Explain with diagram
LIFE CYCLE OF GYMNOSPERMS
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Word gymnosperms, gymnos − naked, sperma − seeds
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Ovules not enclosed by any ovary wall
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Seeds formed after fertilization are not covered (i.e., naked).
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Spores produced within sporangia, borne on sporophylls, which form strobili or cones o
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Male and female strobili may be borne on same tree ( Pinus ) or on different trees (Cycas).
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Megaspore mother cell divides mieotically to form four megaspores.
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Megaspore mother cell is a differentiated cell of nucellus. Nucellus protected by envelopes is known as an ovule.
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Male and female gametophytes do not have independent existence, hence remain within sporangia.
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Steps in fertilization:
Pollen grain released from microsporangium
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Carried by air currents
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Come in contact with ovules
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Discharge of pollen content on mouth of archegonia
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Fertilization
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Formation of zygote
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Development of naked seed
LIFE CYCLE OF ANGIOSPERMS
There is alternation of generations between haploid gametophyte (male gametophyte and female gametophyte) and diploid sporophyte (main plant body) in the life cycle of a plant.
The anthers, which contain microsporangia, produce microspores by meiosis, and the microspores develop into male gametophytes called pollen.
The ovaries, which posses megasporangia simultaneously produce megaspores which grow into female gametophytes, called ovules. It is a seven celled structure with one large central cell containing two polar nuclei and one egg cell.
By the process of pollination, the pollen is transferred to the stigma of the pistil. The pollen grain germinates and a pollen tube grows down into the ovary. Eventually, two sperm nuclei travel down the pollen tube. One sperm nuclei fertilizes the egg forming zygote (which grows into an embryo) and the other two polar nuclei forming triploid cell (endosperm) which often serves as food for the embryo. This is called double fertilization. The Zygote which begins the next sporophyte generation, on complete development forms a seed. When the seed germinates, the sporophyte matures and the life cycle begins again.
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