Chapter 22 The Land Plants

1. Chapter 22The Land Plants

2. Algal Ancestors • Plants are multicelled, photosynthetic eukaryotes adapted to life on land • They are close relatives of red algae and green algae, and charophyte algae are considered their sister group red algae chlorophytes charophytes land plants

3. From Algal Ancestors to Embryophytes • Plants and charophyte algae share a mechanism by which a new cell wall forms after cell division and the arrangement of flagella on sperm (among those that have flagellated sperm) • Land plants (embryophytes) have an embryo that develops within a chamber of parental tissues and receives nourishment from its parent during early development

4. Life Cycle of Land Plants • Plants have a life cycle in which a diploid generation alternates with a haploid one • During the diploid generation, a multicellular sporophyte produces spores (haploid cells with thick walls) by meiosis • Spores germinate, undergo mitosis, and grow into multicelled haploidgametophytes that produce gametes by mitosis • Male and female gametes meet (fertilization), forming a diploid zygote which undergoes mitosis and develops into a new sporophyte

5. multicelled sporophyte mitosis zygote Diploid (2n) Phase of Cycle Meiosis Fertilization Haploid (n) Phase of Cycle gametes spores multicelled gametophyte mitosis mitosis Stepped Art Figure 22-2 p360

6. Evolution of Land Plants • Spores (haploid cells with thick walls) that date to 475 million years ago are the earliest fossil evidence of land plants • The oldest fossil spores resemble spores of modern liverworts • Liverworts, mosses, and hornworts are three early nonvascular plant lineages called bryophytes • In bryophytes, the gametophyte is larger and longer-lived than the sporophyte

7. Evolution of Vascular Plants • The first seedless vascular plants evolved about 430 million years ago • In vascular plants, the sporophyte is larger and longer-lived than the gametophyte, and has specialized internal pipelines that transport water and sugars

8. Evolution of Seed Plants • Seed plants evolved about 385 million years ago • Seed plants are vascular plants that hold onto their spores and disperse by releasing seeds • Seed plants are also the only plants that produce pollen • Producing seeds and pollen allowed seed plants to expand into dry habitats that seedless plants could not tolerate

9. Two Lineages of Seed Plants • Two lineages of seed plants survived to the present: gymnoperms and angiosperms (flowering plants) • Gymnosperms include modern conifers such as pines, firs, and spruces • Flowering plants arose from a gymnosperm ancestor in the late Jurassic or early Cretaceous

10. Table 22-1 p360

11. The Major Plant Groups Bryophytes Seedless vascular plants Gymnosperms Angiosperms • No xylem or phloem • Vascular tissue present • Vascular tissue present • Vascular tissue present • Gametophyte predominant • Sporophyte predominant • Sporophyte predominant • Sporophyte predominant • Water required for fertilization • Water required for fertilization • Pollen grains; water not required for fertilization • Pollen grains; water not required for fertilization • Seeds form in a floral ovary that becomes a fruit • Seedless • Seedless • “Naked” seeds whisk ferns, horsetails, ferns hornworts gnetophytes, ginkgos, conifers, cycads monocots, eudicots, and relatives club mosses, spike mosses liverworts mosses ancestral alga

12. Take-Home Message: How do the various plant lineages differ? • Land plants are embryophytes; their embryo forms in a chamber on the parental body and is nourished by the parent • Three early-evolving plant lineages are known collectively as bryophytes; these nonvascular plants do not produce seeds • Vascular plants have internal pipelines that transport water and sugars • The first seed plants were gymnosperms; angiosperms (flowering plants) are the most diverse plant lineage, and the most recently evolved

13. From Haploid to Diploid Dominance • Plants shifted from gametophyte-dominated life cycle (in bryophytes) to sporophyte-dominated life cycle • Flowering plants have large, complex sporophytes – their gametophytes form inside flowers and consist of few cells • Spores are more likely to survive than gametes, so increased spore production provides a greater advantage • A high degree of genetic diversity in sporophyte-dominated lineages facilitated adaptive radiation into diverse habitats

14. sporophyte (2n) gametophyte (n) seed seed bryophytes ferns gymnosperms angiosperms Figure 22-4 p362

15. Structural Adaptations • Life on land favored water conserving features • Sporophytes secrete a waxy covering (cuticle) that reduces evaporative water loss • Closable pores (stomata) that extend across the cuticle allow gas exchange for photosynthesis

