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Ferns
Numbering just over 10,000 species worldwide, ferns, technically called pteridophytes, are more advanced than the bryophytes, in part because they have vascular tissue (xylem and phloem). These plants are capable of growing much larger than the bryophytes (some to more than 80 feet tall), partly as a result of their vascular tissues. They are considered less advanced than the gymnosperms and angiosperms because they do not produce seeds. The typical fern has a horizontal underground stem known as a rhizome; the above-ground structures are fronds or leaves. In many ferns, the newly emerging frond is called a crozier or fiddlehead because it looks like the carved spiral at the end of a violin.
Fern Reproduction [Illustration]
Sporophyte development is very rapid in common garden and woodland ferns. The zygote develops an embryonic root, leaf, stem, and foot. The foot apparently aids in absorbing nutrients from the gametophyte. The stem develops slowly, but ultimately produces leaves. The first few leaves of juvenile ferns differ from those of the mature sporophyte. As growth continues, the leaves finally begin to resemble those of the adult species.
When the minute sporophyte has become established, the gametophyte dies. The gametophyte generation in ferns, as in other seedless vascular plants, is relatively short-lived and simple. The sporophytic phase is dominant in the life cycle. As in all vascular plants, what we recognize as "the plant" is the sporophytic generation, and at maturity it develops sporangia, the spore-producing organs.
In the common ferns, sporangia are borne on the undersides of fronds, or on modified fronds. The first fronds produced in spring are sterile-that is, they lack reproductive structures. The fertile fronds arise later in the season. In some species, such as lady fern (Athyrium filix-femina) the sterile and fertile fronds are similar. In other species-sensitive fern (Onoclea sensibilis), for example-the sterile and fertile fronds are very different.
The fertile regions on fronds that bear sporangia are known as receptacles, and the group of sporangia on a single receptacle is called a sorus. In many species, but by no means in all, the sorus is covered during development by a flap of tissue called an indusium. The shape and arrangement of the sori are different in different species.
Spores arise by meiosis from sporocytes. For the most part, immature sori are a pale, whitish color, although there are exceptions, such as autumn fern (Dryopteris erythrosora), whose sori, which are produced in the fall, are bright red. As the spores inside each sporangium mature, they typically get darker, until they are a deep brown or black. Not all spores mature to a deep brown or black, however. Polypodiums are ripe when they are buttercup yellow; osmundas, when they are green.
The sporangia of ferns is typically a thin-walled case, usually on a stalk, and surrounded by a ring of thick-walled cells known as the annulus, which aids in opening the sporangium when the spores are mature.
The indusium, if present, ultimately shrivels when the spores have matured, exposing the sporangia. The annulus breaks near the base of one side, tearing the sporangium apart, and arches backward. Then the annulus snaps forward abruptly, catapulting the spores into the air. Ferns produce tremendous numbers of spores, but the special requirements of most species for moisture and shade effectively reduce the number of gametophytes that ultimately develop.
Spores that are deposited by air currents on sufficiently moist soil and rocks germinate within five to six days, and the gametophyte begins to develop. Most fern spores also require light for germination; those of the bracken fern (Pteridium aquilinum) are an exception. Early in germination, colorless rhizoidal cells form at the base of the filament of green cells. These rhizoids, or root-like hairs, absorb and carry water and nutrients to the developing gametophyte.
Eventually, the gametophyte becomes heart-shaped, and in some species may be as big as half an inch in diameter. The fern gametophyte was long ago called the prothallus or prothallium, because it was known to be the precursor of the fern plant, even before its sexual function was clearly understood.
As well-nourished, bisexual gametophytes mature they develop antheridia and archegonia, normally on their undersides. The female, egg-producing organs, or archegonia, occur near the notch of the heart-shaped prothallus. The male, sperm-producing organs, or antheridia, occur on the "wings" and opposite the notch. Each archegonium has a chimney-like protuberance that is flared at the top to receive the sperm. Each antheridium is a minute, capsule-like sac where the sperm grow.
The cells inside the neck of a mature archegonium dissolve, creating a moist passageway through which the sperm can swim to the mature eggs. The eggs of several archegonia may be fertilized, but usually only one of the zygotes develops into a juvenile sporophyte.
It is interesting to note that while bisexual prothalli may be capable of fertilizing themselves, experiments have shown that the resulting sporophytes often fail to thrive. This suggests that cross-fertilization, the union of sperm and egg from different prothalli, necessary for increasing genetic variability within species, is promoted in at least some ferns.