Structure and Development of Ovule

The structure in flowering plants that develops into a seed after fertilization is called the ovule.

The gynoecium is the female reproductive part of the flower. It is composed of one or more units called pistils (also called carpels). Each pistil has exactly three parts: the Stigma, the Style, and the Ovary.

Three Parts of a Pistil:

Stage A:

• The ovule begins as a parenchymatous mound arising on the placenta.
• The outer layer of cells forms the epidermis.
• A single cell within the mound differentiates as the megaspore mother cell (MMC).

Stage B:

• The megaspore mother cell becomes more prominent and enlarges within the mound.
• The mound continues to grow and develop into the nucellus.

Stage C:

• The inner integument initiates from the epidermal layer, arising as a ring below the nucellar apex.
• The outer integument begins to initiate from the sub-epidermal layer.
• The nucellar epidermis becomes distinctly visible.

Stage D:

• Both integuments grow and cover the nucellus, leaving a small opening called the micropyle.
• The megaspore mother cell undergoes meiosis to form a linear tetrad of megaspores.

Fig. 15 A-D. Development of ovule: A-C. successive stages in the development of ovule; D. showing a linear tetrad of megaspores

The ovule (megasporangium) arises from the placenta inside the ovary. It is the most critical structure in plant reproduction - after fertilisation, it develops into a seed, while the ovary develops into the fruit.

Structure of an ovule

In anatropous ovules, the micropyle is directed downward toward the funicle. This is the most common ovule type in angiosperms. The funicle becomes fused with the ovule body, forming a ridge called the raphe.

(a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed); (b) Multicarpellary, syncarpous pistil of Papaver ; (c) A multicarpellary, apocarpous gynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule

• The ovule is an integumented megasporangium that develops into a seed after fertilisation.
• It is attached to the placenta by a stalk called the funicle, and the junction between them is the hilum.
• The ovule arises as a parenchymatous mound that grows into a mass of tissue called the nucellus, which contains the megaspore mother cell.
• The nucellus provides nutrition to the developing embryo sac and is enclosed by one or two protective integuments.
• The inner integument forms first from the epidermal layer, while the outer integument forms later from the subepidermal layer.
• The integuments cover the nucellus but leave a small opening at the tip called the micropyle, through which the pollen tube enters.
• The basal part of the ovule, opposite the micropyle, is called the chalaza.
• The megaspore mother cell undergoes meiosis to form megaspores, one of which develops into the embryo sac (female gametophyte).