16. cuticle (waxy layer at leaf surface) A Light micrograph showing waxy cuticle (stained pink) at a leaf surface. Figure 22-5a p363

17. stoma (opening across cuticle) B One stoma. Figure 22-5b p363

18. Vascular Tissues • Moving substances from roots to other body regions requires vascular tissues, a system of internal pipelines • Xylem is a vascular tissue that distributes water and mineral ions • Phloemis a vascular tissue that distributes sugars made in photosynthetic cells

19. Vascular Tissues • Ligninstiffens the walls of xylem cells and provides structural support • Leaves increase the surface area for intercepting sunlight and for gas exchange, and contain veins of vascular tissue

20. xylem A Colorized micrograph of xylem (hollow tubes) in cross-section. Figure 22-6a p363

21. xylem phloem B Light micrograph of vascular tissue of a squash stem with lignin stained red. Figure 22-6b p363

22. Pollen and Seeds • Bryophytes and seedless plants release spores – only seed-bearing vascular plants release pollen grains and seeds • A pollen grainis a walled, immature gametophyte that will give rise to the sperm • A seedconsists of an embryo sporophyte and some nutritive tissue enclosed inside a waterproof coat • Angiosperms disperse seeds inside a fruit

23. Dispersal Methods: Spores and Seeds

24. Take-Home Message: What adaptations contributed to plant diversification? • Plant life cycles shifted from a gametophyte-dominated in bryophytes to a sporophyte-dominated in vascular plants • Life on land favored water-conserving features such as a cuticle; in vascular plants, vascular tissue (xylem and phloem) distributes material through leaves, stems, and roots • Bryophytes and seedless vascular plants release spores; • Seed plants release embryos inside protective seeds; in flowering plants, seeds form inside floral tissue that later develops into a fruit

25. 22.4 Bryophytes • Bryophytes include three land plant lineages (liverworts, hornworts, and mosses) with a gametophyte-dominated life-cycle • None of the bryophytes have true vascular tissue reinforced by lignin

26. A Moss: Polytrichum

27. Peat Moss • Peat mosses (Sphagnum) are the most ecologically and commercially important bryophytes • In peat bogs, ancients peat mosses become compressed and form a carbon-rich material called peat, used as fuel • Freshly harvested peat moss is dried and added to planting mixes to help soil retain moisture

28. Peat Bog in Ireland

29. Liverworts • Liverworts may be the most ancient land plant lineage • Reproduces asexually by producing gemmae in cups on the gametophyte • During sexual reproduction, sperm swim to eggs that have formed in the gametangium of a female plant • Fertilization produces a sporophyte that remains attached to the female gametophyte

30. Take-Home Message:What are bryophytes? • Bryophyte is the name for three lineages of plants: mosses, liverworts, and hornworts; all are low-growing with no lignin-reinforced vascular tissues; all have flagellated sperm that require a film of water to swim to eggs, and all disperse by releasing spores • Bryophytes are unique among land plants in having a life cycle in which the gametophyte is the dominant generation; the sporophyte remains attached to the gametophyte even when mature

31. 22.5 Seedless Vascular Plants • Seedless vascular plants are the oldest vascular plant lineages – like bryophytes, they have flagellated sperm that swim to eggs, and they disperse by releasing spores directly into the environment • A sporophyte with lignified vascular tissue (xylem and phloem) dominates the life cycle

32. Two Lineages of Seedless Vascular Plants • Lycophytes (club mosses, spike mosses) and monilophytes (whisk ferns, horsetails, ferns) diverged from a common ancestor before leaves and roots had evolved • Lycophytes form spores along the sides of branches – their leaves have one unbranched vein • Monilophytes have spores at branch tips – their leaves have branching veins

33. Club Mosses • Club mosses (Lycopodium) have a horizontal stem (rhizome) that runs along the ground • Roots and upright stems with tiny leaves grow from the rhizome • Spores form inside a soft, cone-shaped strobiluscomposed of modified leaves

34. Ferns • Ferns are 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

35. Take-Home Message:What are seedless vascular plants? • Club mosses and relatives belong to one seedless vascular lineage (lycophytes) • Ferns, horsetails and whisk ferns belong to the other (monilophytes) • A sporophyte with vascular tissues (xylem and phloem) dominates their life cycle, and they disperse by releasing spores

