The Plant Kingdom: Seedless Plants Chapter 27

Learning Objective 1 What are some environmental challenges of living on land? How do some plant adaptations meet these challenges?

Colonization of Land by Plants Required anatomical, physiological, and reproductive adaptations Waxy cuticle protects against water loss Stomata for gas exchange needed for photosynthesis

Alternation of Generations 1 2 parts of plant life cycle haploid gametophyte generation diploid sporophyte generation Gametophyte plant produces gametes by mitosis gametes fuse (fertilization) to form zygote (first stage of sporophyte generation)

Basic Plant Life Cycle

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Spore Sperm Egg HAPLOID (n) GAMETOPHYTE GENERATION Meiosis Fertilization DIPLOID (2n) SPOROPHYTE GENERATION Figure 27.2: The basic plant life cycle. Plants have an alternation of generations, spending part of the cycle in a haploid gametophyte stage and part in a diploid sporophyte stage. Depending on the plant group, the haploid or the diploid stage may be greatly reduced. Zygote Embryo Sporophyte Fig. 27-2, p. 582

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Sperm Egg Fertilization Spore Meiosis DIPLOID (2n) SPOROPHYTE GENERATION Sporophyte Zygote Embryo Figure 27.2: The basic plant life cycle. Plants have an alternation of generations, spending part of the cycle in a haploid gametophyte stage and part in a diploid sporophyte stage. Depending on the plant group, the haploid or the diploid stage may be greatly reduced. Stepped Art Fig. 27-2, p. 582

Alternation of Generations 2 Zygote develops into multicellular embryo protected and nourished by gametophyte Mature sporophyte plant develops from the embryo produces sporogenous cells (spore mother cells)

Alternation of Generations 3 Sporogenous cells undergo meiosis to form spores first stage in gametophyte generation

KEY CONCEPTS Plants undergo an alternation of generations between multicellular gametophyte and sporophyte generations

Gametangia 1 Most plants have multicellular gametangia with protective jacket of sterile cells surrounding gametes

Gametangia 2 Antheridia Archegonia gametangia that produce sperm cells gametangia that produce eggs

Gametangia

Developing sperm cells Sterile cells Antheridium Figure 27.3: Plant gametangia. Shown are generalized moss gametangia. Fig. 27-3a, p. 583

Egg Archegonium Sterile cells Fig. 27-3b, p. 583 Figure 27.3: Plant gametangia. Shown are generalized moss gametangia. Fig. 27-3b, p. 583

Vascular Transport System In ferns and other vascular plants xylem conducts water and dissolved minerals phloem conducts dissolved sugar

KEY CONCEPTS Adaptations to life on land that have evolved in plants include a waxy cuticle to prevent water loss; multicellular gametangia; stomata; and for most plants, vascular tissues containing lignin

Learning Objective 2 From which green algal group are plants hypothesized to have descended? Describe supporting evidence

Charophytes Plants probably arose from charophytes a group of green algae Based on molecular comparisons of DNA and RNA sequences close match between charophytes and plants

Plant Evolution

NONVASCULAR BRYOPHYTES VASCULAR SEEDLESS PLANTS VASCULAR SEED PLANTS Gymnosperms Club mosses Angiosperms Hornworts Liverworts Mosses Ferns Evolution of seeds Figure 27.4: Plant evolution. Cladograms such as this one represent an emerging consensus that is open to change as new discoveries are made. Although the arrangement of nonvascular, seedless vascular, and seed plant groupings is widely recognized, the order in which the hornworts, liverworts, and mosses evolved is not yet resolved. Evolution of dominant sporophyte, vascular tissue Evolution of cuticle, multicellular gametangia, multicellular embryos Green algal ancestor Fig. 27-4, p. 584

KEY CONCEPTS Biologists infer that plants evolved from aquatic green algal ancestors known as a charophytes

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Learning Objective 3 What features distinguish bryophytes from other plants?

