* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Plant Kingdom: Seedless Plants
History of phycology wikipedia , lookup
Pollination wikipedia , lookup
Plant tolerance to herbivory wikipedia , lookup
Venus flytrap wikipedia , lookup
Plant defense against herbivory wikipedia , lookup
Fertilisation wikipedia , lookup
Plant use of endophytic fungi in defense wikipedia , lookup
Cultivated plant taxonomy wikipedia , lookup
History of herbalism wikipedia , lookup
History of botany wikipedia , lookup
Ornamental bulbous plant wikipedia , lookup
Historia Plantarum (Theophrastus) wikipedia , lookup
Plant physiology wikipedia , lookup
Plant morphology wikipedia , lookup
Evolutionary history of plants wikipedia , lookup
Sustainable landscaping wikipedia , lookup
Flowering plant wikipedia , lookup
27 The Plant Kingdom: Seedless Plants Moss-covered rocks. Shown is Elowa Falls, Columbia River Gorge, Oregon. A Darrell Gulin /Getty Images bout 445 million years ago (mya), the ocean was filled with vast numbers of fishes, mollusks, and crustaceans as well as countless microscopic algae, and the water along rocky coastlines was home to large seaweeds. Occasionally, perhaps, an animal would crawl out of the water onto land, but it never stayed there permanently because there was little to eat on land—not a single blade of grass, no fruit, and no seeds. During the next 30 million years, a time corresponding roughly K EY C ONCE P TS Biologists infer that plants evolved from aquatic green algal ancestors known as a charophytes. Adaptations to life on land that have evolved in plants include a waxy cuticle to prevent water loss; multicellular gametangia; stomata; and for most plants, vascular tissues containing lignin. to the Silurian period (see Table 21-1), plants appeared in abundance and colonized the land. Where did they come from? Although plants living today exhibit great diversity in size, form, and habitat (see photograph), biologists hypothesize that they all evolved from a common ancestor that was an ancient green alga. Recall from Chapter 25 that red algae, green algae, and land plants are collectively classified as “plants.” Biologists infer this common Plants undergo an alternation of generations between multicellular gametophyte and sporophyte generations. ancestor because modern green algae share many biochemical Mosses and other bryophytes lack vascular tissues and do not form true roots, stems, or leaves. plants contain the same photosynthetic pigments: chlorophylls a In club mosses and ferns, lignin-hardened vascular tissues that transport water and dissolved substances throughout the plant body have evolved. and metabolic traits with modern plants. Both green algae and and b and carotenoids, including xanthophylls (yellow pigments) and carotenes (orange pigments). Both store excess carbohydrates as starch and have cellulose as a major component of their cell walls. In addition, plants and some green algae share certain details of cell division, including formation of a cell plate during cytokinesis (see Chapter 10). 581 Land plants are complex multicellular organisms that range in plant kingdom consists of hundreds of thousands of species that size from minute, almost microscopic duckweeds to massive giant live in varied habitats, from frozen arctic tundra to lush tropical rain sequoias, some of the largest organisms that have ever lived. The forests to harsh deserts to moist stream banks. ■ ADAPTATIONS OF PLANTS Developing gametes Learning Objectives 1 2 Discuss some environmental challenges of living on land, and describe how several plant adaptations meet these challenges. Name the green algal group from which plants are hypothesized to have descended, and describe supporting evidence. What are some features of plants that have let them colonize so many types of environments? One important difference (a) between plants and algae is that a waxy cuticle covers the aerial portion of a plant. Essential for existence on land, the cuticle helps prevent desiccation, or drying out, of plant tissues by evaporation. Plants that are adapted to moister habitats may have a very thin layer of wax, whereas those adapted to drier environments often have a thick, crusty cuticle. (Many desert plants also have a reduced surface area, particularly of leaves, which minimizes water