The Land Plants

Chapter 23

Closest Living Relative to a Land Plant

• DNA Evidence suggests that charophyte green algae are the sister group to land plants, implying that land plants evolved from oceanic algae.

• Continental movements that caused global climate to become drier favored groups that were better adapted to drought (seed plants)

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23.2 Evolutionary Trends of Land Plants

Land Problem: Gravity

Favored Adaptations:

Cell Walls, Water Vacuole, Tough tissues like lignin and bark

Land Problem:

Obtaining Nutrients

Favored Adaptation: Roots and Shoots

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• Vascular System - tissue reinforced by lignin that distributes materials through leaves, stems, and roots of sporophytes
• Xylem: Distributes water and minerals
• Phloem: Distributes products of photosynthesis

Land Problem:

Drying Out

Favored Adaptations:

Waxy cuticles on their leaves.

Stomata that close to prevent water loss.

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Land Problem:Reproduction

Favored Adaptation:

• Plants shifted from gametophyte-dominated life cycle (in bryophytes) to sporophyte-dominated life cycle (in other plants)
• Gametophyte: Haploid stage that forms gametes by mitosis
• Sporophyte: Diploid stage that forms spores by meiosis; a sporangium helps protect and disperse spores

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Land Problem: Fertilization

Favored Adaptation:

Pollen are dry capsules that carry sperm in the air to another plant.

Land Problem:

Protecting the Offspring

Favored Adaptation:

• Seed - An embryo sporophyte and some nutritive tissue enclosed inside a waterproof coat

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23.1 Evolution on a

Changing World Stage

Diversity of Modern Land Plants

• Land plants (embryophytes) evolved from a lineage of green algae (charophytes) after the ozone layer made life on land possible

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Timeline of Land Plant Evolution

• Nonvascular Bryophytes include the earliest land plant lineages.

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• Vascular seedless plants evolved next (Tracheophytes)

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• The first Vascular seed plants were gymnosperms

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• Vascular seed plants with flowers evolved last (Angiosperms)

Early Seedless Vascular Plants (Extinct)

• Cooksonia and Psilophyton

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23.3 The Bryophytes

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• Bryophytes include three land plant lineages – liverworts, hornworts, and mosses – with a gametophyte-dominated life-cycle

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Characteristics of Bryophytes

• Nonvascular (no xylem or phloem)

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• No Roots - Gametophyte attaches to soil or surfaces by rootlike rhizoids

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• Sperm swim through water to eggs

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• The sporophyte forms on, and is nourished by, the dominant gametophyte

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• Spores are the dispersal form

Liverworts

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• Liverworts
• Reproduces sexually, or asexually by producing gemmae in cups on the gametophyte

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Hornworts

• Have a hornlike sporophyte with its base embedded in gametophyte tissue; spores form in an upright capsule (sporangium)
• Sporophyte has chloroplasts
• Gametophyte has nitrogen-fixing bacteria

Mosses

• Sporophyte consists of a capsule (sporangium) embedded in gametophyte
• Mosses are the most diverse group of bryophytes; peat mosses (Sphagnum) are ecologically and commercially important

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23.4 Seedless Vascular Plants (Tracheophytes)

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• A sporophyte with lignified vascular tissue (xylem and phloem) dominates the life cycle

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• Two lineages of seedless vascular plants
• Lycophytes (club mosses, spike mosses)
• Monilophytes (whisk ferns, horsetails, ferns)

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Lycophytes

• Club moss (Lycopodium)
• Spores form inside a soft, cone-shaped strobilus

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• Spike moss (Selaginella) “resurrection plant”
• The most drought-tolerant vascular plants

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Monilophytes

• Whisk ferns (Psilotum)
• Have rhizomes (underground stems) but no roots
• Photosynthetic stems appear leafless
• Spores form in fused sporangia at tips of branches

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Monilophytes

• Horsetails (Equisetum)
• Have rhizomes, hollow stems with silica deposits, and nonphotosynthetic leaves
• Photosynthesis occurs in stems and leaflike branches
• Spores in strobili form tiny gametophytes

Monilophytes

• Ferns
• The most diverse seedless vascular plants
• Most sporophytes have leaves and roots that grow out from rhizomes
• Spores are dispersed from clusters of sporangia (sori) on lower surfaces of frond leaves
• Many live as epiphytes attached to another plant

Early Seed Vascular Ferns (Extinct)

23.7 Gymnosperms

• Gymnosperms
• Vascular seed plants with “naked” seeds
• One of the two modern lineages of seed plants

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• Gymnosperms include conifers (such as pines), cycads, ginkgos, and gnetophytes

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Gymnosperm Life Cycle

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• Gymnosperms release pollen and seeds

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• Ovules form in in strobili or, in the case of conifers, in woody cones

23.8 Angiosperms— �The Flowering Plants

• Angiosperms are the most diverse plant lineage and the only plants that make flowers and fruits

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• In the Mesozoic, angiosperms began adaptive radiation to all land and many aquatic habitats

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Keys to Angiosperm Success

• Short life cycles and rapid growth

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• Specialized reproductive structures (flowers)

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• Specialized pollination and dispersal structures
• Wind and animal pollinators
• Fruits that float or stick
• Seeds that survive animal digestive tracts

Specialized Angiosperm Structures

• A flower is a specialized reproductive shoot
• Seeds develop inside the ovaries (chambers that enclose ovules) of flowers
• After fertilization, an ovary becomes a fruit

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Pollination and Coevolution

• Pollinators
• Animals (such as insects that feed on pollen) move pollen grains from male parts of one flower to female parts of another

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• Coevolution
• Over time, plants and their animal pollinators jointly evolved; changes in one exerts selection pressure on the other