Pteridophytes of Wisconsin: Ferns and Fern Allies
Glossary of selected Pteridophyte terminology
adaxial. Refers to the surface of the frond that faces the axis of the stem or faces inward when the fronds are arranged in a roughly circular clump. Many people intuitively call this the upper or top surface of the frond because as the fronds lean slightly outward they bend over to some degree, making the adaxial surface the upper or top surface.
auricle. An ear-like projection of a pinna or pinnule.
bi-pinnate blade. A blade that is divided into pinnae and each pinnae is again fully divided into smaller segments (called pinnules).
blade. The flat, green portion of a frond. Some fern books refer to the blade as the lamina (plural: laminae).
costa. Plural: costae. The central axis or midrib of a pinna.
fertile frond. A frond bearing sori. Contrast with a sterile frond which has no sori.
frond. A leaf of a fern. The fern stems are often at or below the soil surface and are very inconspicuous, so the fronds are the portion of ferns that most people see. Fronds may be as small as one inch in length or as much as 3 feet or more and they may range from undivided to divided several times into very small segments.
glands. Tiny structures found on the surface of some fern fronds. They often appear as very short hairs with an enlarged distal (free) end. Glands are very fragile and may be worn away on older fronds. Observation of glands often requires the use of a hand lens.
hydathode. In some taxa, such as Dryopteris, the veins end in raised, broadened tips, without reaching the margin of the blade segments. These "hydathodes" are often whitish in color, and better observed on the adaxial (upper) surface of the frond.
indusium. Plural: indusia. A covering over a sorus. Indusia are often thin and fragile and may be easily broken. The shape of indusia, how they attach to the frond, and their location on the frond are often important characters in keys to the ferns.
pinna. Plural: pinnae. A segment of a blade produced by one level of division. If a blade is once pinnate, the segments formed are pinna. Each pinna can be further divided into segments called pinnules.
pinnate. A term describing a common method of division of fern fronds, in which the blade is divided into segments arising from both sides of a central axis (rachis). Each segment of the blade could then be divided again in a pinnate fashion, making the blade bi-pinnate or twice pinnate. The process can be continued to further levels of division--see "bipinnate" and "tripinnate" for examples.
pinnatifid. Similar to pinnate, except the blade is deeply divided but not deeply enough to produce separate segments, i.e. at least a narrow strip of green blade remains along the rachis (central leaf axis).
pubescent. A general word referring to any kind of hairiness of the plant parts. In some fern books the word "indument" is used to describe all hairs and scales collectively.
rachis. The central axis of the frond; it might be called a mid-rib.
scales. Small, flat, leafy structures sometimes attached to the rachis or stipe of a frond, or to the costae. They are generally brown or blackish rather than green.
simple leaf. A leaf with an undivided blade.
sorus. Plural: sori. A cluster of sporangia (the structures which produce spores). The shape of the sorus and its location on the frond are often important characters in identifying ferns.
sporangium. Plural: sporangia. The roughly spherical structures which produce spores on the surface of fern fronds. When the sporangia are mature they open to release the spores. Usually several to many sporangia are attached near one another in conspicuous clusters. These clusters of sporangia are known as sori.
sterile frond. A sterile frond is a frond that bears no sporangia, and therefore has no sori. In some species the sterile fronds are the same shape ans size as the fertile fronds. In other species sterile and fertile fronds may differ in size and or shape.
stipe. A continuation of the rachis below the base of the blade. It is in effect the petiole of the leaf (frond).
tri-pinnate. A blade that is divided into three levels of segments. The blade is divided into segments called pinnae, each of the pinnae is again divided into segments, and those segments are again divided.
ultimate segment. The smallest level of segments into which a blade is divided. If a blade is once pinnate, the pinnae would be the ultimate segments. If the blade was twice pinnate into pinnae and pinules, then the pinnules would be the ultimate segments, etc.