loss.) Plants obtain the carbon they need for photosynthesis from the atmosphere as carbon dioxide (CO2). To be fixed into organic molecules such as sugar, CO2 must first diffuse into the chloroplasts that are inside green plant cells. Because a waxy cuticle covers the external surfaces of leaves and stems, however, gas exchange through the cuticle between the atmosphere and the interior of cells is negligible. Tiny pores called stomata (sing., stoma), which dot the surfaces of leaves and stems of almost all plants, facilitate gas exchange; algae lack stomata. The sex organs, or gametangia (sing., gametangium), of most plants are multicellular, whereas the gametangia of algae are unicellular (❚ Fig. 27-1). Each plant gametangium has a layer of sterile (nonreproductive) cells that surrounds and protects the delicate gametes (eggs and sperm cells). In plants, the fertilized egg develops into a multicellular embryo (young plant) within the female gametangium. Thus, the embryo is protected during its development. In algae, the fertilized egg develops away from its gametangium; in some algae, the gametes are released before fertilization, whereas in others the fertilized egg is released. (b) Unicellular gametangium Figure 27-1 and plants Sterile cells Multicellular gametangium (c) (d) Generalized reproductive structures of algae (a, b) In algae, gametangia are generally unicellular. When the gametes are released, only the wall of the original cell remains. (c, d) In plants, the gametangia are multicellular, but only the inner cells become gametes. The gametes are surrounded by a protective layer of sterile (nonreproductive) cells. Gametophyte Spore Sperm Egg HAPLOID (n) GAMETOPHYTE GENERATION Meiosis Fertilization DIPLOID (2n) SPOROPHYTE GENERATION Zygote Embryo Sporophyte The plant life cycle alternates haploid and diploid generations Plants have a clearly defined alternation of generations in which they spend part of their lives in a multicellular haploid stage and part in a multicellular diploid stage (❚ Fig. 27-2).1 The haploid portion of the life cycle is called the gametophyte generation because it gives rise to haploid gametes by mitosis. When two gam1 For convenience we limit our discussion to plants that are not polyploid, although polyploidy is very common in the plant kingdom. We therefore use the terms diploid and 2n (and haploid and n) interchangeably, although these terms are not actually synonymous. 582 Chapter 27 Figure 27-2 The basic plant life cycle Plants have an alternation of generations, spending part of the cycle in a haploid gametophyte stage and part in a diploid sporophyte stage. Depending on the plant group, the haploid or the diploid stage may be greatly reduced. etes fuse, the diploid portion of the life cycle, called the sporophyte generation, begins. The sporophyte generation produces haploid spores by the process of meiosis; these spores represent the first stage in the gametophyte generation. www.thomsonedu.com/biology/solomon Let us examine alternation of generations more closely. The haploid gametophytes produce male gametangia, known as antheridia (sing., antheridium), in which sperm cells form, and /or female gametangia, known as archegonia (sing., archegonium), each bearing a single egg (❚ Fig. 27-3). Sperm cells reach the female gametangium in a variety of ways, and one sperm cell fertilizes the egg to form a zygote, or fertilized egg. The diploid zygote is the first stage in the sporophyte generation. The zygote divides by mitosis and develops into a multicellular embryo, the young sporophyte plant. Embryo development takes place within the archegonium; thus, the embryo is protected as it develops. Eventually the embryo grows into a mature sporophyte plant. The mature sporophyte has special cells called sporogenous cells (spore-producing cells, also called spore mother cells) that divide by meiosis to form haploid spores. Fertilization of egg by sperm cell ¡ zygote ¡ embryo ¡ mature sporophyte plant ¡ sporogenous cells ¡ meiosis ¡ spores All plants produce spores by meiosis, in contrast with algae and fungi, which may produce spores by meiosis or mitosis. The spores represent the first stage in the gametophyte genera- Developing sperm cells Sterile cells Antheridium tion. Each spore divides by mitosis to produce a multicellular gametophyte, and