0632 C24Open BDOL-829900 8/4/04 11:22 PM Page 632 Reproduction in Plants Color-enhanced SEM Magnification: 1800ϫ What You’ll Learn ■ You will compare and contrast the life cycles of mosses, ferns, and conifers. ■ You will sequence the life cycle of a flowering plant. ■ You will describe the charac- teristics of flowers, seeds, and fruits. Why It’s Important Much of life on Earth, including you, depends on plants. Humans eat plants and some people eat organisms that eat plants. If plants did not reproduce and continue their species, the food sources for much of life on Earth would disappear. Understanding the Photo Pollen grains of seed plants— coniferophytes and antho- phytes—contain male reproductive cells. Botanists and paleobotanists can use the unique structures of pollen grains to identify living or extinct seed-plant species. This color-enhanced SEM photograph shows pollen grains from several species of living anthophytes. Visit ca.bdol.glencoe.com to • study the entire chapter online • access Web Links for more information and activities on reproduction in plants • review content with the Interactive Tutor and self- check quizzes 632 RMF/Visuals Unlimited 0633-0640 C24S1 BDOL-829900 8/4/04 12:03 PM Page 633 Life Cycles of Mosses, 24.1 Ferns, and Conifers SECTION PREVIEW Objectives Review the steps of alter- nation of generations. Life Cycles Make the following Foldable to help you Survey and identify organize events in the life cycles of mosses, ferns, methods of reproduction and conifers. and the life cycles of mosses, ferns, and conifers. STEP 1 Fold a sheet of paper in half STEP 2 Turn the paper so the Review Vocabulary lengthwise. Make the back part about fold is on the bottom, then fold gametophyte: haploid 5 cm longer than the front part. it into thirds. form of an organism in alternation of genera- tions that produces gametes (p. 516) New Vocabulary vegetative reproduction STEP 3 Unfold and cut only the top STEP 4 Label the Foldable protonema layer along each fold to make three tabs. as shown. megaspore microspore Life Cycles micropyle Mosses Ferns Conifers Illustrate and Write As you read Chapter 24, illustrate and write about the life cycles of each group of plants behind the appropriate tab. Alternation of Generations Male As you learned earlier, plant life cycles include an alter- gamete (n) Female nation of generations. As shown in Figure 24.1, an alter- gamete (n) Spores (n) nation of generations consists of a sporophyte stage and a GAMETOPHYTE (n) gametophyte stage. All cells of a sporophyte are diploid. Certain cells of a Meiosis Fertilization sporophyte undergo meiosis, which produces haploid spores. These spores undergo cell divisions and form a SPOROPHYTE (2n) multicellular, haploid gametophyte. Some cells of a game- Mitosis and tophyte differentiate and form haploid gametes. The cell division female gamete is an egg and the male gamete is a sperm. When a sperm fertilizes an egg, a diploid zygote forms. This is sexual reproduction. The zygote can undergo cell divisions and form an embryo sporophyte. If the embryo develops to maturity, the cycle can begin again. Figure 24.1 This basic life cycle pattern is the same for most plants. In an alternation of generations, the game- However, there are many variations on this pattern within tophyte (n) stage produces gametes, and the plant kingdom. For instance, recall that in mosses the the sporophyte (2n) produces spores. gametophyte is the familiar form, not the sporophyte. 24.1 LIFE CYCLES OF MOSSES, FERNS, AND CONIFERS 633 0633-0640 C24S1 BDOL-829900 8/4/04 12:04 PM Page 634 In others, such as flowering plants, generation lives longer and can sur- the gametophyte is microscopic. vive independently of the other gen- Most people have never even seen eration. In most plant species the the female gametophyte of a flower- sporophyte is the dominant plant. ing plant. Botanists usually refer to the bigger, more obvious plant as the Asexual reproduction dominant generation. The dominant Most plants also can reproduce asex- ually by a process called vegetative reproduction. In this type of repro- duction, new plants are produced from existing plant organs or parts of organs. The new plants have the same genetic Experiment makeup as the original plant. For Growing Plants Asexually Plants are capable of reproduc- example, some thallose liverwort ing asexually. Reproductive cells such as egg or sperm are not gametophytes can produce cuplike needed in asexual reproduction. Plant organs such as roots, structures, as shown in Figure 24.2. stems, and even leaves can produce new offspring. Inside, minute pieces of tissue called gemmae ( JEH mee) develop. If gemmae Garlic bulb Potato with eye (storage leaves) (stem & buds) fall from the cup, they can grow into Toothpick other liverwort gametophytes that are Water genetically identical to the thallus. You can learn more about