29 Plants without Seeds: From Sea to Land

Transcription

1 29 Plants without Seeds: From Sea to Land Residents of the coal-producing central Chinese city of Changsha almost never see the sun, because it is hidden behind an atmosphere dense with choking smog. Nine-tenths of the precipitation in Changsha is acid rain. China burns more coal than any other country in the world, and the resulting untreated smoke leads to disastrous conditions such as those in Changsha, the site of a major coal-fired power plant. Coal is used for 75 percent of China s energy needs primarily to generate electricity, but also directly for heating, smelting of metals, and other purposes. The United States produces more than half of its electricity by burning coal, and indeed has the largest coal reserves in the world. Extractable coal reserves in the U.S. exceed the total amount of oil available for pumping in all other countries combined. Where did all this coal come from? Coal comes from the remains of seedless plants that grew in great forests hundreds of millions of years ago. (The two other fossil fuels petroleum and natural gas come from the remains of plankton that lived in ancient oceans.) Plant parts from those forests sank in swamps that were later covered by soil. Over millions of years, as the buried plant material was subjected to intense pressure and elevated temperatures, coal formed. At the time those ancient forests flourished, the plant world also included relatives of today s mosses. These mossy ancestors were the first plant life on dry land. Today, mosses are among the most abundant plants on Earth, yet they seem at first glance to lack adaptations to life on land. Mosses have no advanced internal plumbing system to move water and nutrients within their bodies, and their leafy photosynthetic organs are only one cell thick. They require liquid water in order to reproduce, and indeed, seem at first glance to be highly dependent on external moisture. Mosses and their relatives do have effective adaptations for life in terrestrial environments, however, as is obvious from their wide distribution. Most live in moist habitats, but a few mosses even live in deserts. An Ingredient of Coal-Based Smog When coal burns, it produces the fly ash shown in this artificially colored image. When too much coal burns where the smoke cannot blow away, the result is disastrous smog.

2 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 571 The earliest terrestrial plants invaded the land sometime during the Paleozoic era (see Table 22.1). These plants were tiny, but their metabolic activities helped convert parent rock into soil that could support the needs of their successors. Larger and larger plants evolved rapidly (in geological terms), and by the Carboniferous period ( mya) great forests were widespread. However, few of the trees in those forests were like those we know today. During the tens of millions of years since the Carboniferous, those early trees have been replaced by the modern trees whose adaptations and appearance are familiar to us. In this chapter, we will see how members of the plant kingdom invaded the land and evolved. Our descriptions here will concentrate on those plants that lack seeds. The next chapter completes our survey of the plant kingdom by considering the seed plants, which dominate the terrestrial scene today. The Plant Kingdom Ancestral organism The kingdom Plantae is monophyletic all plants descend from a single common ancestor and form a branch of the evolutionary tree of life. The shared derived trait, or synapomorphy, of the plant kingdom is development from embryos protected by tissues of the parent plant. For this reason, plants are sometimes referred to as embryophytes. Plants retain the derived features that they share with green algae: the use of chlorophylls a and b and the use of starch as a photosynthetic storage product. Both plants and green algae have cellulose in their cell walls. There are other ways to define plant and plant kingdom and still come out with a monophyletic group (clade). For example, combining plants as defined above with a group of green algae called the charophytes results in a monophyletic plant kingdom with several shared derived traits, including the retention of the egg in the parent body. The addition of the chlorophytes (the remainder of the green algae) to the group just described gives another monophyletic group, with synapomorphies including the possession of chlorophyll b, that can be called a plant kingdom. There are no hard-andfast criteria for defining a kingdom (or any other taxonomic rank), so these definitions of the plant kingdom are all valid. In this book, we choose to use the first definition given above, in which the kingdom Plantae comprises only the embryophytes (Figure 29.1). Some botanists refer to a group consisting of the Plantae plus the green algae as the green plant kingdom, to the red algae as the red plant kingdom, and to the stramenopiles as the brown plant kingdom. There are ten surviving phyla of plants The surviving members of the kingdom Plantae fall naturally into ten phyla (Table 29.1). All members of seven of those phyla possess well-developed vascular systems that transport materials throughout the plant body. We call these seven phyla, collectively, the tracheophytes because they all possess conducting cells called tracheids. The tracheophytes constitute a clade. The remaining three phyla (liverworts, hornworts, and mosses), which lack tracheids, were once considered classes of a single larger phylum. In this book we use the term nontracheophytes to refer collectively to these three phyla. The nontracheophytes are sometimes collectively called bryophytes, but in this text we reserve that term for their most familiar members, the mosses. Collectively, the nontracheophytes are not a monophyletic group. They are the three basal clades of the plant kingdom. Life cycles of plants feature alternation of generations A universal feature of the life cycles of plants is the alternation of generations. Recall from Chapter 28 that alternation of generations has two hallmarks: Stramenopiles Red algae Chlorophytes Charophytes Embryophytes Plantae Brown plants Red plants Green plants 29.1 What Is a Plant? There are three ways to define a plant kingdom, depending on which clade is chosen. In this book, we use the most restrictive definition: plants as embryophytes. Here, the two green algal clades are not considered plants. The life cycle includes both multicellular diploid individuals and multicellular haploid individuals. Gametes are produced by mitosis, not by meiosis. Meiosis produces spores that develop into multicellular haploid individuals. If we begin looking at the plant life cycle at a single-cell stage the diploid zygote then the first phase of the cycle