the cycle continues. Plants therefore alternate between a haploid gametophyte generation and a diploid sporophyte generation. archegonia ¡ eggs mature ¡ Spores ¡ gametophyte plants ¡ antheridia ¡ sperm cells Four major groups of plants evolved Recent ultrastructural and molecular data indicate that land plants probably descended from a group of green algae called charophytes or stoneworts (see Fig. 25-16d). Molecular comparisons, particularly of DNA and RNA sequences, provide compelling evidence that charophytes are closely allied to plants. These comparisons among plants and various green algae include sequences of chloroplast DNA, certain nuclear genes, and ribosomal RNA. In each case, the closest match occurs between charophytes and plants, indicating that modern charophytes and plants probably share a recent common ancestor. The plant kingdom consists of four major groups: bryophytes, seedless vascular plants, and two groups of seeded vascular plants: gymnosperms and angiosperms (flowering plants) (❚ Fig. 27-4; see also ❚ Table 27-1, which is an overview of plant phyla). The mosses and other bryophytes are small nonvascular plants that lack a specialized vascular, or conducting, system to transport nutrients, water, and essential minerals (inorganic nutrients) throughout the plant body. In the absence of such a system, bryophytes rely on diffusion and osmosis to obtain needed TABLE 27-1 The Plant Kingdom (a) Each antheridium, the male gametangium, produces numerous sperm cells. Nonvascular plants with a dominant gametophyte generation (bryophytes) Phylum Bryophyta (mosses) Phylum Hepatophyta (liverworts) Phylum Anthocerophyta (hornworts) Vascular plants with a dominant sporophyte generation Seedless plants Phylum Lycopodiophyta (club mosses) Egg Archegonium Sterile cells Phylum Pteridophyta (ferns and their allies, the whisk ferns and horsetails) Seed plants Plants with naked seeds (gymnosperms) Phylum Coniferophyta (conifers) Phylum Cycadophyta (cycads) Phylum Ginkgophyta (ginkgoes) (b) Each archegonium, the female gametangium, produces a single egg. Phylum Gnetophyta (gnetophytes) Seeds enclosed within a fruit Phylum Anthophyta (angiosperms or flowering plants) Figure 27-3 Plant gametangia Class Eudicotyledones (eudicots) Class Monocotyledones (monocots) Shown are generalized moss gametangia. The Plant Kingdom: Seedless Plants 583 Key Point This cladogram shows hypothetical evolutionary relationships among living plants, based on current evidence. The four major groups of plants are bryophytes, seedless vascular plants, and two groups of seed plants— gymnosperms and angiosperms. VASCULAR SEED PLANTS Angiosperms Gymnosperms Ferns VASCULAR SEEDLESS PLANTS Club mosses Mosses Liverworts Hornworts NONVASCULAR BRYOPHYTES Evolution of seeds Evolution of dominant sporophyte, vascular tissue Evolution of cuticle, multicellular gametangia, multicellular embryos Green algal ancestor plant cell wall chemistry, including lignin). The stiffening property of lignin enabled plants to grow tall (which let them maximize light interception). The successful occupation of the land by plants, in turn, made the evolution of terrestrial animals possible by providing them with both habitat and food. Club mosses and ferns (which include whisk ferns and horsetails) are seedless vascular plants that, like the bryophytes, reproduce and disperse primarily via spores. Seedless vascular plants arose and diversified during the Silurian and Devonian periods of the Paleozoic era, between 444 mya and 359 mya. Club mosses and ferns extend back more than 420 million years and were of considerable importance as Earth’s dominant plants in past ages. Fossil evidence indicates that many species of these plants were the size of immense trees. Most living representatives of club mosses and ferns are small. The gymnosperms are vascular plants that reproduce by forming seeds. Gymnosperms produce seeds borne exposed (unprotected) on a stem or in a cone. Plants with seeds as their primary means of reproduction and dispersal first appeared about 359 mya, at the end of the Devonian period. These early seed plants diversified into many varied species of gymnosperms. The most recent plant group to appear is the flowering plants, or angiosperms, which arose during the early Cretaceous period of the Mesozoic