Ovule

An ovule is a structure found in the ovaries of seed plants where the female gametophyte develops. It is essentially the precursor to a seed. Within the ovule, meiosis occurs to produce the female gametophyte, which contains the egg cell or cells. The ovule is typically surrounded by one or more integuments, which eventually develop into the seed coat upon fertilization. After fertilization, the ovule develops into a seed containing the embryo, endosperm, and seed coat. This seed is capable of developing into a new plant under suitable conditions. 

1.0Structure of Ovule

It consists of three parts: the integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore) in its center. The female gametophyte — specifically termed a megagametophyte — is also called the embryo sac in angiosperms.

2.0Parts of Ovule

• Funicle: The ovule is a small structure attached to the placenta by means of a stalk called a funicle. 
• Hilum: The body of the ovule fuses with a funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle. Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore.
• Integument: Each ovule has one or two protective envelopes called integuments. Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised.
• Micropyle: The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the micropyle. The micropyle opening allows the pollen to enter the ovule for fertilization. During seed germination, the seedling's radicle emerges through the micropyle, It also facilitates the entry of water and  gasses.
• Chalaza: Base of the ovule located opposite from the micropyle is called chalaza.
• Nucellus: Mass of cells within the ovule is called the nucellus. Cells of the nucellus have abundant reserve food materials. After fertilization, the nucellus may develop into the perisperm that feeds the embryo. The nucellus  is part of the inner structure of the ovule, forming a layer of diploid cells immediately inside the integuments. It is structurally and functionally equivalent to the megasporangium. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis.
• Megasporogenesis : Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus.The MMC undergoes meiotic division. The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.

• Embryo sac or female gametophyte: Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore. A common pattern of embryo sac development includes a single functional megaspore followed by three rounds of mitosis.

3.0Location of Ovule

In flowering plants, the ovule is situated inside the gynoecium, which is the female reproductive part of the flower. The ovary of the gynoecium generates one or more ovules and eventually develops into the fruit wall. Ovules are connected to the placenta in the ovary by a stalk-like structure known as a funiculus. 

4.0Types of Ovules

On the bases of number of integument:

An integument is a protective layer that surrounds the ovule. After fertilization, the integuments transform into the seed coat. Ovules can be classified into three types based on the number of integuments they have: 

1. Unitegmic: Ovules with one integument. Examples gymnosperms. 
2. Bitegmic: Ovules with two integuments. This is the typical condition for angiosperms. 
3. Ategmic: Ovules with no integuments. Examples include Loranthus, Olax, Liriosma, and Santalum. 

On the bases of position of micropyle, chalaza and funicle:

In angiosperms ovules are of different types on the bases of position of micropyle, chalaza, and hilum relationship and funiculus orientation.

1. Atropous or Orthotropous Ovule: The ovule is upright, with the micropyle, chalaza, and hilum aligned in a straight line, and found in primitive plants like betel, piper, polygonum, as well as in gymnosperms. Raphe is absent in this type.

2. Hemitropous or Hemianatropous Ovule: The ovule bends on the funiculus at a 90-degree angle, positioning the ovule body horizontally. Micropyle and chalaza are in line, but the micropyle is positioned away from the hilum. Examples include plants like ranunculus and primula.

3. Anatropous Ovule: The ovule is completely inverted at a 180-degree angle due to unilateral growth of the funiculus. Micropyle faces downwards, and chalaza and micropyle lie side by side close to each other. It is the most common type of ovule and found in 80% of angiosperm families, excluding Capsella. Examples include members of Malvaceae, Cucurbitaceae, Solanaceae, and Compositae.

4. Amphitropous Ovule: Curvature affects the nucellus, resulting in a horseshoe-shaped embryo sac. Micropyle approaches the hilum closely. Also known as a transverse ovule, found in plants like mirabilis, lemna, and poppy (papaver).
   
5. Campylotropous Ovule: The ovule body is curved, with micropyle and chalaza not in a straight line. Nucellus is curved, but the embryo sac remains straight. Micropyle is close to the hilum, found in leguminosae and cruciferae families (e.g., Capsella).

6. Circinotropous Ovule: The ovule body becomes inverted and then straightened due to funiculus growth, forming a coil around the entire ovule. Micropyle is away from the hilum. Also called a coiled ovule, found in the cactaceae family, such as Opuntia.