3 572 CHAPTER TWENTY-NINE 29.1 Classification of Plants a PHYLUM COMMON NAME CHARACTERISTICS Nontracheophytes Hepatophyta Liverworts No filamentous stage; gametophyte flat Anthocerophyta Hornworts Embedded archegonia; sporophyte grows basally Bryophyta Mosses Filamentous stage; sporophyte grows apically (from the tip) Tracheophytes Nonseed tracheophytes Lycophyta Club mosses Microphylls in spirals; sporangia in leaf axils Pteridophyta Ferns and allies Differentiation between main axis and side branches Seed plants Gymnosperms Cycadophyta Cycads Compound leaves; swimming sperm; seeds on modified leaves Ginkgophyta Ginkgo Deciduous; fan-shaped leaves; swimming sperm Gnetophyta Gnetophytes Vessels in vascular tissue; opposite, simple leaves Pinophyta Conifers Seeds in cones; needlelike or scalelike leaves Angiosperms Angiospermae Flowering plants Endosperm; carpels; much reduced gametophytes; seeds in fruit a No extinct groups are included in this classification. Spore Multicellular gametophyte HAPLOID (n) Meiosis Fertilization DIPLOID (2n) Multicellular sporophyte Gametes Zygote 29.2 Alternation of Generations A diploid sporophyte generation that produces spores alternates with a haploid gametophyte generation that produces gametes by mitosis. features the formation, by mitosis and cytokinesis, of a multicellular embryo and eventually the mature diploid plant (Figure 29.2). This multicellular, diploid plant is the sporophyte ( spore plant ). Cells contained in sporangia (singular, sporangium, spore vessel ) on the sporophyte undergo meiosis to produce haploid, unicellular spores. By mitosis and cytokinesis, a spore forms a haploid plant. This multicellular, haploid plant is the gametophyte ( gamete plant ) that produces haploid gametes. The fusion of two gametes (syngamy, or fertilization) results in the formation of a diploid cell the zygote and the cycle repeats. The sporophyte generation extends from the zygote through the adult, multicellular, diploid plant; the gametophyte generation extends from the spore through the adult, multicellular, haploid plant to the gamete. The transitions between the generations are accomplished by fertilization and meiosis. In all plants, the sporophyte and gametophyte differ genetically: The sporophyte has diploid cells, and the gametophyte has haploid cells. In the three basal plant clades, the gametophyte generation is larger and more self-sufficient, while the sporophyte generation is dominant in those groups that appeared later in plant evolution. Some protist life cycles also feature alternation of generations, suggesting that the plants arose from one of these protist groups. But which one? The Plantae arose from a green algal clade Much evidence indicates that the closest living relatives of the plants are members of a clade of green algae called the charophytes. The charophytes, along with some other green algae and the plants, form a clade that is sister to the chlorophytes (see Figure 29.1), but we don t yet know which charophyte clade is the true sister group to the plants. Stoneworts of the genus Chara are charophytes that resemble plants in terms of their rrna and DNA sequences, peroxisome con-

4 (a) Chara sp. (stonewort) PLANTS WITHOUT SEEDS: FROM SEA TO LAND 573 use different mechanisms for dispersing its gametes and progeny than its aquatic relatives, which can simply release them into the water. How did organisms descended from aquatic ancestors adapt to such a challenging environment? Adaptations to life on land distinguish plants from green algae Most of the characteristics that distinguish plants from green algae are evolutionary adaptations to life on land. Several of these features probably evolved in the common ancestor of the plants: tents, mechanics of mitosis and cytokinesis, and chloroplast structure (Figure 29.3a). On the other hand, strong evidence from morphology-based cladistic analysis suggests that the sister group of the plants is a group of charophytes that includes the genus Coleochaete (Figure 29.3b). Coleochaete-like algae have several features found in plants, such as plasmodesmata and a tendency to protect the young sporophyte. Whether they were more similar to stoneworts or to Coleochaete, the ancestors of the plants lived at the margins of ponds or marshes, ringing them with a green mat. From these marginal habitats, which were sometimes wet and sometimes dry, early plants made the transition onto land. The Conquest of the Land (b) Coleochaete sp The Closest Relatives of Land Plants The plant kingdom probably evolved from a common ancestor shared with the charophytes, a green algal group. (a) Molecular evidence seems to favor stoneworts of the genus Chara as sister group to the plants. (b) Evidence from morphology indicates that the group including this coleochaete alga may be sister to the land plants. Plants, or their immediate ancestors in the green mat, first invaded the terrestrial environment between 400 and 500 million years ago. That environment differs dramatically from the aquatic environment. The most obvious difference is the availability of the water that is essential for life: It is everywhere in the aquatic environment, but hard to find and to retain in the terrestrial environment. Water provides aquatic organisms with support against gravity; a plant on land, however, must either have some other support system or sprawl unsupported on the ground. A land plant must also The cuticle, a waxy covering that retards desiccation (drying) Gametangia, cases that enclose plant gametes and prevent them from drying out Embryos, which are young sporophytes contained within a protective structure Certain pigments that afford protection against the mutagenic ultraviolet radiation that bathes the terrestrial environment Thick spore walls containing a polymer that protects the spores from desiccation and resists decay A mutualistic* association with a fungus that promotes nutrient uptake from the soil Further adaptations to the terrestrial environment appeared as plants continued to evolve. One of the most important of these later adaptations was the appearance of vascular tissues. Most present-day plants have vascular tissues The first plants were nonvascular, lacking both water-conducting and food-conducting tissue. Although the term nonvascular plants is a time-honored name, it is misleading when applied to the entire nontracheophyte group, because some mosses (unlike liverworts and hornworts) do have a limited amount of simple conducting tissue. Thus the more unwieldy name nontracheophyte is more descriptive. The first true tracheophytes possessing specialized conducting cells called tracheids arose later (Figure 29.4). The nontracheophytes (the liverworts, hornworts, and mosses) have never been large plants. Except for some of the mosses, they have no water-conducting tissue, yet some are found in dry environments. Many grow in dense masses (see Figure 29.9a), through which water can move by capillary action. Nontracheophytes also have leaflike structures that readily catch and hold any water that splashes onto them. These plants are small enough that minerals can be distributed throughout their bodies by diffusion. *In a mutualistic association, both partners here, the plant and the fungus profit.

