Colonization of land by plants fundamentally altered the history of life on Earth. A terrestrial environment offers abundant CO2 and solar radiation for photosynthesis. But for at least 500 million years, the lack of water and higher ultraviolet (UV) radiation on land confined green algal ancestors to an aquatic environment. Evolutionary innovations for reproduction, structural support, and prevention of water loss are key in the story of plant adaptation to land. The evolutionary shift on land to life cycles dominated by a diploid generation masks recessive mutations arising from higher UV exposure. As a result, larger numbers of alleles persist in the gene pool, creating greater genetic diversity. Long before seeds and flowers evolved, the seedless plants covered the Earth. In this chapter we consider the evolutionary innovations in seedless plants during the first 100 million years of terrestrial life.
Origin of Land Plants
Green algae and the land plants shared a common ancestor a little over 1 BYA and are collectively referred to as the green plants. DNA sequence data are consistent with the claim that a single individual gave rise to all green plants. The green plants are photoautotrophic, but not all photoautotrophs are plants. The definition of a green plant is broad, but it excludes the red and brown algae. All algae–red, brown, and green–shared a primary endosymbiotic event 1.5 BYA. But sharing an ancestral chloroplast lineage is not the same as being monophyletic, as discussed in chapter 29. Red and green algae last shared a common ancestor about 1.4 BYA. Brown algae became photosynthetic through endosymbiosis with a eukaryotic red alga that had itself already acquired a photosynthetic cyanobacterium, as described in the preceding chapter.
Plants are also not fungi, which are more closely related to metazoan animals. Fungi, however, were essential to the colonization of land by plants, enhancing plants’ nutrient uptake from the soil.
One of the most significant evolutionary events in the billion-year-old history of the green plants is the adaptation to terrestrial living.
Land plants evolved from freshwater algae
Some saltwater algae evolved to thrive in a freshwater environment. Just a single species of freshwater green algae gave rise to the entire terrestrial plant lineage, from mosses through the flowering plants (angiosperms). Given the incredibly harsh conditions of life on land, it is not surprising that all land plants share a single common ancestor. Exactly what this ancestral alga was is still a mystery, but close relatives, members of the charophytes, exist in freshwater lakes today.
The green algae split into two major clades: the chlorophytes, which never made it to land, and the charophytes, which are sister clade to all land plants. Together charophytes and land plants are referred to as streptophytes. Land plants, although diverse, have certain characteristics in common. Unlike the charophytes, land plants have multicellular haploid and diploid stages. Diploid embryos are also land plant innovations. Over time, the trend has been toward more embryo protection and a smaller haploid stage in the life cycle.
Land plants have adapted to terrestrial life
Unlike their freshwater ancestors, most land plants have only limited amounts of water available to them. As an adaptation to living on land, most plants are protected from desiccation–the tendency of organisms to lose water to the air–by a waxy surface material called the cuticle that is secreted onto their exposed surfaces. The cuticle is relatively impermeable, preventing water loss. This solution, however, limits the gas exchange essential for respiration and photosynthesis. Gas diffusion into and out of a plant occurs through tiny mouth-shaped openings called stomata (singular, stoma), which allow water to diffuse out at the same time. Stomata can be closed at times to limit water loss.
Moving water within plants is a challenge that increases with plant size. Members of the land plants can be distinguished based on the presence or absence of tracheids, specialized cells that facilitate the transport of water and minerals. Tracheophytes have specialized transport cells called tracheids and have evolved highly efficient transport systems: water-conducting xylem and food-conducting phloem strands of tissue in their stems, roots, and leaves. Some plants that grow in aquatic environments, including water lilies, have tracheids. Aquatic tracheophytes had terrestrial ancestors that adapted back to a watery environment.
Terrestrial plants are exposed to higher intensities of UV irradiation than aquatic algae, increasing the chance of mutation. Diploid genomes mask the effect of a single, deleterious allele. All land plants have both haploid and diploid generations, and the evolutionary shift toward a dominant diploid generation allows for greater genetic variability to persist in terrestrial plants.
The haplodiplontic cycle produces alternation of generations
Humans have a diplontic life cycle, meaning that only the diploid stage is multicellular; by contrast, the land plant life cycle is haplodiplontic, having multicellular haploid and diploid stages. