30.8: Leaves - Leaf Structure and Arrangment
- Sketch the basic structure of a typical leaf
Structure of a Typical Leaf
Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Leaves also have stipules, small green appendages usually found at the base of the petiole. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin .
Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern . Monocots and dicots differ in their patterns of venation. Monocots have parallel venation in which the veins run in straight lines across the length of the leaf without converging. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. Ginkgo biloba is an example of a plant with dichotomous venation.
Leaf Arrangement
The arrangement of leaves on a stem is known as phyllotaxy . The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, opposite, or whorled. Plants that have only one leaf per node have leaves that are said to be either alternate or spiral. Alternate leaves alternate on each side of the stem in a flat plane, and spiral leaves are arranged in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled.
Key Points
- Each leaf typically has a leaf blade ( lamina ), stipules, a midrib, and a margin.
- Some leaves have a petiole, which attaches the leaf to the stem; leaves that do not have petioles are directly attached to the plant stem and are called sessile leaves.
- The arrangement of veins in a leaf is called the venation pattern; monocots have parallel venation, while dicots have reticulate venation.
- The arrangement of leaves on a stem is known as phyllotaxy; leaves can be classified as either alternate, spiral, opposite, or whorled.
- Plants with alternate and spiral leaf arrangements have only one leaf per node.
- In an opposite leaf arrangement, two leaves connect at a node. In a whorled arrangement, three or more leaves connect at a node.
Key Terms
- petiole : stalk that extends from the stem to the base of the leaf
- lamina : the flat part of a leaf; the blade, which is the widest part of the leaf
- stipule : small green appendage usually found at the base of the petiole