era, about 130 mya. Like gymnosperms, flowering plants reproduce by forming seeds. Flowering plants, however, produce seeds enclosed within a fruit. Review ❚ Figure 27-4 Animated ❚ Plant evolution Cladograms such as this one represent an emerging consensus that is open to change as new discoveries are made. Although the arrangement of nonvascular, seedless vascular, and seed plant groupings is widely recognized, the order in which the hornworts, liverworts, and mosses evolved is not yet resolved. materials. This reliance means that bryophytes are restricted in size; if they were much larger, some of their cells could not obtain enough necessary materials. Bryophytes do not form seeds, the reproductive structures discussed in Chapter 28. Bryophytes reproduce and disperse primarily via haploid spores. Recent molecular and fossil evidence, discussed later in the chapter, suggests that bryophytes may have been the earliest plants to colonize land. The other three groups of plants — seedless vascular plants, gymnosperms, and flowering plants —have vascular tissues and are thus known as vascular plants. The two vascular tissues are xylem, for conducting water and dissolved minerals, and phloem, for conducting dissolved organic molecules such as sugar. A key step in the evolution of vascular plants was the ability to produce lignin, a strengthening polymer in the walls of cells that function for support and conduction (see Chapter 32 for a discussion of 584 Chapter 27 ❚ ❚ What are the most important environmental challenges that plants face living on land? What adaptations do plants have to meet these environmental challenges? From which group of green algae are plants hypothesized to have descended? What is alternation of generations in plants? BRYOPHYTES Learning Objectives 3 4 5 Summarize the features that distinguish bryophytes from other plants. Name and briefly describe the three phyla of bryophytes. Describe the life cycle of mosses, and compare their gametophyte and sporophyte generations. The bryophytes (from the Greek words meaning “moss plant”) consist of about 16,000 species of mosses, liverworts, and hornworts; bryophytes are the only living nonvascular plants (❚ Table 27-2). Because they have no means for extensive internal transport of water, sugar, and essential minerals, bryophytes are typically quite small. They generally require a moist environment for www.thomsonedu.com/biology/solomon Figure 27-18 Figure 27-19 Reconstruction of Rhynia gwynne-vaughanii Reconstruction of Aglaophyton major This leafless fl plant, one of Earth’s earliest vascular plants, is now extinct. It grew about 18 cm (7 in) tall. (Redrawn from D. Edwards, “Evidence for the Sporophytic Status of the Lower Devonian Plant Rhynia gwynnevaughanii,” Review of Palaeobotany and Palynology, Vol. 29, 1980.) Recent evidence indicates that this plant, although superficially fi similar to other early vascular plants, lacked conducting tissues that are characteristic of vascular plants. For this reason, it has been reclassified fi into a new genus and is no longer considered a rhyniophyte. (Redrawn from J. D. Mauseth, Botany: An Introduction to Plant Biology, 2nd ed., Saunders College Publishing, Philadelphia, 1995.) is an ongoing enterprise, and over time, existing knowledge is re-evaluated in light of newly discovered evidence. Aglaophyton majorr is an excellent example of the self-correcting nature of science, which is in a perpetually dynamic state and changes in response to newly available techniques and data. ❚ Review ❚ What adaptations do ferns have that both algae and bryophytes lack? ❚ ❚ ❚ How does one distinguish between megaphylls and microphylls? Which of the following are parts of the sporophyte generation in ferns: frond, sperm cells, egg cell, roots, sorus, sporangium, spores, prothallus, rhizome, antheridium, archegonium, and zygote? Why are whisk ferns and horsetails now classifi fied as ferns? How does heterospory modify the life cycle? S U M M ARY WI T H K EY TERMS Learning Objectives 1 Discuss some environmental challenges of living on land, and describe how several plant adaptations meet these challenges (page 582). ❚ The colonization of land by plants required the evolution of many anatomical, physiological, and reproductive adaptations. Plants have a waxy cuticle to protect against