Question
What are the five types of placentation in flowering plants? For each type, describe the arrangement of ovules and give an example.
(NEET + CBSE Class 12)
Solution — Step by Step
Placentation is the arrangement of ovules on the placenta inside the ovary. The placenta is the tissue that connects ovules to the ovary wall and supplies them with nutrients. Different plant families have different placentation patterns.
| Type | Ovule arrangement | Example |
|---|---|---|
| Marginal | Ovules along one margin (edge) of a single carpel | Pea, bean (Leguminosae) |
| Axile | Ovules on central axis where septa meet, in a multi-chambered ovary | Tomato, lemon, hibiscus |
| Parietal | Ovules on inner wall of ovary, no septa (or septa break down) | Mustard, cucumber |
| Basal | Single ovule at the base of ovary | Sunflower, marigold (Compositae) |
| Free central | Ovules on central column, no septa | Dianthus, Primrose |
The key differentiator is whether septa (partitions) are present:
- Axile: Septa present, ovules at the centre where septa meet
- Parietal: No septa, ovules on the ovary wall
- Free central: No septa, ovules on a free-standing central column
- Marginal: Monocarpellary ovary, ovules along one suture line
- Basal: One ovule sitting at the bottom of the ovary
Placentation Type Classification
flowchart TD
A["Placentation — where are ovules?"] --> B{"Septa present?"}
B -->|"Yes"| C["Axile: ovules on central axis"]
B -->|"No"| D{"Central column present?"}
D -->|"Yes"| E["Free central: ovules on column"]
D -->|"No"| F{"Ovules on wall or margin?"}
F -->|"On ovary wall"| G["Parietal"]
F -->|"Along one margin"| H["Marginal"]
F -->|"Single ovule at base"| I["Basal"]
C --> C1["Example: Tomato, Hibiscus"]
E --> E1["Example: Dianthus, Primrose"]
G --> G1["Example: Mustard, Cucumber"]
H --> H1["Example: Pea, Bean"]
I --> I1["Example: Sunflower"]Why This Works
Placentation type is determined by the number and arrangement of carpels and the presence of septa. It is a key taxonomic feature — plants in the same family usually share the same placentation type. All legumes have marginal placentation because they have a single carpel. All members of Solanaceae (tomato, brinjal) have axile placentation.
Common Mistake
Students confuse axile and free central placentation. Both have ovules in the centre of the ovary. The difference: in axile, septa connect the central axis to the ovary wall (creating chambers). In free central, there are no septa — the central column stands free. Think of it this way: cut the ovary — if you see chambers, it is axile. If you see an open space with a central column, it is free central.