Bryophytes Nonvascular (lack xylem and phloem) unlike other land plants Dominant gametophyte generation unlike other plants Sporophytes remain permanently attached nutritionally dependent on gametophytes

Learning Objective 4 What are the three phyla of bryophytes?

Mosses (Phylum Bryophyta) Gametophytes are green plants that grow from a filamentous protonema

Liverworts (Phylum Hepatophyta) Many gametophytes are flattened, lobelike thalli (others are leafy)

Hornworts (Phylum Anthocerophyta) Have thalloid gametophytes

Fig. 27-5, p. 585 Figure 27.4: Plant evolution. Cladograms such as this one represent an emerging consensus that is open to change as new discoveries are made. Although the arrangement of nonvascular, seedless vascular, and seed plant groupings is widely recognized, the order in which the hornworts, liverworts, and mosses evolved is not yet resolved. Fig. 27-5, p. 585

Nonvascular bryophytes seed plants Vascular seedless plants Vascular Nonvascular bryophytes seed plants Vascular Figure 27.4: Plant evolution. Cladograms such as this one represent an emerging consensus that is open to change as new discoveries are made. Although the arrangement of nonvascular, seedless vascular, and seed plant groupings is widely recognized, the order in which the hornworts, liverworts, and mosses evolved is not yet resolved. Green algal ancestor Fig. 27-5 (1), p. 585

KEY CONCEPTS Mosses and other bryophytes lack vascular tissues and do not form true roots, stems, or leaves

Learning Objective 5 Describe the life cycle of mosses Compare their gametophyte and sporophyte generations

Mosses 1 Green moss gametophyte Fertilization bears archegonia / antheridia at top of plant Fertilization sperm cell fuses with egg cell in archegonium (zygote)

Mosses 2 Zygote grows into embryo develops into moss sporophyte attached to gametophyte

Mosses 3 Meiosis When spore germinates occurs within capsule of sporophyte produces spores When spore germinates grows into a protonema forms buds that develop into gametophytes

Moss Life Cycle

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Antheridia at the tip of the gametophyte shoot Gametophyte plants Buds on protonema 1 Antheridia with sperm cells Spore germinates Spores released 6 Protonema Sperm cell HAPLOID (n) GAMETOPHYTE GENERATION Archegonium with egg 2 Meiosis Fertilization DIPLOID (2n) SPOROPHYTE GENERATION 5 Figure 27.6: The life cycle of mosses. The gametophyte generation is dominant in the moss life cycle. After sexual reproduction, the sporophyte grows out of the gametophyte. See text for a detailed description. Calyptra Zygote Capsule 4 Sporogenous cells that undergo meiosis 3 Sporophyte Embryo Gametophyte plant Fig. 27-6, p. 586

Moss Sporophytes

Capsule Seta Foot Fig. 27-7, p. 587 Figure 27.7: Moss sporophytes. Each consisting of a foot, seta, and capsule, the sporophytes grow out of the top of the gametophytes. Spores are produced within the capsule. Shown is the haircap moss (Polytrichum commune). Fig. 27-7, p. 587

Liverwort Life Cycle

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Antheridiophore Archegoniophore Male thallus Germination of spores and development of young gametophyte 1 Female thallus Antheridia with sperm cells 5 Gemmae cup Spores released Sperm cell Male and female gametophyte plants HAPLOID (n) GAMETOPHYTE GENERATION Archegonia with eggs 2 Fertilization Meiosis DIPLOID (2n) SPOROPHYTE GENERATION 4 Foot Figure 27.8: The life cycle of the common liverwort (Marchantia polymorpha). The dominant generation is the gametophyte, represented by separate male and female thalli. The stalked, umbrella-shaped structures are the antheridiophores, with antheridia that produce sperm cells, and the archegoniophores, with archegonia that each bear an egg cell. See text for a detailed description. Seta Zygote Tissue derived from archegonium Capsule Embryo Sporogenous cells that undergo meiosis 3 Sporophyte Fig. 27-8, p. 588

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Learning Objective 6 What features distinguish seedless vascular plants from algae and bryophytes?

