Polypodium Life Cycle

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Figure 21.16 Common ferns with typically dissected fronds. A. Cinnamon fern. The cinnamon-colored fertile fronds in the center are nonphotosynthetic and produce large numbers of sporangia. B. Holly fern. C. A maidenhair fern. D. 'Ama'uma'u, a small Hawaiian tree fern. (A, B Courtesy Perry J. Reynolds)

that they are actually clusters of sporangia (Fig. 21.19). The sporangia may be scattered evenly over the lower surfaces of the fronds, but they are often confined to the margins. The sporangia are mostly found in numerous discrete clusters called sori (singular: sorus). In many ferns, the sori, while they are developing, are protected by thin individual flaps of colorless tissue called indusia (singular: indusium). As the sporangia mature, the indusium, which often resembles a tiny semi-transparent umbrella attached by its base to the frond surface, shrivels and exposes the sporangia beneath.

Most of the sporangia are microscopic and stalked and look something like tiny transparent baby rattles with a conspicuous row of heavy-walled brownish cells along the edge. This row of cells, that looks like a tiny millipede, is called an annulus. It functions in catapulting spores out of the sporangium with a distinct snapping action influenced by moisture changes in the cells (Fig. 21.20).

Sporocytes undergo meiosis in the sporangia, usually producing either 48 or 64 spores per sporangium. Sporangia of some of the primitive adder's tongue ferns may have up to 15,000 spores, however, and the number of

Chapter 21

young sporophyte gametophyte

Chapter 21

young sporophyte gametophyte

Polypodium Young Sporophyte

germinating spore gametophyte (prothallus)

rhizoid

Figure 21.17 Life cycle of a fern.

germinating spore gametophyte (prothallus)

rhizoid

Figure 21.17 Life cycle of a fern.

Prothallus
Figure 21.18 A crozier (fiddlehead) of a tropical fern.

sporangia is often so great that it has been estimated that a single beech fern plant will produce a total of 50 million spores.

Two kinds of spores are produced in certain aquatic or amphibious ferns such as the clover fern (Marsilea) and Salvinia. Single large megaspores are produced in some sporangia, while numerous tiny microspores are produced in others. The vast majority of fern species, however, produce only one kind of spore.

After the spores have been flung out of their sporangia, they are dispersed by wind; relatively few end up in habitats suitable for their survival. Such habitats include shady, wet ledges and rock crevices or moist soil. The spores can also easily be germinated on damp clay flowerpots in the home or greenhouse. Those that germinate in favorable locations produce little "Irish valentines," or prothalli (sin-

The Seedless Vascular Plants: Ferns and Their Relatives

Flap Indusia

sporangium D.

margin forms a false indusium hair sporangium

Polypodium Sporangia

margin vein blade sporangium D.

blade

Life Cycle Flowering Plants

indusium of sorus opening of-

"pocket" formed by indusium over sorus young sorus young sorus

Polypodium Sporangia

margin vein blade indusium splitting open, revealing sporangia

Figure 21. 19 Pinnae of fern fronds, showing some types of arrangements of sporangia. A. Cheilanthes. B. Polypodium. C. Cystopteris. D. Cibotium. E. Davallia. F. Cyathea.

gular: prothallus), as the green heart-shaped gametophytes of these and other seedless vascular plants are more properly called (Fig. 21.21). These structures often curl slightly at their edges and may be 5 to 6 millimeters (0.25 inch) in diameter; they are visible without a microscope.

Prothalli are only one cell thick, except toward the middle where they are slightly thicker. Antheridia are interspersed among the rhizoids produced on the lower surface of the central area of most prothalli, with archego-nia also being produced, usually closer to the notch of the heart-shaped gametophyte. The archegonia are somewhat flask shaped, with curving necks that protrude slightly above the surface, whereas the antheridia are more spherical and often elevated above the surface on short stalks. In a few species, antheridia and archegonia are produced on separate prothalli.

A single antheridium may produce from 32 to several hundred sperms, each with few to many flagella. Fertilization of an egg takes place within an archegonium and is chemically influenced so that it usually occurs with a sperm from a different prothallus. Only one zygote develops into a young sporophyte on any prothallus, regardless of the number of eggs that may be fertilized. This sporophyte usually has smaller, simpler fronds during its first season of growth, but typical full-sized fronds grow from the persisting rhizomes in succeeding years.

414 Chapter 21

414 Chapter 21

Annulus Cyathea
Figure 21.20 Release of spores from a fern sporangium. A. An intact sporangium. B. Spores being ejected as the sporangium splits; the annulus first draws back and then snaps forward.
Life Cycle Flowering Plant

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  • eija-riitt
    What is the function of annulus of fern?
    3 years ago

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