UNIT 8: ANGIOSPERMS 3 (Early Development of the Plant, Cells, and Tissues)

55 UNIT 8: ANGIOSPERMS 3 (Early Development of the Plant, Cells, and Tissues) th Biology of Plants, Raven et al., 7 Ed. - Chapter 22 OBJECTIVES The fruit is the dispersal vehicle for angiosperms seeds; gymnosperm seeds have no such vehicle. A seed develops after fertilization, becomes dormant, is dispersed, germinates, and grows into a new plant. In this lab, you will examine basic fruit structure, as well as the steps from seed development to early growth as a new plant in both monocots and dicots. At the end of the lab, you should be able to describe the stages of seed development, germination, and the early growth of both monocot and dicot plants. The plant body is composed of four types of tissue: meristems, ground tissues, dermal tissues, and vascular tissues. In this lab you will be observing examples of the last three types of tissues. A knowledge of these tissues is important in understanding the next several units. At the end of this lab, you should know the four tissues of a plant, their functions and the cell types of each. BACKGROUND Seed plants pass through several developmental stages as they grow into a mature plant body. A new plant generation begins at the formation of the zygote which results from the fusion of a sperm and egg cell in the embryo sac of the ovule. This is followed by a rapid and programmed sequence of cell divisions which produces an enlarging embryo. The embryo develops to a prescribed size and form and then becomes dormant. The dormant embryo is surrounded by the ovule and is called a seed. This seed is encased by the ovary or fruit wall of the flower. As the fruit matures, it may become detached from the plant and subsequently the seeds within will be discharged or released from the fruit. The seed breaks dormancy after certain environmental cues are met. The embryo will resume growth by cell division and expansion. This resumption of growth, called germination, will continue in an organized manner resulting in the formation of roots, stems and leaves. The seeds secrete hormones which cause the ovary to begin to develop into a fruit. The ovary or fruit wall is often referred to as the pericarp. Some fruits such as peach, apple, or cucumber, develop fleshy fruits. The pericarp of some fleshy fruits may have three distinct layers, an exocarp (outer layer), a mesocarp (middle layer), and an endocarp (inner layer). Others have dry fruits that may be hard or have papery pericarps. Some dry fruits are dehiscent and split open at maturity to disperse the seeds. Other dry fruits remain closed or are indehiscent. Dry fruits include grains, maples, okra, walnuts, and beans. The angiosperms (flowering plants) are divided into two classes, Monocotyledons and Eudicotyledons; these classes are commonly referred to as monocots and dicots, respectively. These terms refer to numbers of cotyledons in the seeds. These classes differ in their development and morphology. This lab will focus on fruit structure, seed and early plant development, and plant development after germination in the monocots and dicots. Plant growth in length or diameter occurs from the addition of new cells. For many plants, growth is indefinite and never attains a fixed size. The new cells are produced from meristem tissues which mitotically divide continually adding new cells. There are three types of meristems: apical, located at the root and shoot tips, which produce the primary tissues; lateral, encircling the stems and roots between the primary vascular tissue, which produce

56 secondary tissues; and intercalary, found in some monocots usually at base of node and leaves. The cells of a meristem remain relatively small and undifferentiated compared to neighboring cells. The cell derivatives from the meristems form the other three types of tissues. The ground tissue constitutes most of the plant body and functions in storage, metabolism and support. There are three kinds of ground tissue: (1) parenchyma cells are involved in storage and metabolism and can be characterized as living cells with thin walls; (2) collenchyma cells are living with uneven thickened walls and give support to growing regions in the plant body; (3) sclerenchyma tissues are very thick-walled cells which are usually dead at maturity. There are two types, sclereids and fibers, both of which provide structural support in nongrowing regions of the plant. These three ground tissues are considered "simple tissue" because they are composed of only one type of cell. The dermal tissue, or epidermis, is the outermost covering of the plant and functions in water regulation, protection, and gas exchange. The cells of the epidermis are usually thinwalled, simple cells which are living at maturity. The epidermis is frequently covered by a waxy layer called the cuticle. Two specialized cells of the epidermis are the guard cells of the stomata and trichomes. The vascular tissue is composed of xylem and phloem, which are involved in long-distance transport of water and food. The xylem is composed of two cell types, tracheids and vessel elements both of which are dead at maturity. They both transport water and dissolved nutrients from the roots to the stems and leaves. They originate from primary and lateral meristems. The phloem is composed of sieve cells and sieve tube elements that are living at maturity. They both transport dissolved organic molecules throughout the plant and originate from primary and lateral meristems. Both vascular tissues are considered "complex tissue" since they also contain one or more ground tissue cell types. SEED GERMINATION AND PLANT DEVELOPMENT The time duration of dormancy and the requirements to break the dormancy of a seed varies widely among the flowering plants. (See textbook pages 504-505, 614, 637-638.) The development of a plant following germination progresses in a sequential manner: first the root emerges and grows downward; next the stem emerges and grows upward; then the first leaves expand at the tip of the stem. In both dicots and monocots, the cotyledons can remain below or at the substrate level (hypogeous) or become elevated above the substrate surface (epigeous). (See textbook pages 506-507.) Plants of three different ages of both corn and bean, one to four weeks old, will be provided in lab. 1. What physical processes would necessarily be involved in the entrance and absorption of water during germination?