Seedless Vascular Plants Have adaptations that algae and bryophytes lack vascular tissues dominant sporophyte generation Reproduction depends on water as transport medium for motile sperm cells (as in bryophytes)

Learning Objective 7 What are the two phyla of seedless vascular plants?

Club Mosses (Phylum Lycopodiophyta) Sporophytes consist of roots, rhizomes, erect branches, and microphylls (leaves)

Vascular supply to enation Microphyll (one vein) Stem Microphyll Vascular tissue Enation Vein Smooth stem Enation Vascular supply to enation Microphyll (one vein) Figure 27.10: Evolution of microphylls and megaphylls. Dichotomous branching (in b) is branching into two equal halves. Webbing (in b) is the evolutionary process in which the spaces between close branches become filled with chlorophyll-containing cells. Fig. 27-10a, p. 590

Club Mosses

Green algal ancestor Vascular seedless plants Nonvascular bryophytes Vascular seed plants Figure 27.11: Club mosses. (a, Redrawn from M. Hirmer, Handbuch der Paläobotanik, R. Olderbourg, Munich, 1927.) Green algal ancestor Fig. 27-11 (1), p. 591

Strobilus Leaves (microphylls) Fig. 27-11 (a-b), p. 591 Figure 27.11: Club mosses. (a, Redrawn from M. Hirmer, Handbuch der Paläobotanik, R. Olderbourg, Munich, 1927.) Fig. 27-11 (a-b), p. 591

Ferns (Phylum Pteridophyta) Largest and most diverse group of seedless vascular plants Fern sporophyte consists of a rhizome that bears fronds and true roots Includes whisk ferns and horsetails

Ferns

Nonvascular bryophytes Vascular seedless plants Vascular seed plants Figure 27.12: Ferns. Green algal ancestor Fig. 27-12 (1), p. 592

Figure 27.12: Ferns. Fig. 27-12a, p. 592

Figure 27.12: Ferns. Fig. 27-12b, p. 592

Figure 27.12: Ferns. Fig. 27-12c, p. 592

Fern Life Cycle

Underside of enlarged mature gametophyte (prothallus) Germination of spores and development of young gametophyte Egg 4 5 Archegonium Spores released Rhizoids 3 Antheridium Sporangium HAPLOID (n) GAMETOPHYTE GENERATION Sperm cell Meiosis Fertilization Cells within sporangia undergo meiosis DIPLOID (2n) SPOROPHYTE GENERATION Sorus (cluster of sporangia) Zygote 2 6 Figure 27.13: The life cycle of ferns. Note the clearly defined alternation of generations between the gametophyte (prothallus) and sporophyte (leafy plant) generations. See text for a detailed description. Frond 1 Leaf of young sporophyte Development of the sporophyte Leaf cross section Haploid prothallus Fiddlehead Root of young sporophyte Roots Rhizome Underside of a frond Fern (mature sporophyte) Fig. 27-13, p. 593

Whisk Ferns Sporophytes have dichotomously branching rhizomes and erect stems lack true roots and leaves

Vascular seedless plants Nonvascular bryophytes Vascular seed plants Figure 27.14: The sporophyte of Psilotum nudum, a whisk fern. The stem is the main organ of photosynthesis in this rootless, leafless, vascular plant. Sporangia, which are initially green but turn yellow as they mature, are borne on short lateral branches directly on the stems. Green algal ancestor Fig. 27-14a, p. 594

Aerial stem with scalelike outgrowths Sporangia Aerial stem with scalelike outgrowths (no leaves) Figure 27.14: The sporophyte of Psilotum nudum, a whisk fern. The stem is the main organ of photosynthesis in this rootless, leafless, vascular plant. Sporangia, which are initially green but turn yellow as they mature, are borne on short lateral branches directly on the stems. Fig. 27-14b, p. 594

Horsetails Sporophytes have hollow, jointed roots, rhizomes, aerial stems leaves reduced to megaphylls

