UNIT 7: ANGIOSPERMS 2 (Embryogenesis, Seeds. and Fruits)

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1 55 UNIT 7: ANGIOSPERMS 2 (Embryogenesis, Seeds. and Fruits) Textbook Chapter 19 OBJECTIVES By the end of this unit, you should understand the development of the gametophyte generation of the flowering plants. Fruit is one of the characters that make the Phylum Anthophyta the most successful phyla of plants. By the end of the lab, you should know what a fruit is, what its function is, and the different types of fruits. BACKGROUND The flowering plants are considered to be the most successful phylum within the plant kingdom. Although they are the youngest plant phylum (appearing in the fossil record about 120 million years ago), they are the most diverse and the most abundant. The flowering plants are placed in the Phylum Anthophyta. This phylum contains vascular plants that produce seeds and have flowers, fruits, and double fertilization. Flowering plants are often referred to as angiosperms. The term "angiosperm" means "seeds in a vessel" and refers the production of seeds inside of a fruit. Angiosperms have the same basic alternation of generations life cycle as do all plants. However, the haploid gametophytes have been reduced in size to just a few cells. The sporophyte produces flowers, which have all of the plant reproductive parts including megasporophylls, microsporophylls, megasporangia, and microsporangia. The carpel is the megasporophyll or female portion of the flower while the stamen is the microsporophyll or male portion. The carpel is essentially a megasporophyll that has been rolled up so the megasporangia are contained on the inside. A carpel has 3 parts. The lower portion, which contains the megasporangia, is the ovary. The stalk-like portion above the ovary is the style. The top portion of the carpel, where pollen will land, is the stigma. The megasporangia contained in the ovary are called ovules. The outer tissue layer of the ovule is the integument. The number of ovules present in an ovary depends upon the species of plant. Contained in the ovule is a megaspore mother cell. The megaspore mother cell undergoes meiosis to produce 4 megaspores, 3 of which usually disintegrate. The remaining megaspore divides mitotically and grows into the female gametophyte, called an embryo sac. In about 70% of angiosperms, the embryo sac is only 8 cells in size; however, some angiosperm families have embryo sac s with other cell numbers. One of the cell s in the embryo sac is the haploid egg. Another cell is a binucleate cell referred to as the polar nuclei. The stamen, or microsporophyll, has 2 parts. The anther, the upper portion, contains the microsporangia. The anther is held up by a long stalk called the filament. Each microspore mother cell inside the microsporangia undergo meiosis to produce 4 microspores. Because each microsporangium has many microspore mother cells, a single anther may produce thousands of microspores. Microspores divide mitotically to produce a two-celled male gametophyte or pollen grain composed of a vegetative and generative cell. The vegetative cell will form the pollen tube (with no further mitotic divisons). The generative cell will mitotically divide to form two sperm cells (either before or after the pollen is released from the anther). When the pollen is mature, the anthers split open and the pollen is dispersed. A mature angiosperm male gametophyte is composed of three cells.

2 56 Pollination occurs when a pollen grain lands on a stigma. The pollen grain germinates and a pollen tube begins to grow. The tube nucleus controls the growth of the pollen tube as it grows through the stigma, down the length of the style and into the ovary. The pollen tube grows into ovule through the micropyle. After the pollen tube enters the embryo sac, both sperm are released. One sperm fertilizes the egg, forming the diploid zygote. The other sperm fuses with the 2 polar nuclei forming a triploid cell, the primary endosperm nucleus. The zygote grows into the sporophyte embryo while the primary endosperm nucleus divides to produce a triploid tissue. This triploid tissue, the endosperm, stores food for the embryo. The integuments thicken and harden, forming the seed coat. As the ovule develops into the seed, the ovary surrounding it develops into a fruit. The fruit functions as dispersal vessel for the seeds that they contain. Rather than relying on one or two types of seed dispersal methods as do the gymnosperms, angiosperms rely on many different vectors (including a wide variety of animals) to scatter their seeds. After pollination and fertilization, the ovules begin developing into seeds. 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, a exocarp (outer layer), mesocarp (middle layer), and endocarp (inner layer). Others have dry fruits that may be hard or 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. Fleshy fruits are classified, in part, by whether they are simple or complex fruits. Simple fruits develop from the individual flowers that have a single pistil. Simple fruits include peaches, tomatoes, lemons, and apples. Other flowers, such as strawberries or blackberries, are individual flowers with several pistils in each flower. Each pistil develops into a miniature "fruitlet" and collectively the "fruitlets" make up a single fruit called an aggregate fruit. A few fruits actually develop from a number of flowers. Pineapples have small, single-pistiled flowers clustered very close together on the stem. The ovaries of each of the flowers develop into fruits and grow in size. As they enlarge, they fuse with the fruits next to them. When they are ripe, they have fused into a single structure, called a multiple fruit.