57 EXERCISE 1 Corn Germination and Seedling Growth. Working in pairs, carefully remove one plant of each age from the trays and arrange them on your table in order of increasing age. Locate the following structures: primary root, coleoptile, coleorhiza, first foliage leaves, adventitious roots, and periderm. Label the diagram to the right. (When finished, share plant material with another student in class or dispose of in the trash can.) (See textbook page 507.) 2. What is a probable explanation for roots developing before stems during early phases of seed germination? 3. What are adventitious roots? From what plant structure do they arise? 4. What is the function of the coleoptile? Coleorhiza? 5. How does the embryo-seedling obtain its food as long as it remains below the soil surface? After it emerges from the ground? EXERCISE 2 Bean Germination and Seedling Growth. Carefully remove one plant of each age from the trays and arrange in order of increasing age. Locate the following structures: seed coat, hypocotyl, epicotyl, cotyledons, foliage leaves, primary roots, lateral roots, and shoot apex. Label drawing below (see textbook page 506). (When finished, share plant material with another student in class or dispose of in trash can.)

58 6. In what ways do bean germination and seedling development differ from that of corn? 7. What do the prefixes "epi" and "hypo" mean? 8. What portion of the stem of the bean embryo elongates most rapidly to pull the cotyledons above the substrate level? Is this maintained in later phases? Explain. 9. What happens to the seed coat? Does this happen in the corn? 10. List other factors besides water absorption which have an effect on whether a seed will germinate. 11. How does the embryo-seedling obtain its food as long as it remains below the soil surface? After it emerges from the ground? 12. A general gardening rule is to plant seeds no deeper than several diameters of the seed. Explain. EXERCISE 3 Pea Germination and Seedling Growth. Carefully remove one plant of each age from the trays and arrange in order of increasing age. Locate the following structures: seed coat, epicotyl, cotyledons, foliage leaves, primary roots, lateral roots, and shoot apex. Label drawing below (see textbook page 506). (When finished, share plant material with another student in class or dispose of in trash can.)

59 13. In what ways do pea germination and seedling development differ from that of bean? 14. In what ways do pea germination and seedling development differ from that of corn? 15. Is this type of development epigeous or hypogeous? 16. How does the embryo-seedling obtain its food as long as it remains below the soil surface? After it emerges from the ground CELLS AND TISSUES In this lab, you will look at examples of ground tissue and vascular tissue. You will examine primary and secondary meristems in the next three lab units. A. Ground Tissues. Parenchyma, collenchyma, and sclerenchyma. EXERCISE 4 Parenchyma and collenchyma. Fresh Celery Petioles. On the front counter are several celery stalks whose cut ends have been placed in a red food dye. Obtain a piece of the petiole from your instructor. Make a THIN cross section of the celery petiole with a razor blade. Trim away thick areas and make a water mount. Observe with the microscope and locate parenchyma cells and tissue, collenchyma cells and tissue, and bundles of vascular tissue (stained red). 1. How would you characterize the parenchyma cells? Collenchyma cells? 2. What part of the plant is the celery stalk?

60 EXERCISE 5 Sclerenchyma. Pear fruit. The gritty texture felt when eating a pear is caused by clusters of sclereids. Obtain a small piece of pear from your instructor. Place it in a drop of water on a microscope slide. With the blunt end of the wooden handle of a dissecting needle, thoroughly mash the pear into the water. Add a cover slip and observe under the microscope. 3. What is the function of sclerenchyma cells? B. Vascular Tissues. EXERCISE 6 Xylem and Phloem. Cucurbita pepo Stem. Obtain a prepared slide of Cucurbita pepo (Squash) (#31) stem. The slide has both cross and longitudinal sections. Examine the cross section first. Find the large, circular, red stained, thick-walled xylem tissue in the vascular bundles. (See textbook pages 516 and 522.) Within the vascular bundle, find the green stained, thinner-walled phloem tissue that is located on both the inner and outer sides of the xylem. Next, examine the longitudinal section and answer the two questions again. In both sections find and note the locations of: parenchyma, collenchyma, epidermis, and trichomes. Does their location fit their described function? 4. From this view, can you tell if the xylem is composed of vessels or tracheids? 5. The phloem is composed of both sieve-tube elements and companion cells. What is the functional difference between these two cells? Can you find the companion cell? Sieve plate? 6. Why do some of the xylem cells look like stacks of rings or a spring while others have a more solid looking wall surface?