5 574 CHAPTER TWENTY-NINE Ancestral alga Chlorophytes Charophytes Liverworts Protected embryos Hornworts Mosses Nontracheophytes First true vascular tissue Club mosses Ferns and allies Nonseed tracheophytes Kingdom Plantae Tracheophytes First seed plants Gymnosperms Flowering plants Seed plants 29.4 From Green Algae to Plants Three key characteristics that emerged during plant evolution protected embryos, vascular tissues, and seeds are all adaptations to life in a terrestrial environment. Plants with vascular tissue are called tracheophytes. We will examine the adaptations of the tracheophytes later in this chapter, concentrating first on the nontracheophytes. Familiar tracheophytes include the club mosses, ferns, conifers, and angiosperms (flowering plants). Tracheophytes differ from liverworts, hornworts, and mosses in crucial ways, one of which is the possession of a well-developed vascular system consisting of specialized tissues for the transport of materials from one part of the plant to another. One such tissue, the phloem, conducts the products of photosynthesis from sites where they are produced or released to sites where they are used or stored. The other vascular tissue, the xylem, conducts water and minerals from the soil to aerial parts of the plant; because some of its cell walls are stiffened by a substance called lignin, xylem also provides support in the terrestrial environment. Nontracheophyte plants evolved tens of millions of years before the earliest tracheophytes, even though tracheophytes appear earlier in the fossil record. The oldest tracheophyte fossils date back more than 410 million years, whereas the oldest nontracheophyte fossils are only about 350 million years old, dating from a time when tracheophytes were already widely distributed. This finding simply shows that, given the differences in their structures and the chemical makeup of their cell walls, tracheophytes are more likely to form fossils than nontracheophytes are. The Nontracheophytes: Liverworts, Hornworts, and Mosses Liverworts Hornworts Mosses Club mosses Horsetails Whisk ferns Ferns Gymnosperms Flowering plants Most liverworts, hornworts, and mosses grow in dense mats, usually in moist habitats. The largest of these plants are only about 1 meter tall, and most are only a few centimeters tall or long. Why have the nontracheophytes not evolved to be taller? The probable answer is that they lack an efficient system for conducting water and minerals from the soil to distant parts of the plant body. To limit water loss, layers of maternal tissue protect the embryos of all nontracheophytes. All nontracheophyte clades also have a cuticle, although it is often very thin (or even absent in some species) and thus not highly effective in retarding water loss. Nontracheophytes lack the leaves, stems, and roots that characterize tracheophytes, although they have structures analogous to each. Most nontracheophytes live on the soil or on other plants, but some grow on bare rock, dead and fallen tree trunks, and even on buildings. Nontracheophytes are widely distributed over six continents and exist very locally on the coast of the

6 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 575 Gametophytes (n) Archegonium (n) H 2 O Fertilization in nontracheophytes requires water so that sperm can swim to eggs. Photosynthetic filament Bud Rhizoid Protonema with bud Antheridium (n) HAPLOID (n) Gametophyte generation Sperm (n) Egg (n) 29.5 A Nontracheophyte Life Cycle The life cycle of nontracheophytes, illustrated here by a moss, is dependent on an external source of liquid water. The visible green structure of nontracheophytes is the gametophyte. Germinating spore Ungerminated spore Meiosis Capsule The sporophyte is attached to and nutritionally dependent on the gametophyte. DIPLOID (2n) Sporophyte generation Sporophyte (2n) Gametophyte (n) seventh (Antarctica). They are successful plants, well adapted to their environments. Most are terrestrial. Some live in wetlands. Although a few nontracheophyte species live in fresh water, these aquatic forms are descended from terrestrial ones. There are no marine nontracheophytes. Nontracheophyte sporophytes are dependent on gametophytes In nontracheophytes, the conspicuous green structure visible to the naked eye is the gametophyte (Figure 29.5). In contrast, the familiar forms of tracheophytes, such as ferns and seed plants, are sporophytes. The gametophyte of nontracheophytes is photosynthetic and therefore nutritionally independent, whereas the sporophyte may or may not be photosynthetic, but is always nutritionally dependent on the gametophyte and remains permanently attached to it. A nontracheophyte sporophyte produces unicellular, haploid spores as products of meiosis within a sporangium, or capsule. A spore germinates, giving rise to a multicellular, haploid gametophyte whose cells contain chloroplasts and are Egg (n) Fertilization Embryo (2n) thus photosynthetic. Eventually gametes form within specialized sex organs, the gametangia. The archegonium is a multicellular, flaskshaped female sex organ with a long neck and a swollen base, which produces a single egg (Figure 29.6a). The antheridium is a male sex organ in which sperm, each bearing two flagella, are produced in large numbers (Figure 29.6b). Once released, the sperm must swim or be splashed by raindrops to a nearby archegonium on the same or a neighboring plant. The sperm are aided in this task by chemical attractants released by the egg or the archegonium. Before sperm can enter the archegonium, certain cells in the neck of the archegonium must break down, leaving a water-filled canal through which the sperm swim to complete their journey. Note that all of these events require liquid water. On arrival at the egg, the nucleus of a sperm fuses with the egg nucleus to form a zygote. Mitotic divisions of the zygote produce a multicellular, diploid sporophyte embryo. The base of the archegonium grows to protect the embryo during its early development. Eventually, the developing sporophyte elongates sufficiently to break out of the archegonium, but it remains connected to the gametophyte by a foot that is embedded in the parent tissue and absorbs water and nutrients from it. The sporophyte remains attached to the gametophyte throughout its life. The sporophyte produces a capsule, within which meiotic divisions produce spores and thus the next gametophyte generation. The structure and pattern of elongation of the sporophyte differ among the three nontracheophyte phyla the liverworts (Hepatophyta), hornworts (Anthocerophyta), and mosses (Bryophyta). The probable evolutionary relationships of these three phyla and the tracheophytes can be seen in Figure 29.4.

7 Sex Organs in Plants (a) Archegonia and (b) antheridia of the moss Mnium (phylum Bryophyta). The gametophytes of all plants have archegonia and antheridia, but they are much reduced in seed plants. (a) Archegonia develop at the tip of a gametophyte. In the archegonium, the egg will be fertilized and begin development into a sporophyte. (b) Antheridia are also located at the tip of a gametophyte. The large egg cell is in the center of the archegonium. These antheridia contain a large number of sperm. When released, the sperm can be carried by water to an archegonium and then swim down its neck to the egg. Liverworts may be the most ancient surviving plant clade The gametophytes of some liverworts (phylum Hepatophyta) are green, leaflike layers that lie flat on the ground (Figure 29.7a). The simplest liverwort gametophytes, however, are flat plates of cells, a centimeter or so long, that produce antheridia or archegonia on their upper surfaces and anchoring and water-absorbing filaments called rhizoids on their lower surfaces. Liverwort sporophytes are shorter than those of mosses and hornworts, rarely exceeding a few millimeters. The liverwort sporophyte has a stalk that connects capsule and foot. In most species, the stalk elongates and thus raises the capsule above ground level, favoring dispersal of spores when they are released. The capsules of liverworts are simple: a globular capsule wall surrounding a mass of spores. In some species of liverworts, spores are not released by the sporophyte until the surrounding capsule wall rots. In other liverworts, however, the spores are thrown from the capsule by structures 29.7 Liverwort Structures Members of the phylum Hepatophyta display various characteristic structures. (a) Gametophytes. (b) Structures bearing antheridia and archegonia. (c) Gemmae cups. that shorten and compress a spring as they dry out. When the stress becomes sufficient, the compressed spring snaps back to its resting position, throwing spores in all directions. Among the most familiar liverworts are species of the genus Marchantia (Figure 29.7a). Marchantia is easily recognized by the characteristic structures on which its male and female gametophytes bear their antheridia and archegonia (Figure 29.7b). Like most liverworts, Marchantia also reproduces asexually by simple fragmentation of the gametophyte. Marchantia and some other liverworts and mosses also reproduce asexually by means of gemmae (singular, gemma), which are lens-shaped clumps of cells. In a few liverworts, the gemmae are loosely held in structures called gemmae cups, which promote dispersal of the gemmae by raindrops (Figure 29.7c). Hornworts evolved stomata as an adaptation to terrestrial life The phylum Anthocerophyta comprises the hornworts, so named because their sporophytes look like little horns (Fig- The umbrella-like structures bear archegonia. The disc-headed structures bear antheridia. These cups contain gemmae small, lensshaped outgrowths of the plant body, each capable of developing into a new plant. (a) Marchantia sp. (b) Marchantia sp. (c) Lunularia sp.