3 57 EXERCISE 1: PARTS OF FRUITS As noted above, fleshy fruits have several layers. In many fleshy fruits, the pericarp is the part of the fruit that you eat the tissue around the seeds. In accessory fruits, what you eat is derived from tissues other than the ovary. For instance, in the apple, you are eating receptacle tissue. Pericarp is generally composed of several layers: the outermost layer is the exocarp (sometimes called the epicarp) or peel, the middle layer is called the mesocarp, and the innermost layer surrounding or containing the seeds is called the endocarp. Inside the seed are the developing embryo, cotyledons, and food source(s) for the young plant. Figure 1. Cross section of a fleshy fruit and seed (source: Wikimedia Commons issued under the Creative Commons License). Obtain a whole green bean and a peanut. Carefully split both of them along one side and open them. Lay them side by side on the table. 1. What kind of fruits are these? Draw a whole bean or a whole peanut fruit and label the various parts.

4 58 2. What structure does the green bean resemble when opened? 3. Is there any evidence that double fertilization occurred in either the bean or the peanut? Why do you say that? 4. From what part of the flower is the fruit wall (pod or shell) derived? 5. Is the fruit wall part of the sporophyte or gametophyte? What is its ploidy? 6. Remove the "skin" from one of the peanuts. What is the technical name for this structure? 7. From what part of the ovule did the peanut "skin" develop? Is it part of the sporophyte or gametophyte? 8. Do you see any evidence of the developing embryonic plant in either seed? Why do you say that? EXERCISE 2 Dicot Seed. Bean seeds. A. Exterior View. Bean (Phaseolus sp.) generally possesses several seeds in a fruit, i.e., green beans similarly peas in a pod. The fruit wall dries and shrivels releasing the seeds Obtain an imbibed bean seed from the supply on your table and compare it with the bean seed from exercise 1, as well as with the figure below. Locate the HILUM. Next to the hilum is a small pore called the MICROPYLE. Label these parts on the diagram below. B. Interior view. Using the dissecting needle or edge of a razor blade, gently remove the outer covering or SEED COAT. Then carefully separate the two halves. Label the drawing below including these terms: seed coat, embryo, cotyledon, hypocotyl-root axis, and plumule.

5 59 Test for carbohydrates: Place one of the cotyledons on a microscope slide and smash it into a paste with the blunt end of the wooden handle of a dissecting needle. Place a drop of iodine followed by a cover slip over the paste. Examine the slide with the microscope. 9. Iodine is an indicator for what molecule (compound)? 10. Did the iodine stain anything? What structure did the iodine stain, the embryo or endosperm? Why do you say that? Take the other cotyledon and place iodine on it as well. Then, take two more beans. Slice one longitudinally (in the direction different from when you separated the two cotyledons), and slice the other bean crosswise and place a drop or two of iodine on them. 11. Is starch present in any of the sections? How does this compare with the disrupted cotyledon? Why do you think there is any difference (if there is any)? 12. Do the different sections seed stain uniformly?

6 60 EXERCISE 3 Monocot Seed. Corn grain. A. Exterior View. A Corn (Zea mays) grain (caryopsis) is a fruit. It develops from a single flower s ovary having a single ovule. There are many ovaries on a "cob." A short indeterminate branch. The corn grain's detachment scar at the pointed end is where the flower s pedicel was attached to the branch. The covering of the corn grain called pericarp is composed of two fused layers: the outer layer is the fruit wall derived from the ovary wall; the inner layer is the seed coat derived from the ovule wall. This fruit type is characteristic of the grasses. Other monocot fruits may have several ovules in a single ovary like Banana which is a berry. Obtain from the supply on your table an imbibed (swollen with water) corn grain. Locate the silk and pedicel scars and position of the embryo. Label these parts on the diagram below. B. Interior View. Lay the flat surface of the corn grain against the table with the embryo facing up. With a razor blade, cut the corn grain longitudinally into two halves as in the drawing. Examine the cut surface and compare to the prepared slide #1 of a stained corn grain cut in the same plane of view (see textbook ). Label the drawing below. Include: pericarp, endosperm, scutellum (cotyledon), coleoptile, coleorhiza, radicle, and plumule. Place a drop of iodine solution on a clean microscope slide. Using forceps, place the cut surface of one of the corn grain halves in the iodine solution. After one minute, observe the cut surface for any color change. Cut another kernel in half crosswise, and place it in the iodine as well. 13. What structure did the iodine stain black, the embryo or endosperm? Why do you say that? 14. Compare the corn grain with the bean seed. Give similarities and differences.

