Morphology of Flowering Plants · Lesson 3 of 11
Leaf Types and Phyllotaxy
“Distinguish simple and compound leaves and use phyllotaxy to classify leaf arrangement.”
• Determine whether a dissected blade is simple or compound. • Differentiate pinnate and palmate compound leaves by leaflet attachment. • Explain the diagnostic role of axillary buds. • Recognise alternate, opposite and whorled phyllotaxy from specimens.
What makes a leaf 'simple' or 'compound'?
The word simple does not mean small, smooth-edged or uninteresting. A simple leaf has one blade, even when the blade is deeply lobed. Its incisions do not divide the blade into separate leaflets reaching the midrib or principal axis. A compound leaf is divided into distinct leaflets attached to a shared leaf axis or to a common point. A single compound leaf may look like a cluster of small leaves, so identifying the full unit requires checking its connection to the stem.
Look for the axillary bud at the base of the entire leaf. In both simple and compound leaves, the bud is in the axil between the true leaf attachment and the stem. Individual leaflets of a compound leaf do not ordinarily have their own axillary buds. This provides a dependable method for distinguishing leaflets from several simple leaves borne along a twig.
A leaf with one blade that may have incisions but is not separated into distinct leaflets reaching the central axis.
A leaf whose lamina is divided into separate leaflets, all together constituting one leaf.
Pinnate and palmate compound leaves
In a pinnately compound leaf, leaflets occur along a common elongated axis called the rachis. The rachis is the continuation of the leaf's central axis, comparable to the midrib of a simple leaf. Neem is a useful example: several separate leaflets occur in rows along the rachis. In a palmately compound leaf, the leaflets converge at the tip of the petiole, radiating from a common point like fingers from a palm. Silk cotton provides the familiar example.
Do not use leaflet number alone to identify the type. A leaf may have many leaflets in either arrangement. The decisive question is where the leaflets connect: distributed along an extended rachis in pinnate leaves, or gathered at one point in palmate leaves. Also avoid calling a pinnate compound leaf an inflorescence; one involves leaflets, whereas the other involves flowers.
| Observation | Pinnately compound | Palmately compound |
|---|---|---|
| Point of leaflet attachment | Several positions along rachis | One point at petiole tip |
| Shape analogy | Feather-like axis | Hand-like radiating group |
| Example | Neem | Silk cotton |
| Bud position | At base of complete leaf | At base of complete leaf |
Phyllotaxy describes an arrangement on the stem
Phyllotaxy refers to how complete leaves are placed at nodes along a stem or branch. In alternate phyllotaxy, only one leaf occurs at each node, with successive leaves situated at different positions. China rose, mustard and sunflower commonly illustrate this. In opposite phyllotaxy, two leaves arise at a node on opposing sides, as in guava and Calotropis. In whorled phyllotaxy, three or more leaves arise from the same node, as in Alstonia.
These patterns influence how leaves occupy space on a shoot, but the chapter's core identification criterion is the number and position of leaves at a node. An alternate pattern is not defined by the leaves all pointing in random directions; the 'one at each node' criterion is stronger. Likewise, a photograph that shows two neighbouring alternate leaves crossing over one another may misleadingly resemble an opposite pair if their heights differ slightly.
A practical identification sequence
When examining an unfamiliar shoot, first locate a node and its bud. Count complete leaves at that node: one means alternate, two opposite and more than two whorled. Then select one full leaf and determine whether its blade is continuous or divided into leaflets. If divided, follow each leaflet attachment back to a long rachis or to a shared terminal point. Record both descriptions independently: a plant may possess opposite simple leaves, alternate compound leaves or many other combinations.
Leaf complexity (simple or compound) and phyllotaxy (alternate, opposite or whorled) answer different questions. One describes how a leaf blade is subdivided; the other describes where whole leaves arise along the stem.
Connecting descriptions to observations
A sketch of the whole leaf and another of a short stem segment can prevent misidentification. Label the rachis only in the pinnate example and the attachment point in the palmate example. In your stem diagram, mark each node as a small horizontal level and count leaves per level. If several leaves overlap in one photo, rotate the stem or use more than one view before assigning a phyllotaxy category.
Quick Quiz
Quiz
Where is an axillary bud normally found in a compound leaf?
A leaf has five leaflets joined at the end of one petiole. Which form is most likely?
Which observation distinguishes opposite phyllotaxy?
A plant has three complete leaves arising from one node. What is the arrangement?
Which criterion supports calling a leaf pinnately compound?
Practice Problems
- Distinguish a deeply lobed simple leaf from a compound leaf.
- Explain the difference between pinnate and palmate compound leaves with examples.
- Define all three phyllotaxy types and give an example.
- Why is an axillary bud useful when identifying leaflets?
Model Answers
1. A deeply lobed simple leaf retains a single continuous blade across its central region. A compound leaf is divided into distinct leaflets attached to the axis or common attachment point.
2. Pinnate: leaflets distributed along a rachis, such as neem. Palmate: leaflets meet at a single point at the petiole tip, such as silk cotton.
3. Alternate means one leaf per node, such as china rose. Opposite means two leaves per node, such as guava. Whorled means more than two leaves per node, such as Alstonia.
4. A true leaf has a bud at its axil with the stem; individual leaflets do not ordinarily have axillary buds, so the bud identifies the boundary of the complete leaf.
Key Takeaways
Simple and compound describe subdivision of a single leaf, not leaf size. Pinnately compound leaves bear leaflets along a rachis; palmately compound leaves bear leaflets at a common point. Locate the axillary bud to distinguish whole leaves from leaflets. Alternate = one leaf per node; opposite = two; whorled = more than two.