Plant Morphology and Anatomy
What the exam tests
This domain tests external plant form and internal tissue arrangement. Expect questions on root, stem, and leaf modifications; inflorescence types; flower symmetry, aestivation, and placentation; and the anatomy of monocot and dicot stems and roots, including secondary growth.
Core rules and concepts
- Root modifications: storage (carrot), prop roots (banyan), stilt roots (maize), pneumatophores (Rhizophora).
- Stem modifications: rhizome, tuber, bulb, corm; tendrils and thorns are modified stems or branches.
- Leaf modifications: tendril (pea), spine (Opuntia), pitcher (Nepenthes), phyllode (Acacia).
- Inflorescence: racemose has unlimited growth with younger flowers near the apex; cymose has limited growth with the oldest flower at the apex.
- Placentation: marginal (pea), axile (tomato), parietal (mustard), free-central (Dianthus), basal (sunflower).
- Dicot stem: vascular bundles in a ring, open, collateral. Monocot stem: scattered, closed bundles. Monocot root: polyarch xylem. Dicot root: tetrarch xylem.
- Secondary growth occurs in dicot stems and roots through vascular cambium and cork cambium.
Worked example
An ovary has ovules attached to a central axis but no septa. Which placentation is it? The central axis with no partitions is free-central placentation, as in Dianthus. If septa were present and ovules were attached to the axis, it would be axile. The presence or absence of septa is the deciding feature.
Traps that cost marks
Do not call a bulb a root - it is a modified shoot. Do not confuse axile with free-central placentation. Monocot stems usually lack secondary growth because their bundles are closed. A tendril can be a modified leaf or stem, so read the example carefully.