8 The sporophytes of hornworts can reach 20 cm in height. Gametophytes are flat plates a few cells thick. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 577 ure 29.8). Hornworts appear at first glance to be liverworts with very simple gametophytes. These gametophytes consist of flat plates of cells a few cells thick. However, the hornworts, along with the mosses and tracheophytes, share an advance over the liverwort clade in their adaptation to life on land. They have stomata pores that, when open, allow the uptake of CO 2 for photosynthesis and the release of O 2. Stomata may be a shared derived trait (synapomorphy) of hornworts and all other plants except liverworts, although hornwort stomata do not close and may have evolved independently. Hornworts have two characteristics that distinguish them from both liverworts and mosses. First, the cells of hornworts each contain a single large, platelike chloroplast, whereas the cells of other nontracheophytes contain numerous small, lensshaped chloroplasts. Second, of all the nontracheophyte sporophytes, those of the hornworts come closest to being capable of indeterminate growth (growth without a set limit). Liverwort and moss sporophytes have a stalk that stops growing as the capsule matures, so elongation of the sporophyte is strictly limited. The hornwort sporophyte, however, has no stalk. Instead, a basal region of the capsule remains capable of indefinite cell division, continuously producing new spore-bearing tissue above. The sporophytes of some hornworts growing in mild and continuously moist conditions can become as tall as 20 centimeters. Eventually the sporophyte s growth is limited by the lack of a transport system. To support their metabolism, the hornworts need access to nitrogen. Hornworts have internal cavities filled with mucilage; these cavities are often populated by cyanobacteria that convert atmospheric nitrogen gas into a form usable by the host plant. We have presented the hornworts as sister to the clade consisting of mosses and tracheophytes, but this is only one possible interpretation of the current data. The exact evolutionary status of the hornworts is still unclear, and in some phylogenetic analyses they are placed as the most ancient plant clade. Anthoceros sp A Hornwort The sporophytes of hornworts can resemble little horns. Water and sugar transport mechanisms emerged in the mosses The most familiar nontracheophytes are the mosses (phylum Bryophyta). There are more species of mosses than of liverworts and hornworts combined, and these hardy little plants are found in almost every terrestrial environment. They are often found on damp, cool ground, where they form thick mats (Figure 29.9a). The mosses are probably sister to the tracheophytes (see Figure 29.4). Many mosses contain a type of cell called a hydroid, which dies and leaves a tiny channel through which water can (a) (b) The teeth of this moss capsule help expel the thousands of spores The Mosses (a) Dense moss forms hummocks in a valley on New Zealand s South Island. (b) The moss capsule, from which spores are dispersed, grows at the tip of the plant.

9 578 CHAPTER TWENTY-NINE travel. The hydroid may be the progenitor of the tracheid, the characteristic water-conducting cell of the tracheophytes, but it lacks lignin (a waterproofing substance that also lends structural support) and the cell wall structure found in tracheids. The possession of hydroids and of a limited system for transport of sucrose by some mosses (via cells called leptoids) shows that the old term nonvascular plant is somewhat misleading when applied to mosses. In contrast to liverworts and hornworts, the sporophytes of mosses and tracheophytes grow by apical cell division, in which a region at the growing tip provides an organized pattern of cell division, elongation, and differentiation. This growth pattern allows extensive and sturdy vertical growth of sporophytes. Apical cell division is a shared derived trait of mosses and tracheophytes. The moss gametophyte that develops following spore germination is a branched, filamentous structure called a protonema (see Figure 29.5). Although the protonema looks a bit like a filamentous green alga, it is unique to the mosses. Some of the filaments contain chloroplasts and are photosynthetic; others, called rhizoids, are nonphotosynthetic and anchor the protonema to the substratum. After a period of linear growth, cells close to the tips of the photosynthetic filaments divide rapidly in three dimensions to form buds. The buds eventually differentiate a distinct tip, or apex, and produce the familiar leafy moss shoot with leaflike structures arranged spirally. These leafy shoots produce antheridia or archegonia (see Figure 29.6). The antheridia release sperm that travel through liquid water to the archegonia, where they fertilize the eggs. Sporophyte development in most mosses follows a precise pattern, resulting ultimately in the formation of an absorptive foot anchored to the gametophyte, a stalk, and, at the tip, a swollen capsule, the sporangium. In contrast to hornworts, whose sporophytes grow from the base, the moss sporophyte stalk grows at its apical end, as tracheophytes do. Cells at the tip of the stalk divide, supporting elongation of the structure and giving rise to the capsule. For a while, the archegonial tissue grows rapidly as the stalk elongates, but eventually the archegonium is outgrown and is torn apart by the expanding sporophyte. The lid of the capsule is shed after the completion of meiosis and spore development. In most mosses, groups of cells just below the lid form a series of toothlike structures surrounding the opening. Highly responsive to humidity, these structures dig into the mass of spores when the atmosphere is dry; then, when the atmosphere becomes moist, they fling out, scooping out the spores as they go (Figure 29.9b). The spores are thus dispersed when the surrounding air is moist that is, when conditions favor their subsequent germination. Mosses of the genus Sphagnum often grow in swampy places, where the plants begin to decompose in the water after they die. Rapidly growing upper layers compress the deeper-lying, decomposing layers. Partially decomposed plant matter is called peat. In some parts of the world, people derive the majority of their fuel from peat bogs. Sphagnumdominated peatlands cover an area approximately half as large as the United States more than 1 percent of Earth s surface. Long ago, continued compression of peat composed primarily of other nonseed plants gave rise to coal. With their simple system of internal transport, the mosses are, in a sense, vascular plants. However, they are not tracheophytes because they lack true xylem and phloem. Introducing the Tracheophytes Although they are an extraordinarily large and diverse group, the tracheophytes can be said to have been launched by a single evolutionary event. Sometime during the Paleozoic era, probably well before the Silurian period (440 mya), the sporophyte generation of a now long-extinct plant produced a new cell type, the tracheid (Figure 29.10). The tracheid is the principal water-conducting element of the xylem in all tracheophytes except the angiosperms, and even in the angiosperms, tracheids persist alongside a more specialized and efficient system of vessels and fibers derived from them. The evolution of a tissue composed of tracheids had two important consequences. First, it provided a pathway for long-distance transport of water and mineral nutrients from a source of supply to regions of need. Second, its stiff cell walls provided something almost completely lacking and unnecessary in the largely aquatic green algae: rigid structural support. Support is important in a terrestrial environment because plants tend to grow upward as they compete for sunlight to power photosynthesis. Thus the tracheid set the stage for the complete and permanent invasion of land by plants. The tracheophytes feature another evolutionary novelty: a branching, independent sporophyte. A branching sporophyte can produce more spores than an unbranched body, and it can develop in complex ways. The sporophyte of a tracheophyte is nutritionally independent of the gametophyte at maturity. Among the tracheophytes, the sporophyte is the large and obvious plant that one normally notices in nature. This pattern is in contrast to the sporophyte of nontracheophytes such as mosses, which is attached to, dependent on, and usually much smaller than the gametophyte. The present-day evolutionary descendants of the early tracheophytes belong to seven distinct phyla (see Figure 29.10). The tracheophytes have two types of life cycles, one that involves seeds and another that does not. The nonseed tracheophytes (the two basal phyla) include the club mosses and the ferns and their relatives: horsetails and whisk ferns. We will describe these phyla in detail after taking a closer

