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Ferns of the Sierra (1960) by Robert J. Rodin


THE STORY OF FERNS

Ferns differ from our cone bearing and flowering plants because ferns produce spores, but no flowers or seeds or other large specialized reproductive structures. Both groups, ferns and seed plants, have many features in common: Roots, stems, leaves, and a vascular system to conduct water, minerals and foods. Because flowers were absent the ferns and their allies were called Cryptogams (hidden reproduction) by early European botanists. Until recently, ferns and their allies usually have been included in the large category called Pteridophytes. Recently ferns have been included in the subphylum Pteropsida, along with seed plants.

In western America one thinks of ferns as common plants. The actual number of fern species is relatively few compared to the number in humid tropical regions where they occur in vast numbers and often in more specialized forms. Such specializations include very thin, delicate filmy ferns which must live in constant mist or spray, or noble tree ferns that attain great height. Only occasionally in temperate regions, but very commonly in tropical rain forests, do ferns live on trees, and thus are called EPIPHYTES. These epiphytes receive no nourishment from the living tree.

A mature fern plant consists of roots, a stem and FRONDS (leaves). The profusely branched root system serves to absorb mineral nutrients and water from the soil. The roots of some rock ferns, which live in very dry habitats, penetrate to great depths and in mysterious ways grow toward moisture. Our ferns may have short stems above ground, but more often have. an elongated underground stem called a RHIZOME. The structural features of rhizomes, such as, size, degree of branching, length, and presence or absence of scales, are important in the identification of ferns. The green FRONDS of ferns serve not only to manufacture food for the plant, but to support the reproductive structures which produce dust-like asexual SPORES on the undersides of the leaves. The leaves may be quite simple but are often compounded in various ways (see illustrations p. 56).

Leaf and stem characters are used in identification of ferns, but more important is the arrangement of the spore-bearing structures on the underside of the leaf. SPORES are produced in large numbers in special organs called SPORANGIA. SORI in the form of dots or lines are formed from clusters of sporangia. Rarely a sorus may cover much of the undersurface of a frond. Sori may form around leaf margins and in such cases the leaf usually curls around the sorus to protect the young sporangia. Any protective cover over a sorus, whether it be a curled leaf margin, a flap of tissue, or perhaps a miniature umbrella, is called an INDUSIUM (See illustrations p. 55).

The story of a fern does not end here. What happens to the spores that are formed by the millions each year on a fern plant? It would be logical to propose that they grew into new fern plants. When moisture is present and temperatures are suitable, that is what they do — only not into the fern plant with which people are most familiar. This new fern plant is part of an alternating cycle that represents the sexual phase of this plant and is a required phase in the reproductive cycle. The spore develops into a small heart-shaped green plant called a PROTHALLUS (See illustrations p. 51). RHIZOIDS, miniature root-like structures on the lower side absorb water from the soil. Among the rhizoids, numerous spherical male organs are formed called ANTHERIDIA which eventually produce myriads of motile SPERM that swim to the female organ. As many as 20 ARCHEGONIA, the female organs, may occur near the notch of the prothallus. Each archegonium contains one egg which remains in the archegonium during fertilization and afterward. The fertilized egg develops into the familiar fern generation, absorbing some nutrient from the small prothallus until it is large enough to manufacture its own food. Only one fern plant develops from a prothallus, and as we now see, the cycle of life and reproduction is completed. Thus a spore-bearing generation, the familiar fern plant, always alternates with a small sexual generation. There can be no short cut except in a few species where vegetative buds produced on the leaves provide an asexual new plant, much as garden plants are started by leaf or stem cuttings. The long underground stems or rhizomes might also be considered a vegetative method of reproduction because they promote the spreading of some species such as Brake Ferns.

Ancient forests contained ferns as far back as the Coal Age. A few ferns are known in the fossil record even before that time although the ferns were not the most ancient vascular plants.

Ferns related to our Grape Fern, Botrychium, in the Adder’s Tongue Family, Ophioglossaceae, are probably the earliest ferns in history which are related to our living forms. There are some related groups now completely extinct of which the Seed Ferns or Pteridosperms are worthy of mention. These fern-like plants were more common in the Coal Age than true ferns. The illustrations (pp. 50, 54) of fossil ferns are examples of the evidence we have of these ancient plants. More details of their preservation are discussed in the next chapter. Even though ferns were perhaps not a dominant part of the floras making up coal, they contributed to its formation.

Fig. 1 Fossil fern from Mazon Creek, Illinois
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Fig. 1 Fossil fern from Mazon Creek, Illinois, with well preserved sori. This fern was living over 250 million years ago during the Coal Age (Carboniferous Period). Specimen courtesy of Ethel Doerer.

LIFE HISTORY OF A FERN

Life History of a Fern: Pinna, Sporangium (in a sorus), Peltate Indusium, Spores, Spore, Prothallus (sexual phase), Antheridium (male organ under prothallus), Archegonium (female organ), Egg in Archegonium, Motile Sperm, Sexual Reproduction (fertilization), Young Fern Plant, Prothallus, Stem, Roots, Frond (leaf)

[click to enlarge]



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