Ovule structure of Plants & Types of Ovules
- 2. Ovule, the developmental precursor of seeds is the organ of prime importance in angiosperm flowers and it can be traced back in evolution almost 400 million years ago to the earliest seed plants as far as phylogeny is concerned. Having diverse position in ovary, nucellus thickness, number & thickness of integuments, degree and direction of curvature and histological differentiations have drawn the importance of plant biologists. The ovule develops as multicellular placental outgrowth including the epidermal & number of hypodermal cells. With further development, this gives to the nucellus and one or two integuments from its basal region. Angiosperm ovules basically consists of a nucellus and two integuments and sessile on the placenta or attached by its stalk
- 4. Most commonly a vascular strands extends from the placenta through the funiculus to the chalaza, i.e. the area below the base of the nucellus where the integuments depart. The funiculus and the chalaza are intercalary structure and thus well demarcated than the nucellus and integuments. The nucellus represents the mega sporangium, in which the meiocytes undergoes meiosis forming four megaspores, typically only one of which develops into embryo sac representing the megagametophyte.The embryo sac contains basically four or eight nuclei, organized into four or seven cells, depending on whether two or three round mitotic divisions in the developing embryo(Maheswari, 1950;Friedman, 2006).
- 6. A typical angiosperm ovule consists of the followings: Funicle- Stalk of the ovule that connects ovule to the placenta, short & multicellular. Hilum- Point where funicle connects the main body of the ovule. Integuments-Layers that surround the ovule-outer & inner integument’ main function to protect the inner tissues. Micropyle- Opening which present at the top of the integuments called micropyle. It allows entry of the pollen tube into the ovule.
- 8. Nucellus- Integuments surrounds parenchymatous tissues which constitutes the main body of the ovule called nucellus. The nucellus is nutritive in function to the developing embryos. Chalaza- The swollen part of the ovule present opposite to micropyle is called chalaza. Embryo sac-It is present in the nucellus and it contains female gamete called egg. Nutrients from the plants travel through phloem of the vascular system to the funiculus and the outer integument and from there apoplastically and symplastically through the nucellus inside the ovule.
- 9. The ovules are characterized by a considerable diversity in shape, degree of development and structure. Mirbel (1929) for the first time classified ovules under four groups- Orthotropus, Anatropus, Campylotropous & Amphitropous. But later on another two types added making the ovules altogether six(6) types as follows: Orthotropous or atropous Anatropous Hemi-anatropous or Hemitropous Campylotropous Amphitropous Circinotropous
- 11. As stated earlier, Ovules may have diversity,. When only one integument present called unitegmic, if the ovule contains two integuments called bitegmic and very rarely three integuments called tritegmic is present in plants like Asphodelus.In some plants like Santalum, no integument called ategmic is also found to occur. On the basis of the position of micropyle, with respect to funiculus, Ovules are classified under six broad categories as stated earlier. The details are given below.
- 12. Orthotropous Ovule: It is atropous or straight where the micropyle, Chalaza & funiculus , all are in the same line. E.g. Cycas, members of Polygonaceae and Piperaceae. Anatropous Ovule: In 80% genera, this is very common type.In this ovule, the funicle is long , whole body is inverted through 180⁰ , As a result, micropyle comes close to the funicle. Most common almost monocots and dicots like the members of Asteraceae , Solanaceae etc. Hemianatropous or Hemitropous Ovule: The body of the ovule is inverted through 90⁰. As a result the funicle comes to lie at right angle to the nucellus.Micropyle and the chalaza lie in the same plane. Members of primitive family Rananunculaceae like Ranunculus etc.
- 13. Campylotropous Ovule: When the body of the ovule is not inverted but it appears like horse shoe. The micropyle and the chalaza do not lie on the same plane , however the nucellus or embryo sac remain straight. Members of the family like Capparidaceae, Brassicaceae, Caryophyllaceae and Fabaceae. Amphitropous Ovule: It is almost similar to Campylotropous but the nucellus or embryo sac is bent like horse shoe. Members of Alismataceae. Circinotropous Ovule; It is very rare in occurrence. Here, the body of the ovule is bent through 360⁰, so that it takes a complete turn. Micropyle, Chalaza and the nucellus are on the same plane.
- 15. Encyclopedia Britannica An introduction to the Embryology of angiosperms by P. Maheswari A text book of Botany by Hait, Bhattacharya, Ghosh Advanced Plant taxonomy by A.K.Mondal Different websites from Google for images and contents.