Cycas

Cycas

• 1.
  Cycas Gymnosperms
• 2.
  Cycas
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  Life cycle (reproduction methods) Vegetative reproduction Adventitious buds containing reserve food Present on the base of stem on axil of each scale Each bulbil consist of few scales and foliage leaves Develop into new plant
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  Sexual reproduction • Heteropsorous •Male plant is rare • Microspores and mega spores are produced • Spores are produced on sporangia that develop on sporophylls • Sporophyll arrange to form strobili or cones • Male and female cones are formed on seprate plants
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  Male cone • Largestamong all cones of gymnosperms • Male cones are very rare • Single, woody, spindle shapes , about 50 cm in length present at the apex of stem • Each cone has central axis, in which microsporophylls are arranged • It is wedged shaped ( narrow lower part. And broader upper part) • Several hundred microsporangia are born on abaxial ( lower) surface of microsporophyll • These are arranged in soral clusters • The sporophylls at apex and base are sterile
• 7.
  Structure of micrsoporangium Each microsporangiumOval in shape and borne on short stalk Several layered mass of sporogenous cells are present inside The outermost layer isTapetum Each sporangium contains large number of microspores
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  Female cone Megasporphylls arenot organized in to cone It consist of loosely arranged megasporophylls that surrounds the shoot apex like foliage leaves Each megasporophyll is pinnate like Differentiated into upper, broad, lobed leaf like portion that has pointed end, And lower stalk like stipe The ovules are naked and 6-8 ovules are arranged laterally on stipe Ovules are hairy , red and 6 cm in length
• 10.
  Structure of ovule Ovuleis erect and has opening called micropyle The part opposite to micropyle is called chalaza Massive nucellus is surrounded by single integument The integument is differentiated into outer, inner and middle( thick that become stony) during seed formation The nucellus is equivalent to megasporangium and consists of thin walled parenchyma having dense cytoplasm Nucellus grows into a beak called nucellor beak The upper part of beak form pollen chamber and pollens are lodged into it after pollination
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  https://www.youtube.com/watch?v=A0hStZf9_R8 https://www.youtube.com/watch?v=mngWYnRt6Yo https://www.youtube.com/watch?v=lSFiRuiqCGI
• 13.
  Megasporogenesis A deeply lyingcell of nucellus enlarges and functions as megaspore mother cell After meiosis it form tetrad of megaspores Upper three degenerate and lower most enlarges to form embryo sac or female gametophyte
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  The male gametophyte Microsporeis the future male gametophyte Each microspore is uninucleate and its protoplast is surrounded by two layered wall Each microspore divides in to two and daughter nuclei are separated by a wall to form two cells 1: a prothalial cell 2:a large cell
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  Release of pollengrain Large cell divides to form 1: a tube cell 2: and generative cell At this stage the sporophylls are separated , sporangium walls ruptures And release 3 celled semi-germinated male gametophyte called pollen grain They are then carried to female gametophyte where further development completes
• 16.
  Pollination • Transfer ofpollen grains from microsporangia to micropyle of the ovule is called pollination • Some cells at apex from nucellus disintegrate and form a colloidal material called pollination droplet • It protrudes out through micropyle and attach pollens • The drop dries , contract and carries pollens inside pollen chamber, where they remain dormant for 4-6 months
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  Pollen tube During restingperiod following changed takes place Protoplast of each pollen grain protrudes from upper part of microspore and form pollen tube The tube nucleus migrates into pollen tube and the generative cell divides into 1: stalk cell 2: body cell The body cell divides to produce two multiflagellate sperms or antherozoids
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  Antherozoid Each sperm cellconsists of a large nucleus surrounded by a thin layer of cytoplasm The flagella are arranged spirally in a band like structure The pollen tube is haustorial in nature and it digests its way to nucellus to the archegonia
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  Female gametophyte • Thefunctional megaspore in ovule begins to enlarge and is called as embryo sac • The tissue around it become vacuolated and form spongy layer to give nutrition for megaspore • The nucleus of megaspore divides to form 1000 more daughter nuclei • The vacuole appears in center • The nuclei are pushed and arranged in the periphery • Later vacuole disappears and cytoplasm is formed • Now embryo sac contains large number of nuclei and water cytoplasm • Cell wall formation stats and at the end entire gametophyte become cellular • Mature gametophyte is surrounded by two layers , the inner become megaspore wall
• 20.
  Structure of archegonium Amature archegonium consists of 4 neck cells and an egg ( surrounded by tough membrane) The egg in Cycus is largest amongst all living plants. Two to eight archegonia are formed in Cycus revoluta.
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  Seed formation The embryoenlarges and ovule increases in size The outermost layer of integument develops red color and becomes fleshy The middle layer become stony and hard, the inner layer becomes soft These layers constitute seed coat
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  Germination of seed •Seed absorbs water and after that seed coat ruptures at micropyle end emerging its radicle • The plumule is followed • The cotyledons remains inside • Foliage leaves are produced by the activity of apical bud after several years • Seed remains attach with young sporophyte by withered cotyledons for one or two years
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  Videos https://www.youtube.com/watch?v=0_q0b3nncJY