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What Makes a Fern a Fern
by Judith I. Jones
Most gardeners think of ferns as plants with finely divided "feathery" foliage and no flowers. They also describe the foliage of some flowering plants that are not ferns as "ferny." So what key characteristics distinguish true ferns from plants with fern-like foliage? The absence of flowers, fruits and seeds is certainly the most obvious feature. When examined at the appropriate time of the year mature ferns generally have rusty patches on the underside of the foliage. These are the structures that produce spores, the unicellular structures involved in fern reproduction. Linnaeus classified ferns into genera based on the position and shape of these spore-producing organs.
The number of temperate fern genera is not too daunting; only 15 percent of the world's fern species occur in the temperate zone. A very small fraction of these are available to gardeners. To simplify matters even more, the most popular and easily grown species come from a small number of genera. And only a few species have yielded many of the cultivars that gardeners grow (a cultivar is a plant specially selected or bred for horticultural use). Becoming familiar with the ferns native to your area will give you a good foundation for learning about other species.
Fern Structure
To be competent at fern identification you don't need a laboratory full of equipment. All you need are a good 1OX to 2OX hand lens, a few good fern identification guides and, if possible, some help from a local expert or enthusiast. You don't even need the hand lens to see most of the important fern parts.
Although mature ferns consist of roots, stems and leaves, just as other, more familiar plants do, different terms are used to describe the various fern parts.
CROZIERS
One very distinctive feature common to most ferns is the manner in which the newly emerging foliage unfolds in a scroll-like fashion. The newly developing fern leaves are called croziers or fiddleheads. The form of the crozier—ranging from a tightly coiled geometric spiral to a lax shepherd's crook—and the way it gradually expands is often a clue to the identity of the fern. Some ferns that don't exhibit these coiled croziers are the moonworts, staghorn ferns and grape ferns.
FROND ELEMENTS
The most obvious part of any fern is the portion above ground, which is known as the frond. The frond is further delineated into the stipe and the blade or lamina. The stipe is the stem-like portion of the frond that bears no foliage and connects the leafy blade to the mostly underground rhizome. The blade comprises the supporting stalk—or rachis, if compound, and the attached primary leafy segments or pinnae. Pinnae (plural of pinna) may be divided into one or more distinct secondary segments known collectively as pinnules.
Rhizome and fronds are often clothed with a protective covering (indument) of hairs and/or scales. Hairs are only one cell wide and one or more cells long. Scales, which are several cells wide and long but mostly one cell thick, are broader. The absence or presence of hairs and/or scales, especially their overall shape and color, are other clues to the identification of ferns.
Another characteristic used to classify ferns is the veins. Veins run from margin to midvein. Where these branch but do not form a united network of veins, they are referred to as free. While most ferns have free veins, some do have veins that unite and form distinctive networks helpful in identification.
FROND DISSECTION
The overall shape of the blade, the characteristic outline of its margins and the number of segments into which it is divided all help distinguish one fern from another.
Special terms are used to describe the degree of frond dissection: Undissected blades are called simple. A blade that is divided once, completely to the rachis, with each pinna narrowed at the base or stalked where it meets the main rachis is pinnate. Bipinnate blades have two divisions, the pinna, which is attached to the main rachis, and the pinnules, which are attached to the pinna rachis, also called the costa. Tripinnate and quadripinnate are further degrees of dissection. To determine the degree of dissection or "pinnateness," start counting from the largest division, the pinna, and continue counting each complete division after that. Another way is to follow the stipe to the rachis at the base of the first pinnae and count the rachis as zero. Then count the number of branches off the rachis. For example, a bipinnate fern has two branches off the rachis: the pinna is one and the pinnule is two, so the frond is bipinnate.
There is another kind of dissection to consider—partial dissection of a pinna or pinnule. This trait of not being divided all the way to a narrow attachment to the main rachis or pinna rachis is termed pinnatifid. Thus, pinnate-pinnatifid means that the blade is divided more than one time but not quite into two distinct divisions. Blades may also be more divided at the base and less divided at the top.
This isn't as complicated as it sounds. Look closely at a few ferns from your garden. A little practice will make the botanical lingo much less baffling.
RHIZOMES
An understanding of the structure and function of the rhizome is crucial to growing ferns successfully. Rhizomes are also key to the vegetative propagation of ferns. However inconspicuous the rhizome may be, it not only provides a vital link between the roots and the frond, but also determines the plant's habit. The rhizome may be erect, holding the fronds in a close, vase-like cluster, or it may recline or creep horizontally, with fronds arising in an irregular cluster fashion. The symmetry of the fronds arising from upright rhizomes is certainly a major factor in our visual appreciation of ferns that display this habit. An erect rhizome gives rise to a crown.
Reclining rhizomes look like upright rhizomes that have toppled over. Some, such as Dryopteris stewartii, produce croziers arranged in a curious spiral-like curve, but are similiar in appearance to their erect counterparts when their fronds are fully flushed. Horizontally creeping rhizomes may be closely or loosely branched, affecting the manner in which their fronds arise. Long-established plants may appear somewhat different in habit because their extensive colonies of overlapping rhizomes produce a denser foliage arrangement.
