Reproduction

Small sporangia that are fused together in threes and resemble miniature yellow pumpkins are produced at the tips of very short stubby branches in the upper parts of the angular stems. Spores released from the sporangia germinate slowly in the soil, in tree bark or tree fern "bark" crevices, or in other similar habitats. In Hawaii, they also grow in old lava flows.

The gametophytes, which lack pigmentation, develop from the spores beneath the soil surface and are easily overlooked. Sometimes they resemble tiny, transparent dog bones that are only about 2 millimeters (0.08 inch) wide and seldom more than 6 millimeters (0.25 inch) long. They are cylindrical and, like the aerial stems, may branch dichotomously. They have no chlorophyll and absorb their nutrients via one-celled, rootlike rhizoids aided by mycorrhizal fungi. Archegonia and antheridia develop randomly over the surface of the same gametophyte. After a sperm unites with an egg in an archego-nium, the zygote develops a foot and a rhizome. As soon as the rhizome becomes established, upright stems are produced, and the rhizome separates from the foot (Fig. 21.2).

Whisk ferns are native to tropical and subtropical regions. In the United States, they are found in Florida, Louisiana, Texas, Arizona, and Hawaii. They are extensively cultivated in Japan and sometimes become a weed in greenhouses around the world. Tmesipteris, a close relative, has leaflike appendages; it is native to Australia and the South Pacific.

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