36. 22.6 History of the Vascular Plants • Seed plants dominate modern forests, but before they evolved, forests of seedless nonvascular plants stood tall • 450 mya (Ordovician period): Cooksonia may be one of the earliest lineages; it had a simple branching pattern, and no leaves or roots; spores formed at branch tips • By the Devonian period, plants such as Psilophyton had evolved more complex branching pattern

37. Coal Forests • In the Carboniferous, plants with lignin-reinforced tissues flourished, died, and became compacted into coal, a nonrenewable fossil fuel • It took millions of years of photosynthesis, burial, and compaction to form coal, which is why fossil fuels are said to be nonrenewable sources of energy

38. Rise of the Seed Plants • The first gymnosperms evolved late in the Devonian • Cycads and ginkgos were early gymnosperms; conifers evolved later • Angiosperms (flowering plants) descended from a gymnosperm ancestor by about 120 mya, during the reign of the dinosaurs

39. Reproductive Traits of Seed-Bearing Plants • Seeds and pollen allowed gymnosperms and angiosperms (flowering plants) to survive and thrive in drier habitats • Microspores become sperm-producing male gametophytes (pollen grains) • Megaspores develop into egg-producing female gametophytes inside ovules • Pollen is carried from one seed plant to the ovule of another (pollination) even in the absence of water

40. Seeds • After pollination and fertilization, an ovule matures into a seed • A seed contains a multicelled embryo sporophyte and stored food that the embryo can draw on during early development • Because the sperm of seed plants do not need to swim through a film of water to reach eggs, these plants can reproduce even during dry times

41. pollen sac (2n) ovule (2n) meiosis meiosis microspores (n) in pollen sac megaspores (n) in ovule develop into develop into sperm-producing male gametophytes (pollen grains) egg-bearing female gametophytes in ovule release and pollination delivers sperm to fertilization zygote (2n) in ovule develop into seed (embryo sporophyte in ovule) Stepped Art Figure 22-20 p369

42. Secondary Growth • Some seed plants undergo secondary growth (growth in diameter) and produce wood • Wood is lignin-stiffened tissue that strengthens and protects older stems and roots

43. Timeline of Major Events

44. Take-Home Message: What are the major events in the evolutionary history of vascular plants? • Seedless vascular plants arose and became widespread during the Carboniferous; coal is the remains of some of these ancient plants • The first seed plants evolved in the late Devonian; because they produced pollen and seeds, they were able to live in drier places than other plants • An ability to produce wood allowed some seed plants to grow very tall

45. 22.7 Gymnosperms • Gymnosperms are vascular seed plants that produce seeds on the surface of ovules • Many gymnosperms enclose their seeds in a fleshy or papery covering – their seeds are said to be “naked” because they are not inside a fruit • Gymnosperms include conifers (such as pines), cycads, ginkgos, and gnetophytes

46. Conifers • Conifersare trees and shrubs with woody cones • Conifers typically have needlelike or scalelike leaves with a thick cuticle, and are resistant to drought and cold • Most conifers remain evergreen • Conifers include redwoods, pines, and bristlecone pines

47. Lesser-Known Lineages • Cycads have palmlike or fernlike leaves and live mainly in the dry tropics and subtropics (e.g. “sago palms” ) • The only living ginkgospecies is Ginkgo biloba, the maidenhair tree – one of the few deciduous gymnosperms • Gnetophytesinclude tropical trees, desert shrubs, and leathery vines (e.g. Ephedra and Welwitschia)

48. A Representative Life Cycle • A pine tree is a sporophyte that produces its spores on specialized strobili called cones • The tree makes two types of cones: small, soft pollen cones; and large, woody, ovulate cones • Microspores develop into pollen grains; megaspores develop into egg-producing female gametophytes • After pollination, a pollen tube grows and delivers a sperm cell to the egg nucleus, forming a zygote

49. Take-Home Message:What are gymnosperms? • Gymnosperms include conifers, ginkgos, and gnetophytes • These seed-bearing vascular plants produce seeds on strobili • In the case of conifers, the seed-bearing strobilus is a woody cone

50. 22.8 Angiosperms: The Flowering Plants • Angiospermsare the most diverse plant lineage and the only plants that make flowers and fruits • A flower is a specialized reproductive shoot that consists of modified leaves arranged in three concentric whorls