10 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 579 Nontracheophytes Common ancestor Club mosses Tracheids; branching, independent sporophyte Horsetails Whisk ferns Pteridophytes Nonseed tracheophytes Multiflagellate sperm, complex leaves Ferns Cycads Tracheophytes Seeds Conifers Ginkgos Gymnosperms Seed plants Gnetophytes Flowers, carpels, triploid endosperm Angiosperms The Evolution of Today s Plants The nine phyla of extant tracheophytes are divided between those that produce seeds and those that do not. look at tracheophyte evolution. The five phyla of seed plants will be described in the following chapter. Tracheophytes have been evolving for almost half a billion years The evolution of an effective cuticle and of protective layers for the gametangia (archegonia and antheridia) helped make the first tracheophytes successful, as did the initial absence of herbivores (plant-eating animals) on land. By the late Silurian period, tracheophytes were being preserved as fossils that we can study today. Two groups of nonseed tracheophytes that still exist made their first appearances during the Devonian period ( mya): the lycopods (club mosses) and the pteridophytes (including horsetails and ferns). Their proliferation made the terrestrial environment more hospitable to animals. Amphibians and insects arrived soon after the plants became established. Trees of various kinds appeared in the Devonian period and dominated the landscape of the Carboniferous. Mighty forests of lycopods up to 40 meters tall, horsetails, and tree ferns flourished in the tropical swamps of what would become North America and Europe (Figure 29.11). The remnants of those forests are with us today as huge deposits of coal. In the subsequent Permian period, the continents came together to form a single gigantic land mass, called Pangaea. The continental interior became warmer and drier, but late in the period glaciation was extensive. The 200-million-year reign of the lycopod fern forests came to an end as they were replaced by forests of seed plants (gymnosperms), which dominated until other seed plants (angiosperms) became dominant less than 80 million years ago. The earliest tracheophytes lacked roots and leaves The earliest known tracheophytes belonged to the nowextinct phylum Rhyniophyta. The rhyniophytes were among the only tracheophytes in the Silurian period. The landscape at that time probably consisted of bare ground, with stands of rhyniophytes in low-lying moist areas. Early versions of the structural features of all the other tracheophyte phyla appeared in the rhyniophytes of that time. These shared features strengthen the case for the origin of all tracheophytes from a common nontracheophyte ancestor.

11 29.11 An Ancient Forest This reconstruction is of a Carboniferous forest that once thrived in what is now Michigan. The dominant trees are lycopods of the genus Lepidodendron; ferns are also abundant. In 1917, the British paleobotanists Robert Kidston and William H. Lang reported their finding of well-preserved fossils of tracheophytes embedded in Devonian rocks near Rhynie, Scotland. The preservation of these plants was remarkable, considering that the rocks were more than 395 million years old. These fossil plants had a simple vascular system of phloem and xylem. Some of the plants had flattened scales on the stems, which lacked vascular tissue and thus were not comparable to the true leaves of any other tracheophytes. These plants also lacked roots. They were apparently anchored in the soil by horizontal portions of stem, called rhizomes, that bore water-absorbing rhizoids. These rhizomes also bore aerial branches, and sporangia homologous with the nontracheophyte capsule were found at the tips of these branches. Their branching pattern was dichotomous; that is, the shoot apex divided to produce two equivalent new branches, each pair diverging at approximately the same angle from the original stem (Figure 29.12). Scattered fragments of such plants had been found earlier, but never in such profusion or so well preserved as those discovered by Kidston and Lang. Sporangia Dichotomous branching Rhizoids An Ancient Tracheophyte Relative This extinct plant, Aglaophyton major (phylum Rhyniophyta), lacked roots and leaves. It had a central column of xylem running through its stems, but true tracheids were lacking. The rhizome is a horizontal underground stem, not a root. The aerial stems were less than 50 cm tall, and some were topped by sporangia. Other very similar rhyniophytes such as Rhynia did have tracheids. Rhizome

