Structure of ovule
The Angiospermic ovule consists of a central body the nucellus and a basal stalk by which it is attached to the placenta the funicle. The nucellus is protected and enclosed by one or two sheaths of the integuments. The micropyle is an aperture in the integumentary sheath that exposes the nucellus’ tip. The nucellus, integuments, and funicle join together in the chalaza of the ovule.
Types of Ovules
The relative position of micropyle and chalaza at maturity, the following types of ovules are as follows
1. Atropous or Orthotropous ovule: In this type, funicle, chalaza, and micropyle are situated on one vertical axis with micropyle directed upwards e,g Polygonum, Piper, and in Gymnosperms.
2. Anatropous: These ovules are also called inverted. They are 180-degree curved ovules. As the body of the ovule is so bent that micropyle is now directed downwards and lies close to the hilum.
3. Campylotropous ovule: This is a transversely positioned ovule on the funicle. The micropyle is oriented downwards and is not in a straight line with chalaza because the body is bent. It is found in gram, bean, mustard, sunflower, etc
4. Hemitropous ovule: In this type, the body of the ovule is situated at right angles to the funicle e,g Ranunculus
5. Amphitropous ovule: The ovule is curved at (180 ) and the embryo sac becomes horseshoe-shaped. Amphitropous ovules occur in members of Alisma, Butomas, Poppy, etc.
6. Circinotropous ovule: This is a characteristic type of ovule found in the family Cactaceae, e,g Opuntia. Here the ovule is straight first, due to more growth on one side the ovule gets inverted, but this unilateral growth continues till the ovule becomes straight again. The funiculua completes a full circle around the ovule’s body in this manner.
Megasporogenesis is the process of haploid megaspore production within the megasporangium by meiotic division of diploid megaspore mother cells (MMC) (ovule). The embryo-sac is formed by successive mitotic divisions of the haploid megaspore.
Embryo sac and its Development
The first cell of the female gametophyte is the functional megaspore. It grows largely along the micropyle-chalazal axis and is mostly chalazal. The nucleus undergoes three mitotic divisions and forms eight nuclei. Out of the Eight nuclei, 4 are present at the micropylar end and 4 at the chalazal end. One nucleus from each group comes in the center to form 2 polar nuclei. The remaining three nuclei at the micropylar end make up the egg apparatus, whereas the remaining three nuclei at the chalazal end make up three antipodal cells. The mature female gametophyte or embryo sac is the entire structure with two polar nuclei, three antipodals, one egg, and two synergids. the Monosporic 8- nucleate embryo sac or polygonum type of embryo sac arises from a single megaspore and has 8 nuclei
Types of Embryo sacs (Female Gametophytes)
It is of the following types
1. Monosporic Embryo sac: A single megaspore produces a monosporic embryo sac. Monosporic embryo sacs are of the following types
A.Monosporic 8-nucleate or polygonum type: Its nucleus divides three times, resulting in eight nuclei. It’s also known as the Polygonum type because it was first described by Strasburger in the polygonum divaricatum.
B. Monosporic 4-nucleate or Oenothera type: This type of embryo sac usually develops from the micropylar megaspore. The megaspore nucleus divides twice and forms only four nuclei, of these three organized into egg apparatus, and the fourth functions as the polar nucleus. As a result, the Oenothera type of embryo sac lacks antipodals.
2. Bisporic Embryo sac: In this kind, two megaspore nuclei participate in the development of the embryo sac. The four nuclei formed from one megaspore nucleus are genetically distinct from the four derived from the second megaspore nucleus in this type of embryo sac. e,g Allium, Endymion,
3. Tetrasporic embryo sac: the four megaspore nuclei form the basis of this embryo sac. The tetrasporic form of embryo sac is genetically more heterogeneous than the bisporic type of embryo sac. e,g Adoxa, Drusa, Plumbago, Frittilaria.