Summary

1. Four phyla of seedless vascular plants are recognized: Psilotophyta (whisk ferns); Lycophyta (club mosses and quillworts); Equisetophyta (horsetails and scouring rushes); and Polypodiophyta (ferns).

2. Whisk ferns (Psilotum) are the simplest of all living vascular plants, consisting of evenly forking green stems that have small protuberances called enations, but no leaves; roots are also lacking. The stem contains a central cylinder of xylem and phloem.

3. Whisk fern spores germinate into tiny gametophytes, with antheridia and archegonia scattered over their surfaces. The zygote develops a foot and a rhizome. Upright stems are produced when the rhizome separates from the foot.

4. Tmesipteris, an Australian relative of whisk ferns, has leaflike appendages. Fossil plants resembling whisk ferns have been found in Silurian geological formations.

5. There are two genera of club mosses with living members. Ground pines (Lycopodium) develop sporangia in the axils of sporophylls. Several sporophytes may be produced from one gametophyte. Spike mosses (Selaginella) are heterosporous and have a ligule on each microphyll; microspores develop into male game-tophytes with antheridia, and the megaspores develop into female gametophytes with archegonia.

6. Quillworts (Isoetes) are heterosporous and have quill-like microphylls that arise from a cormlike base. The corms have a cambium that remains active for many years.

7. Some fossil relatives of club mosses were large dominant members of the forests and swamps of the Carboniferous period.

8. Club moss spores have been used for flash powder, medicinal purposes, as talcum powder, and to staunch bleeding. The plants themselves have been used as ornamentals, novelty items, Christmas ornaments, and for intoxicating purposes.

9. Horsetails and scouring rushes (Equisetum) accumulate deposits of silica in their epidermal cells and have made good scouring material. They occur in both unbranched and branched forms. The stems are jointed and ribbed and have tiny scalelike leaves in whorls at each joint.

10. The stems of Equisetum are centrally hollow and contain cylindrically arranged carinal and vallecular canals. The stems arise from rhizomes that branch extensively below the surface of the ground.

11. Some species of Equisetum produce non-photosynthetic stems. Conelike strobili are produced in the spring in all species. Equisetum spores have ribbonlike elaters that are sensitive to changes in humidity.

12. Equal numbers of male and female gametophytes are produced, but female gametophytes may become bisexual. The development of more than one sporophyte from a gametophyte is common.

13. Ancient relatives of horsetails were the size of trees when they flourished in the Carboniferous period of 300 million years ago.

14. Horsetails have been used for food after the parts containing silica were removed, but they are not recommended for human consumption. They have also been used medicinally as a diuretic, as an astringent, and in the treatment of venereal disease and tuberculosis. Other uses include a hair wash, a mineral indicator, and a metal polish. Cannel coal consists primarily of spores of giant horsetails that were reduced to carbon.

15. Fern leaves (fronds) are typically divided and feathery in appearance but vary greatly in form. They usually first appear as croziers that unroll and expand.

16. Patches of sporangia appear on the lower surfaces of fern fronds. The sporangia commonly occur in sori, which may be protected by indusia. Each sporangium has an annulus that functions in catapulting mature spores out of the sporangium.

17. Fern gametophytes (prothalli) develop after spores germinate. Most prothalli contain both archegonia and antheridia; only one zygote develops into a sporophyte.

18. Possible ancestors of ferns are found in Devonian deposits estimated to be 375 million years old.

19. Ferns are used as ornamentals, air filters, a source of "bark" for growing orchids and other plants, a source of stuffing materials for bedding, in tropical construction, as food, and in numerous folk medicinal applications. Other uses include basketry and weaving material,

Chapter 21

ingredient in brewing ale, and ingredient in the preparation of chamois leather. One floating fern forms dense mats and is believed to suffocate mosquito larvae.

20. Fossils are recognizable prehistoric organic objects that are formed in different ways. Molds, casts, compressions, and imprints are formed when material buried by silt or other sediment has hardened into rock and the organic material has slowly been washed away by water.

21. Petrifactions are uncompressed rocklike materials in which the original cell structure has been preserved. Coprolites are fossilized dungs that may contain pollen grains and other plant and animal parts. Unaltered fossils are those of plants or animals that may have fallen into bodies of oil or water or snowfields and were not subjected to decay.

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