Fern roots can help in the identification of a species. For example, the roots of Osmunda regalis, the royal fern, are spongy, overlapping mats. Healthy roots are a lovely, translucent caramel color with creamy yellow growing tips. Older roots are dark brown or black but still have creamy yellow root tips.
FERTILE FRONDS
The first fronds produced in spring are sterile fronds. The fertile fronds, those bearing sporangia, the spore-producing organs, arise later in the season. Fertile fronds may be the same shape as sterile fronds. On some species they may be a slightly modified version of the sterile frond or even a completely different shape and structure altogether. For example, the sterile and fertile fronds of Athyrium filix-femina are similar. Those of Osmunda claytoniana are slightly different, and those of Matteuccia struthiopteris are entirely different. When a fern exhibits this condition of distinctly different sterile and fertile fronds it is called dimorphic. Observing whether the sterile and fertile fronds are identical or different is another tool in the identification of ferns.
SPORE-PRODUCING ORGANS Remember that the system of classification upon which fern identification is based revolves around their spore-producing organs. Individual clusters of sporangia are referred to as sori, in the singular sorus, and they may be grouped in round, oblong, linear or other configurations. The sori may be scattered along the veins or vein endings or cover the entire underside of the frond. Some sori may be enveloped by a special tissue-thin mantle known as the indusium (in the plural, indusia). Marginal sori are frequently overlapped by a flap of folded blade tissue which is called a false indusium. The shape and arrangement of the sori as well as the shape and presence of the indusia or false indusia are of key importance in fern identification.
The Life Cycle of Ferns
Because ferns lack flowers, their reproductive cycle was long shrouded in mystery. It wasn't until the 17th century that botanists came to realize that spores give rise to more ferns. In the mid-19th century when the microscope was refined, spore germination finally could be studied scientifically.
ALTERNATING GENERATIONS
A two-phase reproductive cycle known as the alternation of generations is characteristic of ferns. The conspicuous phase that we recognize as a fern and in which spores are produced is known as the sporophyte generation. The sexual phase, a tiny plant by comparison, is called the gametophyte. To understand this complex life history, let's begin with the formation of spores within the sporangia of the sporophyte generation.
SPORES
Most ferns produce 64 spores within each sporangium. In the center of the developing sporangium there is a single cell that divides to produce all the spores. It divides into identical cells, a process known as mitosis. These cells continue to divide until there are 16 cells, called spore mother cells. Now each spore mother cell undergoes a special kind of nuclear division known as meiosis, during which one cell produces four cells, each with only half the number of chromosomes (thus, if the mother cell contains 14, each daughter cell will contain only seven). Not only is the chromosome number reduced, but there is also interchange of genetic material between chromosomes. Thus the four new cells, destined in ferns to become spores, will be genetically different from each other and from the original mother cell. Set free from the sporophyte each spore, given the right environmental conditions, will grow by mitosis into a gametophyte (prothallus). It will produce eggs and/or sperm cells (see below). Fusion of these in pairs produces zygotes, each having the original chromosome number (say 14, using the example above) and the capability of growing into a sporophyte (see the illustration on the opposite page). Some ferns undergo more mitotic divisions before meiosis.
PROTHALLUS
A spore released from a parent plant that lands on moist soil begins to divide. As the cell divides it gradually forms a spongy, roughly heart-shaped cushion of cells. Visible to the naked eye, this is the gametophyte plant or prothallus (the plural is prothalli), on which the sexual organs involved in fertilization emerge. Like the more conspicuous spore-producing plant, the prothallus has root-like hairs, called rhizoids, that absorb and conduct nutrients and water to the cells.
While the edges of this "cushion" may be only one cell thick, the center is several cells thick and it is here that the sexual organs are generally found. Although prothalli may be bisexual, they are usually either male or female. The female egg-producing organs, archegonia, (the singular is archegonium) occur near the notch of the heart-shaped prothallus. The male sperm-producing organs, antheridia, (the singular is antheridium) occur on the "wings" and opposite the notch of the prothallus. Each archegonium has a chimney-like protuberance that is flared at the top to receive the sperm, which fertilizes the egg at the base. Each antheridium is a minute, capsule-like sac where the sperm grow.
The life cycle diagram above illustrates the current understanding of the reproductive process known as out-crossing—that is, fertilization between two different gametophytes (as opposed to self-crossing or self-fertilization). Although each prothallus may be capable of fertilizing itself, experiments have shown that the resulting sporophytes are inbred and often fail to thrive. Cross-fertilization helps ensure genetic diversity.
When an egg is fertilized by a spermatozoid the chromosome number doubles, returning to what it was in the parent sporophyte. The fertilized egg, or zygote, begins to divide to form the young sporophyte. Initially, a short-lived primary root grows. It is supplanted by roots growing from the newly forming rhizome. With a food- and water-absorbing system in place, the first fronds emerge. These bear only a remote resemblance to their mature parents, especially in the more dissected species. Successive fronds increasingly resemble those of the parent plants.
Judith Jones is a self-taught fern specialist. Her nursery, Fancy Fronds, in Seattle, has introduced six new ferns to the American nursery trade. She has lectured throughout North America and England and her nursery's displays of hardy ferns have won four gold medals at flower shows in the Northwest. She is a self-proclaimed pteridomaniac.