Life cycle of gymnosperms

Life cycle of gymnosperms

• 1.
  LIFE CYCLE OFGYMNOSPERMS Gymnosperm Definition: Naked seeded non-flowering plants are called gymnosperms. LIFE CYCLE: Gymnosperm shows a clear alternation of sporophytic and gametophytic generations. The Plant body is sporophytic generation.The gametophytic is very much reduced. It had completely lost independent existence.
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  SPOROPHYTE (VEGETATIVE PLANT): Theplant body is a sporophyte. It is well differentiated into roots, stem and leaves. 1. Roots: Primary or tap root is well developed. Root develops root caps. Corraloid Roots and mycorrhizal Roots are found in some genera. Roots provided mechanical support to plants. Roots also absorb water and minerals for plants. 2. Leaves: Leaves of two types: a) Microphyllous or scale leaves: These are sac like leaves.These are much smaller in size.These are found in Ephedra. Scale leaves are associated with the foliage leaves other plants. b) Megaphyllous or foliage leaves: These are large and well developed leaves. Both simple and compound leaves are found. The foliage leaves are without a reticulate venation. The branching of the veins is dichotomous type. There leaves are modified to withstand the xerophytic conditions. These leaves have tough cuticle, sunken stomata mesarch bundle. These are found in Cycas and Pinus. Leaves of Cycas may reach the length of six feet. 3. Stem: The stem is woody. Vascular bundles are collateral(arrange in rings) and open. They are arrange in ring in the primary stem. Secondary growth takes place by the activity of the cambium. It produces the Secondary Xylem(woody). Xylem is formed onf Xylem parenchyma and tracheid.
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  They have notrue vessels except the order Gnetales companion cells. Primary cambium may remain active throughout the life of the plant. The secondary and tertiary cambia are found. It produces alternate zones of wood. The activity of phellogen produces bark on the surface. Secondary Growth: Secondary growth occurs in gymnosperms. The girth of plants is increased during secondary growth.Vascular cambium is involved in secondary growth. Secondary growth produces Secondary wood. It produces secondary Xylem. Secondary growth produces distinct annual rings. The plants having active secondary growths have reduced pith and cortex. Reproduction: Gymnosperms reproduce by seed formation. Vegetative reproduction is absent in gymnosperms. Only Cycas reproduce vegetatively by formation of bulbil like structure Cones or Strobili: Gymnosperms are heterosporous. They produce micro and megaspore. They develop definite unisexual cones or strobili. There are two types of strobili or cones. 1. Male cone 2. Female cone Male cones are usually smaller in size than the female cones. These cones show a great variation. a) Male cone: The male cones are smaller in size. They are produced in clusters on an axis(stem). Males cone contains many microsporangia. These microsporangia produce many microspores by meiosis. The microspores germinates within its own wall to form a small inconspicuous male gametophyte. b) Female cone:
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  The female conehas large size and they are more conspicuous. The female cone contains many microsporangia. Each female cone is composed of a large number of scales called megasporophylls. These megasporophylls are spirally arranged an woody in texture. An ovule is present at base of each scale. Ovule is a megasporangium. Sporangia: (a) Microspore and Microsporangia: Microspores (pollen grains) are produced in Microsporangia(anthers). They are borne on microsporophyll (stamens). Microsporophylls are spirally arranged on the central axis of thje male cone. Microsporangia have Microspore mother cells. Each cells divided by meiosis to form four haploid microspores. Microspores are dispressed by wind. (b) Megaspores and Megasporangia : The Megaspores(embryo sac) are produced in megasporangia(ovules). They are borne on the megasporophylls(carpels). The ovules are covered by a single integument. It has single megaspore mother cell. This cell divides meiotically to form four megaspores. These megaspores degenerate only one remains functional.
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  Gametophytes: Male Gametophyte: Microspore representsthe beginning of the gametophytic stage. It germinates to give rise to male prothallus within the spore wall. It has only one prothalial cells. It cuts of generative cells. This generative cell represents the reduced antheridium. Generative cell divides into a basal stalk cell and a body cell. Body cell gives rise to two male gametes. Pollen tube carries these two male gametes (pollen grain) to the oosphere. Male gametes are non-motile except certain primitive forms as Cycads. Female Gametophyte: The functional megaspore is the beginning of the gametophytic stage. It germinates to form multicellular female peothallus within the spore wall. Superficial cells gives to rise to two or simple archegonia towards the micropylar end. Each archegonia has a simple neck cells and a venter. The venter contains an egg (oosphere) and a venter canal cell. Pollination: Pollination is the transfer of pollan grain from male cone to micropylar end of the ovule on female cone. It occurs by wind. In some cases, it may occur in animals. A sticky fluid exudes from the micropyle of the ovule. It is called pollination drop. The microscope wall adheres to this drop of fluid. The liquid become dry. It pulls the male gametophyte down into the ovule. After that micropyle is closed.
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  Formation of PollenTubes and Fertilization: In some primitive gymnosperms, well developed pollen chambers are present at the fee apical end of the nucellus. These chambers contains liquid. Flagellated sperms swim in it and reach the egg. These pollen chambers are found in Ginkgo Cycas and Ephedra. Such chambers are absent in other gymnosperms like pinus. In hese plants, pollen comes in direct contact with the surface of the nucellar beak by pollination. Male gametophyte produce a pollen tube. It is the extension of the inner wall (exine) of pollen. It grows down into the nucellar tissue. It enters the archegonium and releases the non-motile sperms directly into it. The basal end of the pollen tube bursts at the time of Fertilization. It liberates the two flagellated sperms above the female gametophyte. One or both sperms may enter an archegonium.Only sperm fertilize the egg. Formation of Embryo and Seed: Oospore is the beginning of gametophyte generation. The oospore gives rise to embryo. Embryo formation is the most important characteristics of gymnosperms. It leads to formation of the seed. Wall formation starts after the nuclear division. Then the embryo becomes cellular. The embryo differentiates into the suspensor radical, hypocotyls, plumule, and cotyledons. Prothalial tissues provide nourishment to developing embryo. Integument is transformed into the seed coat. The unutilized prothalial cell becomes endosperm.
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  Polyembryony: Formationn of severalembryos in a single gametophyte is called Polyembryony. It is common in gymnosperms. In this case, more than one archegonium are formed. Their eggs are fertilized by sperm. Thus several zygotes are produced. It produces several embryos. Cleavages Polyembryony is characteristics of a majority of conifers. In this case, certain embryo becomes separated from one another. They give rise to a system of four or more distinct embryos. Competition starts between the various embryos. It eliminates all embryos except one. The surviving embryo grows and develops into seed.