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hus far, our survey of all organisms traditionally

T regarded as plants has taken us from simple one-celled prokaryotic organisms, through more specialized eukaryotic protists and fungi, and on to the bryophytes. We now take up several phyla of plants whose members have internal conducting tissues but do not produce seeds.

In the introduction to the Plant Kingdom in Chapter 20, we mentioned that the ancestors of the bryophytes and the vascular plants probably were multicellular green algae that became established on the land over 400 million years ago. As green organisms moved ashore, they developed several new features, including sterile jackets of cells around gametangia, embryos within protective tissues, stomata for gas exchange, and a cuticle. Most of these features at least partly protected vital parts from drying out. Bryophytes have no significant internal tissues for conducting water, and any water used internally is absorbed directly through the above-ground parts. External water is required for fertilization.

During the early stages of vascular plant evolution, internal conducting tissues (xylem and phloem) began to develop, true leaves appeared, and roots that function in absorption as well as anchorage developed. At the same time, gametophytes became progressively smaller and more dependent on sporo-phytes that became progressively larger. Initially the sporo-phyte also afforded gametophyte protection.

Unlike conifers and flowering plants, the primitive vascular plants discussed in this chapter do not produce seeds. They include the ferns and a number of their relatives often referred to as "fern allies." Four phyla of seedless vascular plants are recognized: 1. Phylum Psilotophyta (whisk ferns). Features unique to members of this phylum of vascular plants include sporophytes that have neither true leaves nor roots, and stems and rhizomes that fork evenly.

2. Phylum Lycophyta (club mosses and quillworts). The stems of these plants are covered with microphylls (leaves with a single vein whose trace is not associated with a leaf gap—see Chapter 6). Most microphylls are photosynthetic.

3. Phylum Equisetophyta (horsetails and scouring rushes). The sporophytes of these plants have ribbed stems containing silica deposits and whorled scalelike microphylls that lack chlorophyll.

4. Phylum Polypodiophyta (ferns). The sporophytes of ferns have megaphylls (leaves with more than one vein and a leaf trace that is associated with a leaf gap—see Chapter 6) that are often large and much divided.

Fossils, discussed at the end of the chapter, give us clues to the ancestry of some of these plants and also to the origins of seed plants discussed in the chapters to follow.

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