Q.The arrangement of ovules within the ovary is known as placentation. What does the term placenta refer to? Draw various types of placentations in the flower as seen in T.S. and V.S.

The placenta refers to the tissue inside the ovary to which the ovules are attached. The arrangement of ovules on this placenta within the ovary is what is called placentation, and different flowering plants show different placentation patterns depending on how the placenta is positioned and how many ovules it bears.

The commonly recognised types of placentation are:

• Marginal — the placenta forms a ridge along the fused margin of a single carpel, with ovules borne in one or two rows along it (as in a pea pod).
• Axile — the placenta occupies the central axis of a multi-chambered (multilocular) ovary, at the point where the septa (partitions) of the carpels meet, with ovules attached along this central column in each chamber (as in a hibiscus-type ovary).
• Parietal — the placenta lies along the inner wall of a single-chambered ovary formed from more than one carpel, with ovules attached to the ovary wall (as in mustard).
• Free central — the placenta forms a central column that is not connected to the ovary wall by septa, with ovules attached all around this free-standing column, in a one-chambered ovary (as in Dianthus).
• Basal — the placenta develops at the base of the ovary, usually bearing a single ovule (as in sunflower). …

Within a flower's ovary, ovules are not scattered at random — they are attached to a specific region of tissue called the placenta. The particular pattern in which the placenta (and hence the ovules) is arranged within the ovary is what is meant by placentation, and this pattern is a reliable, characteristic feature used to distinguish different plant families.

The main recognised types of placentation are:

• Marginal placentation: found in ovaries formed from a single carpel, where the placenta develops as a ridge along the fused margin of that carpel, and the ovules are borne in one or two rows along this ridge.
• Axile placentation: found in ovaries with more than one chamber (locule), where the placenta occupies the central axis, exactly where the partitions (septa) separating the chambers meet, and ovules are attached along this central column within each chamber.
• Parietal placentation: found in a single-chambered ovary that has nevertheless been formed by the fusion of more than one carpel, where the placenta develops along the inner wall of the ovary rather than at the centre.
• Free central placentation: found in a single-chambered ovary where the placenta forms a central column that stands free, not connected to the ovary wall by any septa, with the ovules attached all around this free column.
• Basal placentation: the placenta develops at the very base of the ovary, and typically bears just a single ovule there. …

Method: Sorting the Five Placentation Types by Two Structural Questions

Rather than listing all five placentation types in a row, sort them using just two yes/no structural questions, which naturally organises the five types into a small decision tree and clarifies what "placenta" itself means along the way.

Define the term first, as the foundation the sorting depends on: the placenta is the tissue inside the ovary to which ovules are physically attached — think of it as the ovary's internal "anchor point(s)" for its ovules.

Sorting Question 1 — Is the ovary single-chambered (unilocular) or multi-chambered (multilocular)?

• If MULTI-chambered → the placenta sits at the central axis, where the partitions (septa) between chambers meet, with ovules attached along this axis in each chamber → this is Axile placentation.
• If SINGLE-chambered → proceed to Question 2.

Sorting Question 2 (only for single-chambered ovaries) — Where exactly does the placenta sit: at the margin, along the wall, in a free-standing central column, or only at the very base?

• Along the fused margin of a single carpel → Marginal.
• Along the inner ovary wall (formed from more than one fused carpel, but staying one chamber) → Parietal.
• On a central column NOT connected to the wall by any septa → Free central.
• Only at the very base of the ovary, typically bearing one ovule → Basal. …