Botany: An introduction to plant biology, 5 ed. Mauseth. Chapter 22

Transcription

1 63 UNIT 5: GYMNOSPERMS th Botany: An introduction to plant biology, 5 ed. Mauseth. Chapter 22 OBJECTIVES This lab considers the gymnosperms, vascular plants that produce seeds but lack flowers and fruits. There are four phyla of extant gymnosperms: Ginkgophyta, Cycadophyta, Coniferophyta, and Gnetophyta. Most of this lab will focus on Phylum Coniferophyta, the conifers. By the end of the lab you should know the characteristics of the phyla, understand the pine life cycle, and know the different reproductive structures of a pine. BACKGROUND The gymnosperms are those vascular plants that produce seeds but do not have flowers or fruits. There are four phyla of extant gymnosperms: Ginkgophyta (ginkgos), Cycadophyta (cycads), Coniferophyta (conifers), and Gnetophyta (gnetophytes). The term "gymnosperm" means naked seed and refers to the fact that the seeds are not enclosed in a protective tissue, what is called a fruit in the Angiosperms. Gymnosperm seeds have a thick seed coat which protects its contents and also helps regulate seed germination. Inside, the seed has a sporophyte embryo and a food reserve for the growing embryo. The production of seeds as a dispersal unit is considered to be an evolutionary advance over the use of spores for dispersal. As a dispersal unit, the seed is a self-contained plant which has a better chance of survival than a single-celled spore. Also the gametophytes of non-seed plants require water for fertilization but seed plants do not. Because the structures look so different, it is sometimes difficult to see the relationships between the life cycle of the conifers and the life cycles of the ferns and fern allies. However, the trend we have seen previously (the gametophyte phase becoming smaller and more dependent with the sporophyte becoming larger and more dominant) continues in the seed plants. In the life cycle of Selaginella, the megaspores and microspores were dispersed. The female gametophyte remained in the megaspore and the male gametophyte in the microspore. In the seed plants, the female gametophyte also develops in the megaspore which is retained in the megasporangium. Similarly, the male gametophytes develop inside of the microsporangium and are dispersed. In the conifers, the cones are strobili. The sporophylls are located between accessory structures called bracts. The structure usually referred to as a pine cone is the female strobilus. The megasporophylls and megasporangia are located between woody bracts. The small, papery male cones that contain the microsporophylls and microsporangia are usually unnoticed by casual observers. The megasporangium never opens to release the megaspore. In the female cone the megaspore mother cell undergoes meiosis inside the megasporangium to produce 4 megaspores, three of which usually degenerate. A very small female gametophyte develops from the remaining megaspore, still inside the megasporangium. In the male cone, the microspores are produced by meiosis inside the microsporangium. The microspores divide by mitosis and develop into a four-celled male gametophyte, or pollen grain. The pollen is released and is carried by wind to the female cones. The megasporangia produce a droplet of sticky fluid called a pollination droplet. The pollen grain sticks to the pollination droplet. As the droplet evaporates, it contracts and pulls the pollen into the megasporangium through an opening called the micropyle. The pollen grain grows a pollen

2 64 tube which delivers and releases its sperm cells near the female gametophyte. After fertilization, the zygote divides by mitosis to form the new sporophyte embryo. The embryo is still enclosed in the megaspore which is still in the megasporangium. This structure, the embryo and the indehiscent megasporangium, is the seed. EXERCISE 1: PHYLUM CYCADOPHYTA At the front or side bench of the lab are pots containing Zamia floridana (cycads). Examine the plants. Look for the presence of reproductive structures. 1. Superficially these cycads resemble the ferns. What characteristics would you look for to determine whether or not they are ferns? 2. What characters distinguish the Cycadophyta from the other gymnosperms? 3. If reproductive structures were present, what is the term that describes these structures? EXERCISE 2: PHYLUM CONIFEROPHYTA: PINE VEGETATIVE STRUCTURES PART A: THE LEAF Examine the slides of the pine leaf (needle) cross section (slides #1 and #2). 4. How many vascular bundles are in the center of the leaf. What differences do you see between the two slides? PART B: THE STEM Examine both the slide of pine wood (#3 through #5) and the pieces of pine wood that are on the lab benches. Wood is made of xylem, the water conducting tissue. 5. Do you see any evidence of wood in the slides? Which one(s)? Why do you say that? 6. Gymnosperms, such as pines, are considered to be very well suited to life in a terrestrial environment. In the pine needle and the wood you should be able to find a number of structures and/or adaptations that support this conclusions. What are some of these adaptations?

3 65 EXERCISE 4: PHYLUM CONIFEROPHYTA: PINE REPRODUCTIVE STRUCTURES 7. Are pines monoecious (an individual plant will have both male and female cones) or dioecious (an individual plant that will have all male cones or all female cones)? Why do you say that? 8. In what ways do the strobili of pines and Selaginella resemble each other? How are they different? 9. What is the difference between pollination and fertilization? In the pines, how much time may elapse after pollination before fertilization occurs? Can you have one without the other in the gymnosperms? 10. How does the sperm reach the egg in the gymnosperms? 11. What are the advantages of having seeds as a dispersal unit rather than spores? PART A: MALE PINE CONES Examine the slide of the longitudinal section of a microsporangiate or male strobilus (slide #6). Draw and label the following structures: microsporophylls, microsporangia, microspores, male gametophytes/pollen grains.

4 66 PART B: PINE POLLEN Make a wet mount of the pine pollen from the container on your lab bench (if there s enough!). Compare to slide (#7). 12. Pine pollen is carried to the female cone by the wind. What aspect of the pine pollen helps in its transport? Draw a pollen grain and indicate those structures. 13. Approximately how many cells are in the mature male gametophytes? What is their function? PART C: FEMALE PINE CONES Examine the examples of megasporophyllus strobili (female pine cones) located throughout the lab. Then secure the prepared slide (slide #8) of the female pine cones. Observe the slides. In the gymnosperms, the female gametophyte is covered by the nucellus (megasporangium tissue) and integuments (protective tissue layers). This entire structure is called an ovule and will, if fertilized, develop into a seed. The megasporophyll on which an ovule develops is referred to as an ovuliferous scale. 14. In the female pine cones, what are the hard woody structures? 15. Were there archegonia present in the female gametophytes in the slide? 16. How many seeds are produced on each bract? How many megasporangia were on each bract?

5 67 label (numbers are fine!) 1. meiosis 2. fertilization 3. embryo 4. seed 5. haploid 6. diploid 7. megastrobilus (section and whole) 8. megasporophyll 9. megasporangia 10. megasporocyte 11. archaegonia 12. female gametophyte 13. microstrobilus (section and whole) 14. microsporophyll 15. microsporangia 16. microspore 17. antheridia 18. male gametophyte 19. mature sporophyte 20. nutritive tissue

6 68

7 69 TERMINOLOGY TO BE FAMILIAR WITH 1. archegonia 2. bract 3. cone 4. Coniferophyta 5. Cycadophyta 6. egg 7. embryo 8. fertilization 9. fruit 10. Ginkgophyta 11. Gnetophyta 12. gymnosperm 13. integuments 14. megasporangia 15. megaspore 16. megasporocyte/ megaspore mother cell 17. megasporophyll 18. micropyle 19. microsporangia 20. microspore 21. microsporocyte/ microspore mother cell 22. microsporophyll 23. needle 24. nucellus 25. ovule 26. ovuliferous scale 27. pollen/pollen grain 28. pollen tube 29. pollination 30. pollination droplet 31. prothallial cells 32. seed 33. seed coat 34. sperm 35. spores 36. sporophyte 37. strobilus 38. wood