water loss and stomata for gas exchange needed for photosynthesis. ❚ Plant life cycles have an alternation of generations in which they spend part of their life cycle in a haploid game- tophyte generation and part in a diploid sporophyte generation. The gametophyte plant produces gametes by mitosis. During fertilization these gametes fuse to form a zygote, the fi first stage of the sporophyte generation. The zygote develops into a multicellular embryo that the gametophyte protects and nourishes. The mature sporophyte plant develops from the embryo and produces sporogenous cells (spore mother cells). These undergo meiosis to form spores, the first stage in the gametophyte generation. The Plant Kingdom: Seedless Plants 597 ❚ Most plants have multicellular gametangia with a protective jacket of sterile cells surrounding the gametes. Antheridia are gametangia that produce sperm cells, and archegonia are gametangia that produce eggs. ❚ Ferns and other vascular plants have xylem to conduct water and dissolved minerals and phloem to conduct dissolved sugar. 2 Name the green algal group from which plants are hypothesized to have descended, and describe supporting evidence (page 582). ❚ Plants probably arose from a group of green algae called charophytes. This conclusion is based in part on molecular comparisons of DNA and RNA sequences, which show the closest match between charophytes and plants. Explore plant evolution by clicking on the figure in ThomsonNOW. Summarize the features that distinguish bryophytes from other plants (page 584). ❚ Unlike other land plants, bryophytes are nonvascular and lack xylem and phloem. Bryophytes are the only plants with a dominant gametophyte generation. Their sporophytes remain permanently attached and nutritionally dependent on the gametophytes. 4 Name and briefly describe the three phyla of bryophytes (page 584). ❚ Mosses (phylum Bryophyta) have gametophytes that are green plants that grow from a filamentous protonema. ❚ Many liverworts (phylum Hepatophyta) have gametophytes that are flattened, lobelike thalli; others are leafy. ❚ Hornworts (phylum Anthocerophyta) have thalloid gametophytes. 5 Describe the life cycle of mosses, and compare their gametophyte and sporophyte generations (page 584). ❚ The green moss gametophyte bears archegonia and/or antheridia at the top of the plant. During fertilization, a sperm cell fuses with an egg cell in the archegonium. The zygote grows into an embryo that develops into a moss sporophyte, which is attached to the gametophyte. Meiosis occurs within the capsule of the sporophyte to produce spores. When a spore germinates, it grows into a protonema that forms buds that develop into gametophytes. 3 Watch the life cycles of the mosses and liverworts by clicking on the figures in ThomsonNOW. Discuss the features that distinguish seedless vascular plants from algae and bryophytes (page 589). ❚ Seedless vascular plants have several adaptations that algae and bryophytes lack, including vascular tissues and a dominant sporophyte generation. As in bryophytes, reproduction in seedless vascular plants depends on water as a transport medium for motile sperm cells. 7 Name and briefly describe the two phyla of seedless vascular plants (page 589). ❚ Sporophytes of club mosses (phylum Lycopodiophyta) consist of roots, rhizomes, erect branches, and leaves that are microphylls. ❚ Ferns (phylum Pteridophyta) are the largest and most diverse group of seedless vascular plants. The fern sporophyte consists of a rhizome that bears fronds and true roots. Phylum Pteridophyta also includes whisk ferns and horsetails. Sporophytes of whisk ferns have dichotomously branching rhizomes and erect stems; they lack true roots and leaves. Horsetail sporophytes have roots, rhizomes, aerial stems that are hollow and jointed, and leaves that are reduced megaphylls. 8 Describe the life cycle of ferns, and compare their sporophyte and gametophyte generations (page 589). ❚ Fern sporophytes have roots, rhizomes, and leaves that are megaphylls. Their leaves, or fronds, bear sporangia in clusters called sori. Meiosis in sporangia produces haploid spores. The fern gametophyte, called a prothallus, develops from a haploid spore and bears both archegonia and antheridia. 6 Watch the life cycle of the ferns by clicking on the figure in ThomsonNOW. 