Characteristics

Nontracheophytes

Hepatophyta

Anthocerophyta

Bryophyta

Liverworts Hornworts Mosses

No filamentous stage; gametophyte flat Embedded archegonia; sporophyte grows basally Filamentous stage; sporophyte grows apically (from the tip)

Tracheophytes

Nonseed tracheophytes

Lycophyta Pteridophyta

Club mosses Ferns and allies

Microphylls in spirals; sporangia in leaf axils Differentiation between main axis and side branches

Seed plants

Gymnosperms Cycadophyta Ginkgophyta Gnetophyta Pinophyta

Cycads Ginkgo Gnetophytes Conifers

Compound leaves; swimming sperm; seeds on modified leaves Deciduous; fan-shaped leaves; swimming sperm Vessels in vascular tissue; opposite, simple leaves Seeds in cones; needlelike or scalelike leaves

Angiosperms Angiospermae

Flowering plants

Endosperm; carpels; much reduced gametophytes; seeds in fruit

a No extinct groups are included in this classification.

a No extinct groups are included in this classification.

features the formation, by mitosis and cytokinesis, of a multicellular embryo and eventually the mature diploid plant (Figure 29.2). This multicellular, diploid plant is the sporo-phyte ("spore plant").

Cells contained in sporangia (singular, sporangium, "spore vessel") on the sporophyte undergo meiosis to produce haploid, unicellular spores. By mitosis and cytokinesis, a spore forms a haploid plant. This multicellular, haploid plant is the gametophyte ("gamete plant") that produces haploid gametes. The fusion of two gametes (syngamy, or fer-

Multicellular

Multicellular

Haploid And Diploid Plants

Multicellular sporophyte

29.2 Alternation of Generations A diploid sporophyte generation that produces spores alternates with a haploid gametophyte generation that produces gametes by mitosis.

Multicellular sporophyte

29.2 Alternation of Generations A diploid sporophyte generation that produces spores alternates with a haploid gametophyte generation that produces gametes by mitosis.

tilization) results in the formation of a diploid cell—the zygote—and the cycle repeats.

The sporophyte generation extends from the zygote through the adult, multicellular, diploid plant; the gametophyte generation extends from the spore through the adult, multicellular, haploid plant to the gamete. The transitions between the generations are accomplished by fertilization and meiosis. In all plants, the sporophyte and gametophyte differ genetically: The sporophyte has diploid cells, and the gametophyte has haploid cells. In the three basal plant clades, the gametophyte generation is larger and more self-sufficient, while the sporo-phyte generation is dominant in those groups that appeared later in plant evolution.

Some protist life cycles also feature alternation of generations, suggesting that the plants arose from one of these pro-tist groups. But which one?

The Plantae arose from a green algal clade

Much evidence indicates that the closest living relatives of the plants are members of a clade of green algae called the charophytes. The charophytes, along with some other green algae and the plants, form a clade that is sister to the chloro-phytes (see Figure 29.1), but we don't yet know which charo-phyte clade is the true sister group to the plants. Stoneworts of the genus Chara are charophytes that resemble plants in terms of their rRNA and DNA sequences, peroxisome con-

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 573

Leaf Type Peroxisomes Charophytes

(b) Coleochaete sp.

29.3 The Closest Relatives of Land Plants The plant kingdom probably evolved from a common ancestor shared with the charo-phytes, a green algal group. (a) Molecular evidence seems to favor stoneworts of the genus Chara as sister group to the plants. (b) Evidence from morphology indicates that the group including this coleochaete alga may be sister to the land plants.

(b) Coleochaete sp.

29.3 The Closest Relatives of Land Plants The plant kingdom probably evolved from a common ancestor shared with the charo-phytes, a green algal group. (a) Molecular evidence seems to favor stoneworts of the genus Chara as sister group to the plants. (b) Evidence from morphology indicates that the group including this coleochaete alga may be sister to the land plants.

tents, mechanics of mitosis and cytokinesis, and chloroplast structure (Figure 29.3a). On the other hand, strong evidence from morphology-based cladistic analysis suggests that the sister group of the plants is a group of charophytes that includes the genus Coleochaete (Figure 29.3b). Coleochaete-like algae have several features found in plants, such as plasmod-esmata and a tendency to protect the young sporophyte.

Whether they were more similar to stoneworts or to Coleochaete, the ancestors of the plants lived at the margins of ponds or marshes, ringing them with a green mat. From these marginal habitats, which were sometimes wet and sometimes dry, early plants made the transition onto land.

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Responses

  • Rosario Lettiere
    WHAT CHARACTERISTICS CAUSE cHARA TO RESEMBLE LAND PLANTS?
    4 years ago
  • kati
    What characteristics cause Chara to resemble a land plant?
    4 years ago
  • ALFRED
    How does chara superficially resemble a land plant?
    11 months ago

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