Dichotomous end branches Thicker main stem Dichotomous end branches Equal branches Vascular tissue Thinner side branch Figure 27.10: Evolution of microphylls and megaphylls. Dichotomous branching (in b) is branching into two equal halves. Webbing (in b) is the evolutionary process in which the spaces between close branches become filled with chlorophyll-containing cells. Dichotomously branching stems Overtopping (unequal branching) Planation (branching in same plane) Webbing of side branch system Megaphyll (many veins) Fig. 27-10b, p. 590

Horsetails

Nonvascular bryophytes seedless plants Vascular Vascular seed plants Green algal ancestor Figure 27.15: Horsetails. (a, Redrawn from L. Emberger, Les Plantes Fossiles, Masson et Cie, Paris, 1968.) Fig. 27-15a, p. 594

Strobilus Vegetative shoots Reproductive shoots Fig. 27-15b, p. 594 Figure 27.15: Horsetails. (a, Redrawn from L. Emberger, Les Plantes Fossiles, Masson et Cie, Paris, 1968.) Reproductive shoots Fig. 27-15b, p. 594

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KEY CONCEPTS In club mosses and ferns, lignin-hardened vascular tissues that transport water and dissolved substances throughout the plant body have evolved

Learning Objective 8 Describe the life cycle of ferns Compare sporophyte and gametophyte generations

Fern Sporophytes Roots, rhizomes, leaves are megaphylls Leaves (fronds) bear sporangia in clusters (sori) Meiosis in sporangia produces haploid spores

Fern Gametophyte Fern Gametophyte (prothallus) develops from haploid spore bears both archegonia and antheridia

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Learning Objective 9 What is the difference between the generalized life cycles of homosporous and heterosporous plants?

Homospory Production of one kind of spore in bryophytes, most club mosses, most ferns including whisk ferns and horsetails Spores give rise to gametophyte plants produce both egg cells and sperm cells

Heterospory 1 Production of two kinds of spores (microspores and megaspores) in some club mosses and ferns in all seed plants

Heterosporous Life Cycle

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Megaspore Gametophyte Archegonium Microspore Antheridium HAPLOID (n) GAMETOPHYTE GENERATION Sperm Egg Meiosis Fertilization DIPLOID (2n) SPOROPHYTE GENERATION Microsporocyte Figure 27.16: The basic life cycle of heterosporous plants. Two types of spores, microspores and megaspores, are produced during the life cycle of heterosporous plants. Zygote Megasporocyte Microsporangium Embryo Megasporangium Sporophyte Fig. 27-16, p. 595

Heterospory 2 Microspores Megaspores give rise to male gametophytes that produce sperm cells Megaspores give rise to female gametophytes that produce eggs

Selaginella Life Cycle

HAPLOID (n) GAMETOPHYTE GENERATION DIPLOID (2n) SPOROPHYTE GENERATION Male gametophyte develops inside microspore wall Single antheridium in male gametophyte produces many sperm cells 3 Ruptured megaspore wall Microspores 5 Sperm cell Archegonium containing egg Female gametophyte develops and protrudes from megaspore wall 4 Longitudinal section through archegonium Megaspores HAPLOID (n) GAMETOPHYTE GENERATION Egg Meiosis Fertilization Microsporangium with microsporocytes DIPLOID (2n) SPOROPHYTE GENERATION Figure 27.17: The life cycle of spike moss (Selaginella). Spike moss is heterosporous, producing two types of spores in one strobilus. The megaspores develop into female gametophytes, and the microspores become male gametophytes. See text for a detailed description. Megasporangium with megasporocyte Female gametophyte Strobilus First leaves 1 Leaf (microphyll) Stem Zygote Stem Longitudinal section through strobilus 6 Root Root Young sporophyte (attached to female gametophyte) Mature sporophyte Fig. 27-17, p. 596

Evolution of Heterospory Essential step in evolution of seeds Rhynia Aglaophyton