents of each cell migrate through a conjugation tube into a female cell in the adjacent filament. The cell contents fuse, forming a zygote. Here the c ell contents function as gametes. Since the cells are identical in size this is an example of isogamy.

The zygote develops a thick, pigmented, protective wall to form a zygospore. This structure survives the unfavourable season at the bottom if the pond, where the parent cell walls rot away. When temperatures again rise the zygospore nucleus undergoes meiosis, but of the four nuclei produced, three abort. One haploid nucleus divides mitotically to form a fresh filament, which may be either male or female in its subsequent conjugation behaviour.

Phylum Filicinophyta (the ferns), Dryopteris

In mature ferns spores are produced on the underside of the leaves, in structures called sori. These consist of clusters of stalked sporangia, protected by a flap of tissue, the indusium. Meiosis occurs during spore production. The haploid spores are ejected from the sporangia and are dispersed by the wind. Ferns have just one type of spore (homosporous), and the leaves bearing the sporangia are often otherwise indistinguishable from the vegetative leaves.

The spores germinate on soil in moist conditions, and a tiny, heart-shaped plate of cells, the fern prothallus, is formed. The prothallus bears the sex organs, the antheridia and archegonia which produce sperms and egg. Fertilisation of sperm and egg produce a zygote which divides and grows into a new sporophyte plant.Phylum Coniferophyta (conifers)

Life cycle of Pinus

The conifer plant is the sporophyte generation, producing male and female cones on the same tree. Male cones are small, and consist of numerous scale leaves (microsporophylls) arranged around a central axis, each bearing two pollen sacs (microsporangia). The pollen grains (microspores) develop large air-filled sacs, which aid in wind dispersal. Male cones drop from the tree after the pollen has dispersed.

Female cones are larger than the male cones, and they remain attached for three years. On the upper surface of each megasporophyll are two naked ovules (not enclosed in an ovary). The ovules consist of a mass of cells arranged around a single megaspore mother cell. During the first year, the mother cell undergoes meiosis to form four megaspores, only one of which survives. In this condition, pollination occurs. The surviving megaspore develops into an embryo sac (megagametophyte=female gametophyte) containing two archegonia. During the second year the pollen grains send out pollen tubes through the ovule tissue, and male nuclei fuse with the egg cells in the archegonia. As a result two or more embryos start to form in each embryo sac, but all but one abort. The embryonic plant becomes surrounded by a food store. In the third year the formation of a winged seed is completed, and the seeds are dispersed before the brown, scaly cones drop from the tree.

In comparing reproduction in conifers with that in flowering plants, it is possible to see a cone as the forerunner of the inflorescence of flowering plants.The life cycle of the flowering plant

Microspores are produced by microsporophylls (stamens). The pollen sacs of the anthers are microporangia, producing microspores (the pollen grains). The microgametophyte (male gametophyte) in the angiosperms is reduced to the generative nucleus, and the male gametes are the two male nuclei.

The megaspore is formed within the megasporophyll (carpel). The embryo sac is a megaspore, and the megagametophyte is reduced to antipodal and synergid cells. The female gamete is the egg cell.

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