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X-section of an ovary Ovary contains a cavitylined with an epidermal layer. Ovules develop from the epidermal cells and are contained within the cavity of the ovary, attached to its inner surface by a short stalk- funiculus
Ovary bears ovules on a ridge on the ovary wall called placenta
Parts of ovule
1. nucellus- central body with vegetative cells enclosing thesporogenous cells
2. 1 or 2 integuments (unitegmic or bitegmic) enclosing the Nucellus
3. funuculus-stalk connecting ovule with the placenta.
4. chalaza-where nucellus, integuments and the funiculus merge
Megaspore mother cell dev. from surrounding nucellar tissue and undergoes meiotic division to form megaspore. Nucellus considered as a megasporangium
Funiculus
nucellus
Chalaza- region where integuments fuse with funiculus
Development of embryo sac and female gamete (in an anatropous ovule)
•A hypodermal cell of the nucellus enlarges and becomes differentiated into a megaspore mother cell or megasporocyte.
•This diploid megaspore mother cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores,
•3 of which degenerate and the 4th becomes the functional megaspore in monosporic types,
•all 4 become functional in tetrasporic types
Female Gametophyte
• The nucleus of the megaspore undergoes three successive mitotic divisions forming eight nuclei.
•The megaspore enlarges into an oval shaped structure called the embryo sac. The eight nuclei of the embryo sac arrange themselves in 3 groups.
Darkly stained cluster of cellsis the nucellus
Inner and outer integumentsbegin as ridges of tissue
Inner integument
Outer integument
Differential growthcauses them to curve so micropyle is bent around funiculus
micropyle
funiculus
funiculus
placenta
Outer and inner integuments completely overgrow the nucellus
Megaspore mother cell dev. from surrounding nucellar tissue and undergoes meiotic division to form megaspore. Nucellus considered as a megasporangium
Funiculus
nucellus
Chalaza- region where integuments fuse with funiculus
Outer and inner integuments completely overgrow the nucellus
Developing ovules of Lilium. Ovule emerges from the placenta as conical protuberance with the first sporogenous cell,called archesporial cell. Integuments formed by periclinal div. of epidermis
Or megaspore mother cell
Micropyle
Inner integument
Outer integument
placenta
funiculus
Outer and inner integument completely overgrow the nucellusExcept for the micropyle. (nucellus=central portion of ovule inside the integuments )
--Begins with elongation of the functional megaspore, usually at chalazal end.-- initially megaspore is non-vacuolate but later small vacuoles appear which may fuse to form large vacuole.
Development of embryo sac
Embryo sac After 3 mitotic divisions
Egg and synergids
Sporogenous cell develops directly from hypodermal nucellar cell- large dense cytoplasm, large nucleus.In others, hypodermal cell divides into outer parietal and inner primary sporogenous cell.
meiosis
Hypodermalarchesporialcell
meiosis
meiosis
Chalazal region
Post meiosis: survivingMegaspore will divide bymitosis
A, D. (bel mutant) exposed nucellus and a single integument
F. Larger mutant ovuleOuter integument has many cells.
A. First megaspore mitosis yields binucleate embryo sac. Spindle of first nuclear div oriented along the long axis of the cell. Wall formation Does Not follow the nuclear division. Both nuclei divide 2x, forming 4 in B then 8 in C
B. Large vacuole appears between the two daughter nuclei. As cell expands, nuclei are pushed toward opposite poles of the cell. Both nuclei from each pole divide twice
D. The 8 nuclei arrange themselves in two clusters of 4 nuclei one at each opposite ends. One nucleus from each end migrates towards the middle, called polar nuclei (named for where they came from, not where they end up).
C.8-nucleate state . All 8 nuclei are present in a common cytoplasm, they move around probably from remnants of spindle fibers from earlier divisions.
Chalazal trio called antipodals( Latin “against the foot”) at opposite end of the egg and antipodals
Egg apparatus consists of larger egg flanked by two smaller cells called synergids (greek for “helpers” or cooperators
The large binucleate
Chalazal=basal part of ovulewhere integument and nucellusare joined
megasporogenesis
Megagameto-genesis
Mutants in ovule determination
1. bell (bel1)- ovule lacks inner integument
2. Aberrant testa shape (ats)- no clear distinction between inner and outer integument
3. Extreme types of integument mutations:
aintegumenta (ant)
huellenhos (hll)Do not develop integument and embryo is disrupted.
highly vacuolatedamount of cytoplasm is
limited cytoplasm spread as a thin layer surrounding vacuole
cytoplasm contains very little ER, limited no. of plastids, mitochondria and dictyosomes but high ribosomes which are randomly distributed rather than aggregated as polyribosomes
cell wall does not extend over the entire cell
EGG
Synergids- limited life span, wilt after fertilization. Probably involved in nutrition of egg. has extensive wall ingrowth at micropylar region called filiform apparatus
metabolically active
Antipodals-transient existence , minimal cytoplasmic organelle, show nuclear abnormalities like
endoreplication polar nuclei-metabolically active, extensive ER, numerous plastids, mitochondria, dictyosomes and polysomes, has large quantities of starch, proteins and lipids
Megasporogenesis and microsporogenesis
megasporocyte microsporocyte
Generates megaspore and microspores
meiosis