Introduction to the identification of ferns
The following explanations are offered for those beginning the study of ferns. An understanding of the terms and concepts introduced here may be helpful or necessary to successfully identify ferns.
Ferns do not have aerial stems in the manner of many other vascular plants. Instead, the leaves arise directly from an underground stem (rhizome) or a very short vertical stem at or near the soil surface. Therefore, fern stems are often very inconspicuous and the portions of ferns most often noticed are the leaves. Each fern leaf is called a frond, and each fern plant may have from one to many fronds. If an individual fern plant has more than one frond, they are often arranged in a conspicuously clumped arrangement, or less commonly they may be spread out along an underground stem.
Fronds in some Wisconsin fern species may be as small as one inch long, or as large as 3 or more feet long in other species. The fronds of many species are herbaceous, dying back at the end of each growing season. New leaves are produced in the next growing season from the stem that overwinters at or below the soil surface. A few ferns have fronds which remain green over winter. The structure of fern fronds ranges from simple (the blade is undivided) to 3 or 4 times compound (the blade is divided into smaller parts and perhaps each part is again divided, etc.). A common form of blade division, known as pinnate produces leaflets called "pinnae" (singular: pinna) attached to an elongate central axis called the rachis. Each pinna may be again divided (into "pinnules") and so on, extending to 3 or 4 levels of division referred to as "once pinnate" for one level of division, "twice pinnate" if the units of the first division are again divided, etc. Blades which are deeply lobed, but not fully divided into pinnae are said to be pinnatifid. The level to which a blade is divided is often an important character in the identification of fern species.
A portion of the central axis of the blade often extends below the blade, corresponding to the petiole of some angiosperm plants, and is referred to as a stipe. At its lower end the stipe is attached to the stem. The rachis and stipe may bear scales, hairs or glands (together comprising the category known as "indument") that may also be important in the keys.
Ferns do not produce flowers or seeds, but rather reproduce by spores. One of the most important characteristics used to identify ferns involves the structures in which each species produces spores. The spores are very small and are produced inside structures called sporangia which may be located on the surface of otherwise unaltered fronds. In some species the sporangia are produced only on a conspicuously modified portion of a frond (e.g. only at the tip, or in the mid portion of a frond), or the entire frond producing sporangia may be different in appearance from fronds not producing sporangia. Fronds producing sporangia are said to be fertile and fronds not producing sporangia are said to be sterile.
The sporangia open at maturity and the spores are dispersed to start new plants. The sporangia are aggregated on the fronds in groups called sori (singular = sorus), the shape of which may vary from species to species (e.g. round, elongate). In many species each sorus is covered by a thin, often membranous structure called an indusium (plural = indusia). Indusia are fragile and may be torn and fragmented and therefore may appear to be absent on older fronds for which the mature sporangia have opened to release the spores. In some species the sori are not covered by a true indusium and are located beneath the inrolled margins of the fronds, referred to as false indusia. Some species lack indusia of any type. The shape and location of the sori and the nature of indusia are often important in the identification of ferns.