The Pistil, Megasporangium (Ovule), and Embryo Sac

So far, you have studied the male side of the flower: the stamen, microsporangium, and pollen grain. Now it is time to look at the female counterpart. While the androecium produces pollen, the gynoecium (the female reproductive part of the flower) houses the ovules that will eventually become seeds. Let us explore how the gynoecium is organised, what a pistil looks like from the inside, and what makes the ovule such a carefully engineered structure.

The Gynoecium: One Pistil or Many?

The gynoecium can be built in different ways depending on the species. In some flowers, there is just a single pistil. This condition is called monocarpellary (having one carpel or pistil). Other flowers have more than one pistil, and these are described as multicarpellary (having multiple carpels).

When a flower is multicarpellary, the pistils can be arranged in one of two ways:

• Syncarpous (fused together) — the multiple pistils are joined into a single compound structure. Papaver (poppy) is a good example: its several carpels are fused so tightly that the gynoecium looks like one unit (Figure 1.7b).
• Apocarpous (remaining free) — each pistil stays separate from the others. In Michelia (a member of the magnolia family), you can see distinct, individual pistils sitting together on the thalamus (Figure 1.7c).

Think of it this way: syncarpous is like fingers clasped tightly together into a fist, while apocarpous is like fingers spread apart, each one independent.

Three Parts of a Pistil: Stigma, Style, and Ovary

Fig 1.7: (a) Dissected Hibiscus flower showing the pistil; (b) Syncarpous pistil of Papaver; (c) Apocarpous gynoecium of Michelia; (d) Diagrammatic view of a typical anatropous ovule

Every pistil, whether it stands alone or is part of a larger gynoecium, has three distinct regions (Figure 1.7a):

1. Stigma (the receptive tip at the very top) — this is where pollen grains land during pollination. You can think of it as a landing pad designed to catch and hold pollen. Its surface is often sticky or feathery, which helps it trap incoming grains.

2. Style (the elongated, slender stalk beneath the stigma) — this connects the stigma to the ovary below. After a pollen grain germinates on the stigma, the pollen tube must grow the entire length of the style to reach the ovule. In some species, the style is very short; in others, it can be quite long.

3. Ovary (the swollen, bulging base of the pistil) — this is the most important part from a reproductive standpoint, because it contains the ovules. The ovary sits at the bottom of the pistil, resting on the thalamus (the receptacle of the flower).

Inside the Ovary: The Cavity, Placenta, and Ovules

Cut open an ovary, and you find a hollow space called the ovarian cavity (also known as the locule). Lining the inner wall of this cavity is a tissue called the placenta. If you recall the types of placentation from Class XI, you already know that the arrangement of the placenta varies across species.

Growing out from the placenta are the structures that make the ovary so important: the ovules. In botanical terminology, ovules are called megasporangia (singular: megasporangium), because each one is a spore-bearing structure that will produce the female megaspore.

How many ovules does an ovary contain? That depends entirely on the species:

Ovule count	Examples
One ovule per ovary	Wheat, paddy, mango
Many ovules per ovary	Papaya, water melon, orchids

This range is enormous. A mango flower produces just a single ovule inside its ovary, while an orchid ovary may contain thousands. The number of ovules directly determines how many seeds a fruit can potentially produce.

Anatomy of the Ovule: A Structure Built for Protection

Fig 1.7 (d): Detailed view of a typical anatropous ovule

Let us now zoom in on a single ovule (Figure 1.7d) and look at each part from the outside in.

The Funicle and the Hilum: Attachment Point

Each ovule is connected to the placenta by a short stalk called the funicle (the stalk that anchors the ovule to the placenta). At the point where the body of the ovule merges with the funicle, there is a region called the hilum (the junction between the ovule body and the funicle). The hilum marks the exact boundary where the ovule proper begins and its stalk ends.

Integuments: The Protective Wrappers

Surrounding the main body of the ovule are one or two protective layers called integuments (the outer envelopes that shield the inner tissues of the ovule). These integuments wrap around the ovule almost completely, covering it like a coat. However, they do not seal the ovule shut. At the very tip, there is a tiny gap where the integuments do not meet. This opening is called the micropyle (the small pore at the tip of the ovule through which the pollen tube will eventually enter).

The end of the ovule that bears the micropyle is known as the micropylar pole. At the opposite end, you find the chalaza (the basal part of the ovule, directly opposite the micropyle). The chalaza represents the base of the ovule, and the axis running from the micropylar pole to the chalazal pole defines the overall orientation of the structure.

Nucellus: The Nutritive Core

Enclosed within the integuments is a mass of cells called the nucellus (the central tissue of the ovule that stores food reserves and surrounds the embryo sac). The cells of the nucellus are packed with abundant reserve food materials. This nutritive tissue plays a vital role: it nourishes the developing embryo sac and, eventually, the young embryo after fertilisation.

Embryo Sac: The Female Gametophyte

Nestled deep within the nucellus is the most important structure of all: the embryo sac, also known as the female gametophyte (the structure within the ovule that contains the egg cell and other cells needed for fertilisation and seed development). An ovule typically contains just a single embryo sac, and this embryo sac develops from a megaspore (a spore produced by meiotic division inside the megasporangium).

The embryo sac is where the actual female reproductive cells reside. Everything else in the ovule, the funicle, integuments, and nucellus, exists to support and protect this single, critical structure. How the embryo sac develops from a megaspore and what cells it contains will be covered in the next topic.

Putting It All Together

To summarise the structural hierarchy, working from the outside in:

Pistil → Ovary → Ovarian cavity (locule) → Placenta → Ovule (megasporangium) → Integuments → Nucellus → Embryo sac (female gametophyte)

Each layer serves a purpose. The ovary protects the ovules. The integuments shield the nucellus. The nucellus feeds the embryo sac. And the embryo sac carries the egg cell that will fuse with a male gamete to begin a new generation. It is a neatly nested set of structures, each one wrapped inside the next, all working together toward a single goal: successful reproduction.