Placentation: Arrangement of Ovules in the Ovary

Inside the ovary of a flower, ovules that will become seeds are not scattered randomly. They are attached to a special tissue called placenta in a definite arrangement known as placentation. The type of placentation depends on number of carpels, whether ovary is divided into chambers and where placenta develops.

Placentation helps identify plant families, predict how seeds are arranged in fruits like tomato, pea, orange and sunflower, and is used in taxonomy and in practical work like cutting fruits and studying seed distribution.

Key Concepts

Placenta: Ridge of tissue inside ovary bearing ovules, connected by funicle stalk. Locule: Chamber inside ovary. Marginal: Single carpel folded, placenta along ventral suture bearing ovules in two rows as in pea. Ovary is unilocular. Parietal: Two or more carpels united, ovary unilocular, placentae on inner wall where carpels meet as in mustard and papaya. Axile: Multicarpellary syncarpous ovary divided into locules by septa, placenta in centre where septa meet as in tomato, lemon and Hibiscus. Free central: Multicarpellary unilocular ovary, placenta as central column not attached by septa as in Dianthus and Primrose. Basal: Single ovule at base of unilocular ovary as in sunflower and marigold. Superficial: Ovules spread over inner surface of multilocular ovary as in water lily.

How It Works

1. During flower development carpel margins fold and fuse. The pattern of fusion decides whether septa form and where placental ridges arise.
2. Ovules develop on placenta with vascular supply for nutrients. Their position decides seed packing after fertilisation.
3. In axile type septa keep ovules in separate groups, in free central loss of septa leaves a central column, in parietal fusion without septa places placentae on wall.

Worked Example

Cut three ovaries. Sample A transverse section shows 1 chamber, ovules in 2 rows along one side seam. Sample B shows 4 chambers meeting at centre with ovules at centre angles, count ovules = 8 per chamber, total = 32. Sample C shows 1 chamber, single ovule at base. Identify. Step 1: A single carpel marginal placentation, example pea. Step 2: B multilocular with central axis, axile placentation, total ovules 8 times 4 = 32, example tomato. Step 3: C basal placentation, example sunflower. This shows how chamber number and ovule position diagnose type.

Common Misconceptions

• Misconception: Placentation is arrangement of placenta in animals. Correction: In botany it strictly means arrangement of ovules on placenta inside plant ovary.
• Misconception: Parietal and marginal are same because ovules are on wall. Correction: Marginal is monocarpellary with one placenta, parietal is multicarpellary syncarpous with multiple wall placentae but one locule.
• Misconception: Free central has septa. Correction: Septa are absent, ovules are on a free central column.

Real-World Applications

Botanists use placentation in floral keys to separate families like Brassicaceae with parietal and replum from Fabaceae with marginal. Horticulturists predict seeds per fruit in tomato axile ovaries for yield estimation. Food scientists explain why papaya seeds line the wall in parietal placentation and pea seeds line one seam. Seed analysts cut ovaries to verify hybrid purity based on locule number.

Placentation is therefore the pattern of ovule attachment inside the ovary, mainly marginal, parietal, axile, free central, basal and superficial, determined by carpel number and fusion.