1. The Plant Kingdom: Seedless Plants
Chapter 27

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

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

4. 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)

5. Basic Plant Life Cycle

6. 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

7. 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

8. 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)

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

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

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

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

13. Gametangia

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

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

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

17. 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

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

19. 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

20. Plant Evolution

21. 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

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

23. Insert “Evolutionary tree for plants”
plant_tree_v2.swf

24. Explore plant evolution by clicking on the figure in ThomsonNOW.

25. Learning Objective 3
What features distinguish bryophytes from other plants?

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

27. Learning Objective 4
What are the three phyla of bryophytes?

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

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

30. Hornworts (Phylum Anthocerophyta)
Have thalloid gametophytes

31. 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

32. 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 (1), p. 585

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

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

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

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

37. 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

38. Moss Life Cycle

39. 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

40. Moss Sporophytes

41. 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

42. Liverwort Life Cycle

43. 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

44. Insert “Moss life cycle”
moss_life_cycle_v2.swf

45. Insert “Marchantia, a liverwort”
liverwort.swf

46. Watch the life cycles of the mosses and liverworts by clicking on the figures in ThomsonNOW.

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

48. 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)

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

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

51. 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 a, p. 590

52. Club Mosses

53. 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 (1), p. 591

54. 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 (a-b), p. 591

55. 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

56. Ferns

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

58. Figure 27.12: Ferns.
Fig a, p. 592

59. Figure 27.12: Ferns.
Fig b, p. 592

60. Figure 27.12: Ferns.
Fig c, p. 592

61. Fern Life Cycle

62. 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 , p. 593

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

64. 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 a, p. 594

65. 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 b, p. 594

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

67. 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 b, p. 590

68. Horsetails

69. 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 a, p. 594

70. 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 b, p. 594

71. Insert “Seedless vascular plants”
seedless_vascular.swf

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

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

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

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

76. Insert “Fern life cycle”
fern_life_cycle_v2.swf

77. Watch the life cycle of the ferns by clicking on the figure in ThomsonNOW.

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

79. 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

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

81. Heterosporous Life Cycle

82. 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 , p. 595

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

84. Selaginella Life Cycle

85. 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 , p. 596

86. Evolution of Heterospory
Essential step in evolution of seeds
Rhynia
Aglaophyton