9 Compare the generalized life cycles of homosporous and heterosporous plants (page 589). ❚ Homospory, the production of one kind of spore, is characteristic of bryophytes, most club mosses, and most ferns, including whisk ferns and horsetails. In homospory, spores give rise to gametophyte plants that produce both egg cells and sperm cells. ❚ Heterospory, the production of two kinds of spores (microspores and megaspores), occurs in certain club mosses, certain ferns, and all seed plants. Microspores give rise to male gametophytes that produce sperm cells. Megaspores give rise to female gametophytes that produce eggs. The evolution of heterospory was an essential step in the evolution of seeds. T E S T Y O U R U N DE R S TAND ING 1. The bryophytes (a) include mosses, liverworts, and hornworts (b) include whisk ferns, horsetails, and club mosses (c) are small plants that lack a vascular system (d) a and c (e) b and c 2. The waxy layer that covers aerial parts of plants is the (a) cuticle (b) archegonium (c) protonema (d) stoma (e) thallus 3. A strengthening compound found in cell walls of vascular plants is (a) xanthophyll (b) lignin (c) cutin (d) cellulose (e) carotenoid 4. Stomata (a) help prevent desiccation of plant tissues (b) transport water and minerals through plant tissues 598 Chapter 27 (c) allow gas exchange for photosynthesis (d) strengthen cell walls (e) produce male gametes 5. The female gametangium, or , produces an egg; the male gametangium, or , produces sperm cells. (a) antheridium; archegonium (b) archegonium; megaphyll (c) megasporangium; antheridium (d) archegonium; antheridium (e) megasporangium; megaphyll 6. Liverworts and hornworts share life-cycle similarities with (a) ferns (b) mosses (c) horsetails (d) club mosses (e) whisk ferns www.thomsonedu.com/biology/solomon 7. The green, gametangia-bearing moss plant (a) is the haploid gametophyte generation (b) is the diploid sporophyte generation (c) is called a protonema (d) contains cells with single large chloroplasts (e) b and c to conduct 8. Seedless vascular plants have water and minerals and to conduct sugar. (a) cuticle; xylem (b) phloem; stomata (c) phloem; xylem (d) stomata; cuticle (e) xylem; phloem 9. A(an) is a leaf that arose from a branch system. (a) antheridium (b) microphyll (c) megaphyll (d) sorus (e) microspore 10. These plants have vascularized stems but lack true roots and leaves. (a) mosses (b) club mosses (c) horsetails (d) whisk ferns (e) hornworts 11. These plants have hollow, jointed stems that are impregnated with silica. (a) mosses (b) club mosses (c) horsetails (d) whisk ferns (e) hornworts 12. Which of the following statements about ferns is not true? (a) ferns have motile sperm cells that swim through water to the egg-containing archegonium (b) ferns are vascular plants (c) ferns are the most economically important group of bryophytes (d) the fern sporophyte consists of a rhizome, roots, and fronds (e) the diversity of ferns is greatest in the tropics 13. Plants probably descended from a group of green algae called (a) rhyniophytes (b) Calamites (c) epiphytes (d) charophytes (e) club mosses 14. Which of the following is not a characteristic of plants? (a) cuticle (b) unicellular gametangia (c) stomata (d) multicellular embryo (e) alternation of generations 15. In plant life cycles (a) the first products of meiosis are gametes (b) spores are part of the diploid sporophyte generation (c) the embryo gives rise to a zygote (d) the first stage in the diploid sporophyte generation is the zygote (e) the first stage in the haploid gametophyte generation is the prothallus C R I TI C AL TH I N KI NG 1. Which group probably colonized the land first, plants or animals? Explain. 2. What adaptations do bryophytes have that algae lack? 3. Evolution Link. How may the following trends in plant evolution be adaptive to living on land? a. dependence on water for fertilization ¡ no need for water as a transport medium b. homospory ¡ heterospory 4. Analyzing Data. According to the cladogram in Figure 27-4, which plants evolved first: nonvascular bryophytes, seedless vascular plants, or seed plants? Which clade would be considered the outgroup: hornworts, liverworts, or mosses? Additional questions are available in ThomsonNOW at www.thomsonedu.com/ login The Plant Kingdom: Seedless Plants 599