12 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 581 The presence of xylem indicated that these plants were tracheophytes. But were they sporophytes or gametophytes? Close inspection of thin sections of fossil sporangia revealed that the spores were in groups of four. In almost all living nonseed tracheophytes (with no evidence to the contrary from fossil forms), the four products of meiosis and cytokinesis remain attached to one another during their development into spores. The spores separate only when they are mature, and even after separation their walls reveal the exact geometry of how they were attached. Therefore, a group of four closely packed spores is found only immediately after meiosis, and a plant that produces such a group must be a diploid sporophyte and so the Rhynie fossils must have been sporophytes. Gametophytes of the Rhyniophyta were also found; they, too, were branched, and depressions at the apices of the branches contained archegonia and antheridia. Although they were apparently ancestral to the other tracheophyte phyla, the rhyniophytes themselves are long gone. None of their fossils appear anywhere after the Devonian period. Early tracheophytes added new features A new phylum of tracheophytes the Lycophyta (club mosses) also appeared in the Silurian period. Another the Pteridophyta (ferns and fern allies) appeared during the Devonian period. These two groups arose from rhyniophytelike ancestors. These new groups featured specializations not found in the rhyniophytes, including one or more of the following: true roots, true leaves, and a differentiation between two types of spores. THE ORIGIN OF ROOTS. The rhyniophytes had only rhizoids arising from a rhizome with which to gather water and minerals. How, then, did subsequent groups of tracheophytes come to have the complex roots we see today? It is probable that roots had their evolutionary origins as a branch, either of a rhizome or of the aboveground portion of a stem. That branch presumably penetrated the soil and (a) Vascular tissue Sporangia Sporangium Microphyll (b) 1 branched further. The underground portion could anchor the plant firmly, and even in this primitive condition it could absorb water and minerals. The discovery of fossil plants from the Devonian period, all having horizontal stems (rhizomes) with both underground and aerial branches, supported this hypothesis. Underground and aboveground branches, growing in sharply different environments, were subjected to very different selection pressures during the succeeding millions of years. Thus the two parts of the plant axis the aboveground shoot system and the underground root system diverged in structure and evolved distinct internal and external anatomies. In spite of these differences, scientists believe that the root and shoot systems of tracheophytes are homologous that they were once part of the same organ. THE ORIGIN OF TRUE LEAVES. Thus far we have used the term leaf rather loosely. We spoke of leafy mosses and commented on the absence of true leaves in rhyniophytes. In the strictest sense, a leaf is a flattened photosynthetic structure emerging laterally from a main axis or stem and possessing true vascular tissue. Using this precise definition as we take a closer look at true leaves in the tracheophytes, we see that there are two different types of leaves, very likely of different evolutionary origins. The first leaf type, the microphyll, is usually small and only rarely has more than a single vascular strand, at least in plants alive today. Plants in the phylum Lycophyta (club mosses), of which only a few genera survive, have such simple leaves. The evolutionary origin of microphylls is thought by some biologists to be sterile sporangia (Figure 29.13a). The principal characteristic of this type of leaf is that its vascular The Evolution of Leaves (a) Microphylls are thought to have evolved from sterile sporangia. (b) The megaphylls of pteridophytes and seed plants may have arisen as photosynthetic tissue developed between branch pairs that were left behind as dominant branches overtopped them. A branching stem system became progressively reduced and flattened. Overtopping 2 Flat plates of photosynthetic tissue developed between branches. Megaphyll Time Time A sporangium evolved into a simple leaf. 3 The end branches evolved into the veins of leaves.

13 582 CHAPTER TWENTY-NINE strand departs from the vascular system of the stem in such a way that the structure of the stem s vascular system is scarcely disturbed. This was true even in the fossil lycopod trees of the Carboniferous period, many of which had leaves many centimeters long. The other leaf type is found in ferns and seed plants. This larger, more complex leaf is called a megaphyll. The megaphyll is thought to have arisen from the flattening of a dichotomously branching stem system and the development of overtopping (a pattern in which one branch differentiates from and grows beyond the others). This change was followed by the development of photosynthetic tissue between the members of overtopped groups of branches (Figure 29.13b). Megaphylls may have evolved more than once, in different phyla of tracheophytes showing overtopping of branches. (a) Homospory HOMOSPORY AND HETEROSPORY. In the most ancient of the present-day tracheophytes, both the gametophyte and the sporophyte are independent and usually photosynthetic. Spores produced by the sporophytes are of a single type, and they develop into a single type of gametophyte that bears both female and male reproductive organs. The female organ is a multicellular archegonium, typically containing a single egg. The male organ is an antheridium, containing many sperm. Such plants, which bear a single type of spore, are said to be homosporous (Figure 29.14a). A different system, with two distinct types of spores, evolved somewhat later. Plants of this type are said to be heterosporous (Figure 29.14b). One type of spore, the megaspore, develops into a larger, specifically female gametophyte (a megagametophyte) that produces only eggs. The other type, the microspore, develops into a smaller, male gametophyte (a microgametophyte) that produces only sperm. The sporophyte produces megaspores in small numbers in megasporangia on the sporophyte, and microspores in large numbers in microsporangia. The most ancient tracheophytes were all homosporous, but heterospory evidently evolved independently several times in the early descendants of the rhyniophytes. The fact that heterospory evolved repeatedly suggests that it affords selective advantages. Subsequent evolution in the plant kingdom featured ever greater specialization of the heterosporous condition. Homosporous plants produce a single type of spore. (b) Heterospory Spore (n) Gametophyte (n) HAPLOID (n) Meiosis Fertilization Spore mother cell (2n) Sporangium (2n) Heterosporous plants produce two types of spores: a larger megaspore and a smaller microspore. Megaspore (n) Meiosis Spore mother cell (2n) Megasporangium (2n) Microspore (n) Spore mother cell (2n) The spores of homosporous plants produce a single type of gametophyte with both male and female reproductive organs. DIPLOID (2n) Sporophyte (2n) Megagametophyte (å) (n) Microgametophyte (ç) (n) HAPLOID (n) DIPLOID (2n) Microsporangium (2n) Sporophyte (2n) Archegonium (å) (n) Antheridium (ç) (n) Sperm (n) Embryo (2n) Eggs (n) Zygote (2n) The spores of heterosporous plants produce male and female gametophytes. Eggs (n) Sperm (n) Embryo (2n) Fertilization Zygote (2n) Homospory and Heterospory (a) Homosporous plants bear a single type of spore. Each gametophyte has two types of sex organs, antheridia (male) and archegonia (female). (b) Heterosporous plants, which bear two types of spores that develop into distinctly male and female gametophytes, evolved later. Some tracheophyte clades arose and became extinct in the course of evolution. The earliest clades to arise and survive to this day belong to the nonseed tracheophytes.

