Reproduction

If stems or rhizomes are broken up by a disturbance, such as a storm or foraging animals, the fragments can grow into new sporophyte plants. In most species, however, reproduction usually involves a sexual process. In the spring, some species produce special cream- to buff-colored non-photosynthetic stems from rhizomes (Fig. 21.12). Small conelike strobili develop at the tips of these special stems or, in other species, at the tips of regular pho-tosynthetic stems.

The strobili are usually about 2 to 4 centimeters (0.75 to 1.5 inches) long. Hexagonal, dovetailing plates at the surface of the strobilus give it the appearance of an ellipsoidal honeycomb. Each hexagon marks the top of a sporangiophore that has 5 to 10 elongate sporangia connected to the rim. The stalks of the sporangiophores are attached to the central axis of the strobilus. The sporangia surround the sporangiophore stalks and point inward. These hidden sporangia are not visible until maturity when the sporangiophores separate slightly. The spores are then released.

When the sporocytes in the sporangia undergo meiosis, distinctive-appearing green spores are produced. At one pole, the spores have four ribbonlike appendages that are slightly expanded at the tips (Fig. 21.13). The appendages are called elaters (not structurally or otherwise related to the elaters of liverworts and hornworts). The elaters are very sensitive to changes in humidity and aid spores in their dispersal. While spores are being carried by an air current, the elaters are more or less extended like wings. If a spore enters a humid air pocket above a damp area below, the elaters coil, causing the spore to drop in an area that is more likely to support germination and growth.

Germination of spores usually occurs within a week of their release. Lobed, cushionlike, green gametophytes develop and seldom grow to more than 8 millimeters (0.36 inch) in diameter. Rhizoids anchor them to the surface. At first, about half of the gametophytes are male with antheridia, and the other half are female with archegonia. After a month or two, however, the female gametophytes of most species become bisexual, producing only antheridia from then on. When water contacts mature antheridia, sudden changes in

408 Chapter 21

Life Cycle Equisetum

gametophyte

Figure 21.12 Life cycle of a horsetail (Equisetum) that has separate vegetative and reproductive shoots.

gametophyte

Figure 21.12 Life cycle of a horsetail (Equisetum) that has separate vegetative and reproductive shoots.

water pressure cause the sperms produced within to be explosively ejected. Several eggs on a female or bisexual gametophyte may be fertilized, and the development of more than one sporophyte is common.

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  • esmeralda
    What is the cycle of reproduction for flowering plants?
    8 years ago

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