7 Compare what you see with slide #1 (in tray 2). What similarities and differencies do you see? EARLY GROWTH AND DEVELOPMENT IN A DICOT EMBRYO. Capsella bursa-pastoris (shepherd's purse) is a common weed found in lawns and disturbed soil sites. It is ideal for studying early seed growth because it produces many fruits of different ages on the same plant; thus, different developmental stages. See textbook pages EXERCISE 4. Available at your table are prepared slides of longitudinal sections of Capsella fruits at three different stages of maturity (slides #2-5). Examine these slides in alpha numeric order with the microscope. Label the drawings below and include the following terms: fruit, fruit wall, seed, seed coat, suspensor, embryo, cotyledons, endosperm, root tip, root apical meristem, shoot apical meristem, hypocotyl-root axis, protoderm, procambium, ground meristem. NOTE: We will NOT study the early embryo development of monocots.

8 On which slide do you first see the development of the cotyledons? What is this stage called? 17. In the mature embryo, how much of the endosperm is still visible? What happened to it?

9 63 TERMINOLOGY TO BE FAMILIAR WITH 1. angiosperm 23. microsporophyll 2. anther 24. monocots 3. carpel 25. ovary 4. dicots 26. ovule 5. double fertilization 27. petal 6. egg cell 28. pistil 7. embryo sac 29. placentae/placenta 8. endosperm 30. polar nuclei 9. filament 31. pollen tube 10. flower 11. fruit 12. gametophyte 13. generative cell 14. integuments 15. Anthophyta 16. megaspore 17. megaspore mother cell 18. megasporophyll 19. micropyle 20. microsporangium 21. microspore mother cell 22. microspore 32. pollen grain 33. pollination 34. primary endosperm nucleus 35. receptacle 36. seed 37. sepal 38. sperm 39. spores 40. sporophyte 41. stamen 42. stigma 43. style 44. tepal 45. tube cell 46. vegetative cell 47. zygote 48. aggregate fruit 49. carpel 50. complex fruit 51. dehiscent 52. dry fruit 53. endocarp 54. exocarp 55. fleshy fruit 56. indehiscent 57. mesocarp 58. multiple fruit 59. ovule 60. pericarp 61. pistil 62. receptacle 63. seed 64. simple fruit

10 64 Pre-Lab Questions (note: you may have answered some of these last week). 1. What is the ploidy of the microspore mother cells? 2. What cellular process will the microspore mother cells undergo? 3. What is the ploidy of the microspores? 4. Relative to the number of microspore mother cells, how many microspores are there? 5. How many cells are there in each microspore at this stage? 6. Why are these cells arranged as tetrads (groups of four)? 7. How many cells are there in each of these male gametophytes? What is their ploidy? 8. What are the names and the functions of the cells in the male gametophytes? 9. List in order the structures through which a pollen tube must grow on its path to fertilize an egg? 10. Where is the source of the food located which enables the pollen tube to grow? 11. What is the ploidy of the megaspore mother cell? 12. What cellular process will this cell undergo? 13. What is the name of the cell/s it will produce? 14. What is the ploidy of the megaspore? Of the embryo sac? 15. How many mitotic divisions have occurred at this point? 16. What are the outer layers of the ovule called? What will these layers eventually develop into? 17. How many cells are there in this embryo sac? Is this typical of most angiosperms? What is the ploidy of the cells? 18. What is the function of the polar nuclei? 19. What exactly is double fertilization? 20. What is/are the difference(s) between simple and complex fruits? 21. What is/are the difference(s) between an aggregate fruit and a multiple fruit?

11 Is it possible for a fruit to develop if fertilization does not occur? Explain. 23. What structural modifications would need to evolve for Selaginella to develop a fruit as in peas and beans.