Frond: Definition & Structure

Instructor: Catherine Paul

Catherine has taught high school science and has a master's degree in biology.

Learn more about ferns, an ancient plant that has withstood the test of time. Uncover what makes the fern leaf, or frond, distinctive by learning more about its structure, then test your knowledge with a quiz.

Frond Fundamentals

Fern Canyon: It's the location for the movie Jurassic Park: The Lost World. As its name suggests, the movie features dinosaurs that roam across a park. But the park isn't an ordinary one; it's actually a forest plush with primitive ferns. Though the dinosaurs in the movie are fictional, the ferns are not. These ferns have actually withstood the test of time, and have unique characteristics that separate them from other plants.

Ferns
Ferns

The most obvious part of the fern is its leaf, or frond. The frond is considered a megaphyll because it is a large leaf with branching veins (think of 'mega' meaning 'big'). Ferns have thin but large leaves with a wide spread to allow the leaf to optimally capture sunlight energy. Ferns are vascular plants with workings similar to our own circulatory systems. Tubes of xylem carry water, and phloem transports nutrients throughout the leaf.

Frond Structure

Let's compare the fern's basic structure to that of a quill feather pen. A quill has a strong center shaft used to support its outreaching feathers. The frond also has a central upward growing stalk, generally called a petiole. In ferns this structure is called the rachis. The rachis acts as the center support for the entire leaf. Similar to the feather section of the quill, outward from the rachis grows many leaflets, or pinnae. The pinnae itself contains many sub-sections of leaflets called pinnules.

The underside of the frond typically contains the reproductive centers, or sporangia, which produce the spores. Fronds usually have clusters of sporangia called sori that stand out from the green leaf as yellow, orange, brown, or dark dots. Sori are arranged differently on the leaf according to the genus of the fern, with some outwardly lining the edges of the leaf structure. Some sori are protected by a membrane outgrowth of the leaf called indusia.

The Sori of a Fern
The Sori of a Fern

Once the spores are released from a mature plant, they germinate to form the prothallus. The prothallus is a heart-shaped structure containing both the sperm and egg. Fertilization can occur when the sperm receives enough water to swim to the egg. The fertilized embryo grows a root into the ground and a plant leaf above, eventually becoming a mature frond.

The Prothallus Stage of a Fern
The Prothallus Stage of a Fern

The root structure of a mature frond is supported underground by the rhizome. From the rhizome grows many adventitious roots, which are typically located beneath the leaf structures.

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