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Sexual Reproduction in Plants

Reproduction in plants is the production of new individuals by asexual or sexual means. Sexual reproduction in plants involves the process of fusion of gametes which produces new individuals which are genetically different from their parents. 

Sexual reproduction involves two fundamental processes which are meiosis and fertilization. Meiosis is the process of rearrangements of genes and is a reductional division of cells, where the chromosomes are reduced to a haploid set. Fertilization is the process of fusion of the haploid gametes to form a diploid zygote. Most of the plants undergo alternation of generation with two phases, a gametophyte phase and a sporophyte phase. 

In algae, sexual reproduction happens by the means of conjugation where two similar organisms fuse and exchange genetic material and split apart. Algae also undergo the process of alteration of generations. Two different generations haploid and diploid are produced. 

In flowering plants, sexual reproduction takes place by the means reproductive organs called flower. Flowers house both the male and the female reproductive structure of a plant. The male structure is the stamen where the anther produces pollen grains. The female structure is known as carpel contains the ovary, ovules and stigma. The fusion of the male and female gametes results in double fertilization. This results in a zygote and develops in an embryo. It grows into a seed fruit and seeds.

Plants can fertilize by pollination. Plants which are non-flowering like ferns, moss and liverworts reproduce by other forms of sexual reproduction. 

 

Evolution of Sexual Reproduction

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  • Plants are immobile organisms and cannot move to find sexual partners for reproduction. In early plants, abiotic factors like wind, water transported sperms for reproduction.
  • The early plants were aquatic and released sperm into water and were carried by water currents. 
  • In primitive land plants, like mosses and liverworts, sperms were motile and they swam in a thin film of water or were splashed in form of water droplets from the male organs onto the female organs. 
  • More complex and taller plants evolved the process of alternation of generation. 
  • During the Paleozoic era, progymnosperms reproduced by the spores which were dispersed by wind. 
  • The seed plants had pollen grains to protect the sperm during the process of transfer. 
  • Insects were believed to feed on pollen and plants thus evolved to use insects for pollen transfer. 
  • Angiosperms and gymnosperms have heteromorphic alternation of generations with large sporophytes containing reduced gametophytes. Angiosperms developed distinct reproductive organs called flower with carpels and stamens. 

Alternation of Generations

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Alternation of generation is also known as metagenesis, which is used to describe plant life cycle. A mutlicellular haploid gametophyte with haploid set of chromosomes alternates with a mulitcellular sporophyte with diploid chromosomes. Spores are produced by a mature sporophyte by meiosis, which reduces chromosomes to half, from 2n to n. The haploid spores germinate and develop in a haploid gametophyte. The gametophytes at maturity produced gametes by mitosis. The gametes fuse to form zygote which develops into diploid sporophytes. This cycle is the way in which most the algae and land plants undergo sexual reproduction. 

Gametophyte

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Gametophyte is a stage during life cycle of land plants and algae. The gametophyte phase is a haploid multicellular adult stage which gives rise to haploid gametes that are produced from mitotic cell division of spores. Gametes are produced by gametophytes by mitosis, the gametes produced may be male or female or both. The male and female gametes fuse to from diploid zygote; the zygote develops into a mutlicellular sporophyte by mitosis. Sporophytes are produced by the fusion of two haploid gametes, the sporophytes are diploid. The mature sporophyte produces spores meiosis in which the chromosome pairs are separated to form haploid sets. The spores are hence haploid and develop into haploid gametophytes. 

Sporophyte

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Sporophyte is the multicellular diploid stage in the life cycle. The sporophyte develops from the zygote; hence each sporophyte has double set of chromosomes. In angiosperms and gymnosperms, the sporophyte phase is more notable than the gametophyte stage. The sporphyte produces spores by meiosis which develops into a gametophye. The spores and the gametophyte are haploid. The gametophyte produces gametes by mitosis. The fusion of male and female gametes produces a diploid zygote. The zygote develops into a new sporophyte. 

Angiosperm Reproduction

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Angiosperms are dominant forms on land; they reproduce by sexual and asexual means.

Illustration os Angiosperm Reproduction
  • These plants reproduce by reproductive organs called flowers. 
  • The male structure is known as the stamen and the female structure is known as carpel. The anther of the stamen produces pollen grains which contain the male gametophytes.
  • The carpel has stigma, style and ovules that are located in the ovary. 
  • Pollination is the process of transfer of male gametes to the female ovules, for pollination to occur the pollen grains adhere to the stigma of the carpel. 
  • The pollen tube grows through the style of the carpel and the nuclei from the pollen migrate to the ovule to fertilize the egg cell and endosperm nuclei inside the female gametophyte, this process is known as double fertilization.
  • The resultant zygote develops into an embryo.
  • The endosperm and female tissues give rise to the tissues surrounding the developing seed. 
  • The ovary of the female gametophytes develops into a fruit. 
  • Pollination can be self-pollination or cross-pollination. 

Gymnosperm Reproduction

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In gymnosperms both the female gametophyte and the male gametophyte are produced separately. The gymnosperms are dependent on wind for pollination.

Reproduction in Gymnosperms
  • The cones of the gametophyte contain green leafy sporophyte and male and female gametophytes. 
  • Female cones are larger than the male cone and are located higher up in the tree. 
  • The male cones are the microsporophylls in which male gametophytes are produced and are carried by wind to the megaspores or female gametophytes. 
  • The female cone has the megaspore mother cell and divides by meiosis to produced haploid megaspores; the megaspore divides to form the female gametophyte. 
  • The microspore lands on the female cone and forms a pollen tube through the generative cell and meets the female gametophye. 
  • One of the sperm cells fuses with the egg forming a diploid zygote that develops to form the embryo. 

Ferns Reproduction

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  • Sexual reproduction in ferns is by production of large diploid sporophytes with rhizomes, roots and leaves; these fertile leaves known as sporangium, produce spores. 
  • These spores germinate forming short, thin gametophytes. 
  • These gametophytes are also known as thallus, they produce both motile sperm in structures known as antheridia and eggs cells are produced in archegonia. 
  • Rain or dew deposits a film of water, the motile sperm are splashed from the antheridia to the archegonia where the sperm fertilizes the egg. 
  • Fertilization results in the formation of zygote which grows into a new plant which is a sporophyte. 
Illustration of Ferns Reproduction

Bryophytes Reproduction

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  • Bryophytes are plants found in moist locations, they have motile sperm with flagella and they require water to make sexual reproduction possible. 
  • Bryophytes are haploid spores that grow to form multicellular haploid body with leaf-like structures.
  • Gametes are haploid and are produced in antheridia and archegonia by mitosis. 
  • The sperm respond to chemical released by ripe archegonia and swim in a film of water and fertilize the egg to produce zygote. 
  • The zygote grows by mitotic division into a diploid sporophyte. 
  • The diploid sporophyte produces structure called spore capsules that produce spores by meiosis. Variations in their breeding structures is seen some bryophytes species.  
Illustration of Bryophyte Reproduction
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