Lesson Overview

1. Lesson Overview 22.1 What is a Plant?

2. THINK ABOUT IT Plants have adapted so well to so many environments that they dominate much of the surface of our planet.

3. Characteristics of Plants What do plants need to survive?

4. Characteristics of Plants What do plants need to survive? The lives of plants center on the need for sunlight, gas exchange, water, and minerals.

5. The Plant Kingdom Plants are classified as members of the kingdom Plantae. Plants are eukaryotes that have cell walls containing cellulose and carry out photosynthesis using chlorophyll a and b.

6. What Plants Need All plants have the same basic needs: sunlight, a way to exchange gases with the surrounding air, water, and minerals.

7. Sunlight Plants use the energy from sunlight to carry out photosynthesis. Leaves are typically broad and flat and are arranged on the stem so as to maximize light absorption.

8. Gas Exchange Plants require oxygen to support cellular respiration, as well as carbon dioxide to carry out photosynthesis. Plants must exchange these gases with the atmosphere and the soil without losing excessive amounts of water through evaporation.

9. Water and Minerals Land plants have evolved structures that limit water loss and speed the uptake of water from the ground. Minerals are nutrients in the soil that are needed for plant growth.

10. Many plants have specialized tissues that carry water and nutrients upward from the soil and distribute the products of photosynthesis throughout the plant body.

11. The History and Evolution of Plants How did plants adapt to life on land?

12. The History and Evolution of Plants How did plants adapt to life on land? Over time, the demands of life on land favored the evolution of plants more resistant to the drying rays of the sun, more capable of conserving water, and more capable of reproducing without water.

13. Origins in the Water The ancestors of today’s land plants were water-dwelling organisms similar to today’s green algae. Although not as large and complex as many plants, green algae have cell walls and photosynthetic pigments that are identical to those of plants. Green algae also have reproductive cycles that are similar to plants. Studies of the genomes of green algae suggest that they are so closely related to other plants that they should be considered part of the plant kingdom.

14. The First Land Plants The greatest challenge that early land plants faced was obtaining water. They met this challenge by growing close to the ground in damp locations. Fossils suggest the first true plants were still dependent on water to complete their life cycles. One of the earliest fossil vascular plants was Cooksonia, shown here.

15. The First Land Plants Several groups of plants evolved from the first land plants. One group developed into mosses. Another lineage gave rise to ferns, cone-bearing plants, and flowering plants.

16. An Overview of the Plant Kingdom Botanists divide the plant kingdom into five major groups based on four important features: embryo formation, specialized water-conducting tissues, seeds, and flowers.

17. An Overview of the Plant Kingdom The relationship of plant groups is shown below

18. The Plant Life Cycle What feature defines most plant life cycles?

19. The Plant Life Cycle What feature defines most plant life cycles? The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase.

20. The Plant Life Cycle The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase. The shift between the haploid phase and the diploid phase is known as the alternation of generations, as shown in the figure.

21. The Plant Life Cycle The multicellular diploid phase is known as the sporophyte, or spore-producing plant.

22. The Plant Life Cycle The multicellular haploid phase is known as the gametophyte, or gamete-producing plant.

23. The Plant Life Cycle A sporophyte produces haploid spores through meiosis. These spores grow into multicellular structures called gametophytes.

24. The Plant Life Cycle Each gametophyte produces reproductive cells called gametes—sperm and egg cells. During fertilization, a sperm and egg fuse with each other, producing a diploid zygote that develops into a new sporophyte.

25. Trends in Plant Evolution An important trend in plant evolution is the reduction in size of the gametophyte and the increasing size of the sporophyte.

26. Lesson Overview 22.2 Seedless Plants

27. We generally think of plants as growing from seeds, but there are plenty of plants that don’t produce seeds at all. How do they manage to reproduce and grow without them? THINK ABOUT IT

28. What are the characteristics of green algae? Green Algae

29. What are the characteristics of green algae? Green algae are mostly aquatic. They are found in fresh and salt water, and in some moist areas on land. Green Algae

30. Algae are not a single group of organisms. “Algae” applies to any photosynthetic eukaryote other than a land plant. “Green algae” are classified with plants. Green Algae

31. The First Plants Green algae are mostly aquatic. They are found in fresh and salt water, and in some moist areas on land. Ancient green algae shared the ocean floor with corals and sponges..

32. The First Plants Green algae absorb moisture and nutrients directly from their surroundings and do not contain the specialized tissues found in other plants.

33. Life Cycle Many green algae switch back and forth between haploid and diploid phases. However, some may not alternate with each and every generation. In the life cycle of Chlamydomonas, as long as living conditions are suitable, the haploid cell reproduces asexually by mitosis.

34. Life Cycle If conditions become unfavorable, Chlamydomonas can switch to a stage that reproduces sexually. Its cells release gametes that fuse into a diploid zygote (a sporophyte). The zygote has a thick protective wall, permitting survival in harsh conditions.

35. Life Cycle The zygote begins to grow once conditions become favorable. It divides by meiosis to produce four haploid cells that swim away, mature, and reproduce asexually.

36. Multicellularity Green algae can form colonies. Spirogyra forms long threadlike colonies called filaments.

37. Multicellularity Volvox colonies consist of as few as 500 to as many as 50,000 cells arranged to form hollow spheres. Volvox shows some cell specialization and straddles the fence between colonial and multicellular life.

38. Mosses and Other Bryophytes What factor limits the size of bryophytes?

39. Mosses and Other Bryophytes What factor limits the size of bryophytes? Bryophytes are small because they lack vascular tissue.

40. Mosses and Other Bryophytes Mosses have a waxy, protective coating that makes it possible for them to resist drying, and thin filaments known as rhizoids that anchor them to the soil. Rhizoids also absorb water and minerals from the soil.

41. Mosses and Other Bryophytes Mosses, hornworts, and liverworts all belong to a group of plants known as bryophytes. Bryophytes have specialized reproductive organs enclosed by other, non-reproductive cells. Bryophytes show a higher degree of cell specialization than do the green algae and were among the first plants to become established on land.

42. Why Bryophytes Are Small Bryophytes do not make lignin, a substance that hardens cell walls, and do not contain true vascular tissue. Because of this, bryophytes cannot support a tall plant body against the pull of gravity.

43. Life Cycle Bryophytes display alternation of generations. The gametophyte is the dominant, recognizable stage of the life cycle and the stage that carries out most of the photosynthesis. The sporophyte is dependent on the gametophyte for its supply of water and nutrients.

44. Life Cycle Bryophytes produce sperm cells that swim using flagella. For fertilization to occur, the sperm must swim to an egg. Because of this, bryophytes must live in habitats where open water is available at least part of the year.

45. Gametophyte When a moss spore lands in a moist place, it sprouts and grows into a young gametophyte. The gametophyte forms rhizoids that grow into the ground and shoots that grow into the air.

46. Gametophyte Gametes are formed in reproductive structures at the tips of the gametophytes. Eggs are produced in archegonia. Sperm are produced in antheridia.

47. Gametophyte Sperm and egg cells fuse to produce a diploid zygote.

48. Sporophyte The zygote marks the beginning of the sporophyte stage of the life cycle.

49. Sporophyte A sporophyte grows within the body of the gametophyte, depending on it for water and nutrients.

50. Sporophyte Eventually the sporophyte grows out of the gametophyte and develops a long stalk ending in a capsule called the sporangium.