The Life Cycle Of Ferns

The life cycle of a typical fern, shown in Figure 30-2, is similar to the moss life cycle. Like mosses, most ferns are homosporous. And as in mosses, the fern sporophyte grows from the gametophyte. But in the fern life cycle, the sporophyte, not the gametophyte, is the dominant generation. Fern gametophytes are tiny (about 10 mm, or 0.5 in., in diameter), flat plants that are anchored to the soil by rhizoids. Both antheridia and archegonia may form on the lower surface of a fern gametophyte. When water is present, sperm released by antheridia swim to archegonia. One sperm fuses with the egg in an archego-nium, forming a zygote, which is the first cell of a new sporophyte.

The zygote grows into an embryo and then into a young fern sporophyte through mitotic cell division. Once the sporophyte can survive on its own, the gametophyte will die. A mature fern sporo-phyte usually has compound leaves that are known as fronds.

Adult sporophyte

figure 30-2

The life cycle of most ferns alternates between a large sporophyte (the dominant generation) and a small gametophyte. Both the egg and sperm may be produced on the same gametophyte.

figure 30-2

The life cycle of most ferns alternates between a large sporophyte (the dominant generation) and a small gametophyte. Both the egg and sperm may be produced on the same gametophyte.

Fronds grow from an underground stem, or rhizome. In most ferns, certain cells on the underside of the fronds develop into sporangia. In many ferns, the sporangia are clustered together. A cluster of sporangia is called a sorus (soH-ruhs), plural, sori (soH-ree). Cells inside a sporangium undergo meiosis, forming haploid spores. At maturity, the sporangium opens and the spores are catapulted 1 cm (0.4 in.) or more and are then carried away by air currents. When the spores land, they may grow into new gametophytes.

Word Roots and Origins sorus from the Greek soros, meaning "heap"

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