14 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 583 The Surviving Nonseed Tracheophytes The nonseed tracheophytes have a large, independent sporophyte and a small gametophyte that is independent of the sporophyte. The gametophytes of the surviving nonseed tracheophytes are rarely more than 1 or 2 centimeters long and are short-lived, whereas their sporophytes are often highly visible; the sporophyte of a tree fern, for example, may be 15 or 20 meters tall and may live for many years. The most prominent resting stage in the life cycle of a nonseed tracheophyte is the single-celled spore. This feature makes their life cycle similar to those of the fungi, the green algae, and the nontracheophytes, but not, as we will see in the next chapter, to that of the seed plants. Nonseed tracheophytes must have an aqueous environment for at least one stage of their life cycle because fertilization is accomplished by a motile, flagellated sperm. The ferns are the most abundant and diverse group of nonseed tracheophytes today, but the club mosses and horsetails were once dominant elements of Earth s vegetation. A fourth group, the whisk ferns, contains only two genera. In this section we ll look at the characteristics of these four groups and at some of the evolutionary advances that appeared in them. The club mosses are sister to the other tracheophytes Liverworts Hornworts Mosses Club mosses Horsetails Whisk ferns Ferns Gymnosperms Flowering plants The club mosses and their relatives (together called lycopods, phylum Lycophyta) diverged earlier than all other living tracheophytes that is, the remaining tracheophytes share an ancestor that was not ancestral to the Lycophyta. There are relatively few surviving species of club mosses. The lycopods have roots that branch dichotomously. The arrangement of vascular tissue in their stems is simpler than in the other tracheophytes. They bear only microphylls, and these simple leaves are arranged spirally on the stem. Growth in club mosses comes entirely from apical cell division, and branching is dichotomous, by a division of the apical cluster of dividing cells. The sporangia in many club mosses are contained within conelike structures called strobili (singular, strobilus; Figure 29.15). A strobilus is a cluster of spore-bearing leaves inserted on an axis tucked into the upper angle between a specialized leaf and the stem. (Such an angle is called an axil.) Other club mosses lack strobili and bear their sporangia in the axil between a photosynthetic leaf and the stem. This placement contrasts with the apical sporangia of the rhyniophytes. There are both homosporous species and heterosporous (a) Lycopodium obscurum species of club mosses. Although only a minor element of present-day vegetation, the Lycophyta are one of two phyla that appear to have been the dominant vegetation during the Carboniferous period. One type of coal (cannel coal) is formed almost entirely from fossilized spores of the tree lycopod Lepidodendron which gives us an idea of the abundance of this genus in the forests of that time (see Figure 29.11). The other major elements of Carboniferous vegetation were horsetails and ferns. Horsetails, whisk ferns, and ferns constitute a clade Once treated as distinct phyla, the horsetails, whisk ferns, and ferns form a clade, the phylum Pteridophyta (pteridophytes, or ferns and fern allies ). Within that clade, the whisk ferns and the horsetails are both monophyletic; the ferns are not. However, about 97 percent of all fern species, including those with which you are most likely to be familiar, do belong to a Liverworts Hornworts Mosses Club mosses Horsetails Whisk ferns Ferns Gymnosperms Flowering plants single clade, the leptosporangiate ferns. In the pteridophytes and in all seed plants there is differentiation (overtopping) between the main axis and side branches. HORSETAILS GROW AT THE BASES OF STEM SEGMENTS. Like the club mosses, the horsetails are represented by only a few (b) Strobilus Microsporangium Club Mosses (a) Strobili are visible at the tips of this club moss. Club mosses have microphylls arranged spirally on their stems. (b) A thin section through a strobilus of a club moss, showing microsporangia.

15 584 CHAPTER TWENTY-NINE present-day species. All are in a single genus, Equisetum. These plants are sometimes called scouring rushes because silica deposits found in their cell walls made them useful for cleaning. They have true roots that branch irregularly. Their sporangia curve back toward the stem on the ends of short stalks called sporangiophores (Figure 29.16a). Horsetails have a large sporophyte and a small gametophyte, both independent. The small leaves of horsetails are reduced megaphylls and form in distinct whorls (circles) around the stem (Figure 29.16b). Growth in horsetails originates to a large extent from discs of dividing cells just above each whorl of leaves, so each segment of the stem grows from its base. Such basal growth is uncommon in plants, although it is found in the grasses, a major group of flowering plants. Sporangium PRESENT-DAY WHISK FERNS RESEMBLE THE MOST ANCIENT TRACHEOPHYTES. There once was some disagreement about whether rhyniophytes are entirely extinct. The confusion arose because of the existence today of two genera of rootless, spore-bearing plants, Psilotum and Tmesipteris, collectively called the whisk ferns. Psilotum nudum (Figure 29.17) has only minute scales instead of true leaves, but plants of the genus Tmesipteris have flattened photosynthetic organs reduced megaphylls with well-developed vascular tissue. Are these two genera the living relics of the rhyniophytes, or do they have more recent origins? Psilotum and Tmesipteris once were thought to be evolutionarily ancient descendants of anatomically simple ancestors. That hypothesis was weakened by an enormous hole in the geological record between the rhyniophytes, which apparently became extinct more than 300 million years ago, and Psilotum and Tmesipteris, which are modern plants. DNA sequence data finally settled the question in favor of a more modern origin of the whisk ferns from fernlike ancestors. These two genera are a clade of highly specialized plants that evolved fairly recently from anatomically more complex ancestors by loss of complex leaves and true roots. Whisk fern gametophytes live below the surface of the ground and lack chlorophyll. They depend upon fungal partners for their nutrition. Sporangiophore (a) Equisetum arvense Leaves Fertile shoot (b) Equisetum palustre Horsetails (a) Sporangia and sporangiophores of a horsetail. (b) Vegetative and fertile shoots of the marsh horsetail. Reduced megaphylls can be seen in whorls on the stem of the vegetative shoot on the right; the fertile shoot on the left is ready to disperse its spores. even these small leaves have more than one vascular strand, and are thus megaphylls. The ferns constitute a group that first appeared during the Devonian period and today consists of about 12,000 species. The ferns are not a monophyletic group, although, as already mentioned, 97 percent of the species the leptosporangiate ferns do constitute a monophyletic group. The leptosporangiate ferns differ from the other ferns in having sporangia with walls only one cell thick, borne on a stalk. Ferns evolved large, complex leaves The sporophytes of the ferns, like those of the seed plants, have true roots, stems, and leaves. Their leaves are typically large and have branching vascular strands. Some species have small leaves as a result of evolutionary reduction, but Psilotum nudum A Whisk Fern Psilotum nudum was once considered by some to be a surviving rhyniophyte and by others to be a fern. It is now included in the phylum Pteridophyta, and it is widespread in the Tropics and Subtropics.