asexual reproduc- Test tube Carrot Water tion in MiniLab 24.1. (root) Beaker Compare the genetic Water makeups of plants produced from one plant by asexual reproduction to A B C each other and the original plant. Procedure ! Prepare three different plant parts for study using dia- grams A, B, and C as a guide. Figure 24.2 Small cups filled with tiny gemmae have @ Design a data table in which you can diagram your obser- formed on the thallus of this liverwort. vations and list the number of days since the experiment began. # Diagram the plant parts and label them as Day 1. $ Make and record observations every three days. Replace any lost water as needed. % Observe any changes that occur to your plants over the next two weeks. Analysis 1. Observe What experimental evidence do you have that: a. plants can use different structures for asexual reproduction? b. asexual reproduction is a rapid process? c. asexual reproduction requires only one parent? 2. List Describe advantages of asexual reproduction in plants. 634 REPRODUCTION IN PLANTS Runk-Schoenberger/Grant Heilman Photography 0633-0640 C24S1 BDOL-829900 8/4/04 12:05 PM Page 635 Life Cycle of Mosses Motile sperm from an antheridium swim in a continuous film of water to The gametophyte stage is the domi- an egg in an archegonium. If fertiliza- nant generation in mosses. A haploid tion occurs, a diploid zygote forms. moss spore can germinate and grow to The zygote undergoes cell divisions form a protonema (proh tuh NEE muh). It is a small green filament of forming the sporophyte that consists cells that can develop into the gameto- of a stalk with a capsule at the top. The phyte. In some mosses, male and female sporophyte remains attached to and reproductive structures form on sepa- dependent on the gametophyte. Cells rate gametophytes, but in others, male in the capsule undergo meiosis, pro- and female reproductive structures are ducing haploid spores. When the cap- on the same gametophyte. Recall that sule matures, it bursts open and Figure 24.3 the egg-producing, female reproduc- releases spores. If the spores land in a The moss gametophyte tive structure is an archegonium, and favorable environment, they can ger- produces gametes that the sperm-producing, male reproduc- minate and the cycle repeats, as shown join to form a zygote. The zygote develops tive structure is an antheridium. in Figure 24.3. into the sporophyte that produces spores. Spores can germinate and grow into a ga- Developing metophyte, completing Mitosis sporophyte the moss’s life cycle. Sporophyte (2n) Zygote (2n) Gametophyte (n) Fertilization Capsule Sperm SPOROPHYTE Meiosis Egg GENERATION 2n Capsule opens releasing spores GAMETOPHYTE GENERATION n Germinating spores Male Female gametophyte gametophyte Protonema (n) (n) 24.1 LIFE CYCLES OF MOSSES, FERNS, AND CONIFERS 635 (inset)Dwight R. Kuhn 0633-0640 C24S1 BDOL-829900 8/4/04 12:06 PM Page 636 Figure 24.4 Fern sporophytes are common in a forest. However, a fern gametophyte is rarely seen. B Each round cluster of sporangia on the underside of a fern frond is called a sorus. Each sporangium contains spores that are released, sometimes in dramatic fashion. C The heart-shaped fern A Most fern sporophytes can grow gametophyte is usually 25 cm or taller. Differentiate How 5 mm–6 mm across. is a fern’s method of growth different from a moss’s? Stained LM Magnification: 25ϫ Some moss gametophytes also re- producing haploid spores. When en- produce by vegetative reproduction. vironmental conditions are right, the They can break into pieces when dry sporangia open and release haploid and brittle then, when moisture spores, as shown in Figure 24.4B. returns, each piece can grow and form A spore can germinate to form a a protonema then a gametophyte. heart-shaped gametophyte called a prothallus, as shown in Figure 24.4C. The prothallus produces both arche- Life Cycle of Ferns gonia and antheridia on its surface. Unlike mosses, the dominant stage The flagellated sperm released by of the fern life cycle is the sporophyte antheridia swim through a film of stage. The fern sporophyte includes water to eggs in archegonia. If fertil- the familiar fronds you see in Figure ization occurs, the diploid zygote can 24.4A. Fern fronds grow from a rhi- develop into the sporophyte. Initially, zome, which is an underground stem. this developing sporophyte depends On the underside of some fronds are upon the gametophyte for its nutri- sori, which are clusters of sporangia. tion. However, once the sporophyte Meiosis occurs within the sporangia, produces green fronds, it can carry on 636 REPRODUCTION IN PLANTS (tl)Stephen J. Krasemann/DRK Photo, (c)Stan Elms/Visuals Unlimited, (tr)Robert & Linda Mitchell 0633-0640 C24S1 BDOL-829900 8/4/04 12:06 PM Page 637 Figure 24.5 Growing Prothallus In the life cycle of a fern, prothallus the sporophyte genera- tion becomes indepen- dent of the gametophyte.