The fern blade can be divided or "cut" into smaller structures called pinnae, pinnules and pinnulettes depending on the number of cuts. Some ferns have undivided blades which look like tongues or straps ... an unfernlike appearance.
Once-cut: the blade is divided once into pinnae (singular pinna).
Twice-cut: the blade is divided twice into pinnae which into turn, are divided into pinnules.
Thrice-cut: pinnae are divided into pinnules which are then divided into pinnulettes.
The overall shape or silhouette of the frond is an important characteristic when identifying ferns.
Broadest at base
Semi-tapering to base
Fully tapered to base
The basic structure of a fern include the roots and the leaves, appropriately referred to as fronds. A frond usually has two parts ... the blade and the stipe (stem). The stipe supports the blade and is below it, but, it also extends through the blade where it is referred to as a rachis or axis. The blade is often divided (sometimes several times) into smaller and smaller structures, beginning with pinnae (singular pinna). This dividing is referred to as "cutting". Some ferns are cut once, while others may be cut twice or even three times (thrice). The degree to which a blade is cut gives the fern its overall lacy appearance.
Sporangia (singular sporangium) are small, globose (round) structures within which spores are produced. They are difficult to see individually without the aid of a hand-lens or magnifying glass. Sporangia do not occur singly, but in clusters. Clusters of sporangia are referred to sori (singular sorus). Many ferns produce their sori on the bottom of their fronds in which case they commonly called "fruit dots." Sori can take-on many shapes and sizes including circular, crescent shaped, elongated, kidney-shaped, cupped or star-shaped.
Other ferns produce a separate stalked branch terminated by a dense collection of sporangia. Still other ferns produce two totally separate types of fronds, sterile and fertile. This image to the left is the fertile frond of Sensitive Fern.
Still other ferns produce two totally separate types of fronds, sterile and fertile. This image to the left is the fertile frond of Sensitive Fern.