16 PLANTS WITHOUT SEEDS: FROM SEA TO LAND 585 (b) (a) Adiantum pedatum Fern Fronds Take Many Forms (a) The fronds of Northern maidenhair fern form a pattern in this photograph. (b) The fiddlehead (developing frond) of a common forest fern; this structure will unfurl and expand to give rise to a complex adult frond such as those in (a). (c) The tiny fronds of a water fern. Ferns are characterized by fronds (large leaves with complex vasculature; Figure 29.18a). During its development, the fern frond unfurls from a tightly coiled fiddlehead (Figure 29.18b). Some fern leaves become climbing organs and may grow to be as much as 30 meters long. Because they require water for the transport of the male gametes to the female gametes, most ferns inhabit shaded, moist woodlands and swamps. Tree ferns can reach heights of 20 meters. Tree ferns are not as rigid as woody plants, and they have poorly developed root systems. Thus they do not grow in sites exposed directly to strong winds, but rather in Dryopteris intermedia Fern Sori Are Clusters of Sporangia Sori, each containing many spore-producing sporangia, have formed on the underside of this frond of the Midwestern fancy fern. (c) Marsilea mutica ravines or beneath trees in forests. The sporangia of ferns are found on the undersurfaces of the fronds, sometimes covering the whole undersurface and sometimes only at the edges. In most species the sporangia are found in clusters called sori (singular, sorus) (Figure 29.19). The sporophyte generation dominates the fern life cycle Inside the sporangia, fern spore mother cells undergo meiosis to form haploid spores. Once shed, the spores travel great distances and eventually germinate to form independent gametophytes. Old World climbing fern, Lygodium microphyllum, is currently spreading disastrously through the Florida Everglades, choking off the growth of other plants. This rapid spread is testimony to the effectiveness of windborne spores. Fern gametophytes have the potential to produce both antheridia and archegonia, although not necessarily at the same time or on the same gametophyte. Sperm swim through water to archegonia often to those on other gametophytes where they unite with an egg. The resulting zygote develops into a new sporophyte embryo. The young sporophyte sprouts a root and can thus grow independently of the gametophyte. In the alternating generations of a fern, the gametophyte is small, delicate, and short-lived, but the sporophyte can be very large and can sometimes survive for hundreds of years (Figure 29.20). Most ferns are homosporous. However, two groups of aquatic ferns, the Marsileaceae and Salviniaceae, are derived from a common ancestor that evolved heterospory. The megaspores and microspores of these plants (which germinate to produce female and male gametophytes, respectively) are produced in different sporangia (megasporangia and microsporangia), and the microspores are always much smaller and greater in number than the megaspores.

17 586 CHAPTER TWENTY-NINE Mature gametophyte (about 0.5 cm wide) The Life Cycle of a Fern The most conspicuous stage in the fern life cycle is the mature, diploid sporophyte. Egg Meiosis Sporangium A few genera of ferns produce a tuberous, fleshy gametophyte instead of the characteristic flattened, photosynthetic structure produced by most ferns. Like the gametophytes of whisk ferns, these tuberous gametophytes depend on a mutualistic fungus for nutrition; in some genera, even the sporophyte embryo must become associated with the fungus before extensive development can proceed. In Chapter 31 we will see that there are many other important plant fungus mutualisms. All the tracheophytes we have discussed thus far disperse themselves by spores. In the next chapter we will discuss the plants that dominate most of Earth s vegetation today, the seed plants, whose seeds afford new sporophytes protection unavailable to those of the nonseed tracheophytes. Germinating spore Spore tetrad Mature sporophyte (typically m tall) Sori (clusters of sporangia) Roots Rhizoids Antheridium HAPLOID (n) DIPLOID (2n) Horizontal stem Sporophyte Archegonium Archegonial wall Embryo Root Sperm Fertilization Chapter Summary The Plant Kingdom Plants are photosynthetic eukaryotes that develop from embryos protected by parental tissue. Like the green algae, they use chlorophylls a and b and store carbohydrates as starch. Review Figure 29.1 Plant life cycles feature alternation of gametophyte (haploid) and sporophyte (diploid) generations. Both generations include multicellular organisms. Review Figure 29.2 There are ten surviving phyla of plants. The three basal phyla are nontracheophytes, and the remaining seven phyla are tracheophytes. Review Table 29.1 Plants arose from a common green algal ancestor in the charophyte clade, either a stonewort or a member of the group that includes Coleochaete. Descendants of this ancestral charophyte colonized the land. The Conquest of the Land The acquisition of a cuticle, gametangia, a protected embryo, protective pigments, thick spore walls with a protective polymer, and a mutualistic association with a fungus are all defining characters of plants, and all are associated with the adaptation of plants to life on land. Tracheophytes are characterized by possession of a vascular system, consisting of water- and mineral-conducting xylem and nutrient-conducting phloem. Nontracheophytes lack a vascular system. Review Figure 29.4 The Nontracheophytes: Liverworts, Hornworts, and Mosses Nontracheophytes either lack vascular tissues completely or, in the case of certain mosses, have only a rudimentary system of water- and food-conducting cells. The nontracheophyte sporophyte generation is smaller than the gametophyte generation and depends on the gametophyte for water and nutrition. Review Figures 29.5, See Web/CD Tutorial 29.1 The nontracheophytes include the liverworts (phylum Hepatophyta), hornworts (phylum Anthocerophyta), and mosses (phylum Bryophyta). Hornwort sporophytes grow at their basal end. Hornworts, mosses, and tracheophytes have surface pores (stomata) that allow gas exchange and minimize water loss. In mosses and tracheophytes, the sporophytes grow by apical cell division. The hydroids of mosses, through which water may travel, may be ancestral to tracheids, the water-conducting cells of the tracheophytes.