61 EXERCISE 7 Xylem. Xylem in Dogwood Leaf. Your lab instructor will demonstrate how to pull a dogwood leaf apart in order to get the xylem cells of the leaf veins. Pull your leaf and with a razor blade, cut the "strings," and make a wet mount of them. Compare these xylem cells with those of the Cucurbita pepo and the celery stalks. 7. Are these xylem cells vessels or tracheids? How do you know? C. Dermal Tissue. In the next three lab units you will be examining many dermal tissues. Here you will look at a peltate trichome of a leaf. EXERCISE 8 Peltate Trichomes. Elaeagnus Leaves. Your instructor will give you a piece of an Elaeagnus (Russian Olive) leaf. Place a drop of water on a microscope slide. Place the leaf flat on the table, the lower surface up (it has a silvery appearance). With a razor blade scrape the leaf surface then dip the razor blade edge in the water drop on the slide. Place a cover slip on the microscope slide and examine with the microscope. 8. What is the function of the trichomes on a leaf? 9. Are the trichomes composed of cells?

62 TERMINOLOGY TO BE FAMILIAR WITH 1. adventitious roots 33. radial pattern 2. apical-basal pattern 34. radicle 3. apical meristem 35. scutellum 4. coleoptile 36. seed 5. coleorhiza 37. seed coat 6. cotyledon 38. seedling 7. dicotyledon 39. suspensor 8. dermal tissue system 40. taproot 9. differentiation 41. zygote 10. dormant 11. embryo proper 12. endosperm 13. epicotyl 14. epidermis 15. epigeous 16. fruit 17. funiculus 18. germination 19. ground meristem 20. hilum 21. hypocotyl 22. hypocotyl-root axis 23. hypogeous 24. lateral roots 25. meristem 26. monocotyledon 27. plumule 28. primary meristems 29. polarity 30. procambium 31. proembryo 42. albuminous cell 14. apical meristem 15. callose 16. collenchyma 17. companion cell 18. complex tissue 19. dermal tissue system 20. derivative 21. determinate 22. development 23. differentiation 24. epidermis 25. fiber 26. ground tissue 27. ground tissue system 28. growth 29. guard cells 30. indeterminate 31. initials 32. lateral meristems 33. meristem 34. morphogenesis 32. protoderm 35. P-protein 36. parenchyma 37. periderm 38. perforation plate 39. phloem 40. pit 41. primary growth 42. P-protein 43. procambium 44. programmed cell death 45. protoderm 46. rays 47. sclereid 48. sclerenchyma 49. sieve area 50. sieve cells 51. sieve element 52. sieve plate 53. sieve-tube elements 54. simple tissue 55. stomate 56. tissue system 57. tracheary elements 58. tracheid 59. transfer cells 60. trichome 61. vascular tissue system 62. vessel 63. vessel elements 64. xylem QUESTIONS FOR FURTHER THOUGHT 1. What are possible reasons why a seed planted in watered saturated soil will not germinate within the expected period of time? 2. What might be the survival value to a plant species requiring a certain period of cold temperature (i.e., winter conditions) before germination can occur? 3. If all cells of an embryo and seedling are derived from the zygote and thus have the same hereditary background, why do some cells become the apical meristems, others the cotyledon and still others the hypocotyl?

4. Quite early in the development of the embryo it exhibits polarity. What is polarity and what are the consequences of this polarity? 5. If polarity did not exist in an embryo, what would be the shape of the young "seedling"? Explain. 6. For each of the following structures, indicate their function and/or what they may in time develop into: endosperm, plumule, suspensor, cotyledons, radicle, zygote, coleorhiza, and coleoptile. 63

64 PRE-LAB Questions and Tables 1. What structural features enable many fibers and tracheary to lend support to plant organs? 2. What is/are the principal function(s) of the following cell types: parenchyma, collenchyma, sclerenchyma, tracheary elements, sieve-tube elements and companion cells? 3. Describe two features of a xylem vessel elements that enable relatively large quantities of water to move through them as compared to tracheids. 4. What cellular features of the phloem sieve-tube elements result in slower movement of food in comparison to movement of water in xylem vessel elements? Why faster in xylem? 5. What other cell types are found in the xylem tissues besides vessels and/or tracheids? 6. What other cell types are found in phloem besides sieve-tube elements and companion cells?

65 Sieve areas - yes or no Sieve plate - yes or no Organism most often found in Callose present Living at maturity - yes or no Nucleus present at maturity P-protein present Companion cells present Albuminous cells present Other types of cells associated with phloem Difference between 1 and 2 phloem Phloem - Tissue Sieve Elements Sieve Cells Sieve-Tube Elements Xylem - Tissue Water transferred via pit pairs Water transfer via perforation Living at maturity - yes or no More efficient in water movement Secondary cell walls present - yes or no Organism most often found in Other cells associated with xylem Difference between 1 and 2 xylem Tracheids Vessel Elements

66 CELLS AND TISSUES Cell Types Living at Maturity Yes/No Thickened Walls Yes/No Special Shape Describe Tissue Type Function Parenchyma Collenchyma Sclerenchyma Fibers Sclerenchyma Sclereids Tracheid Vessel Elements Sieve-tube Element Companion Cell Sieve Cell Albuminous Cell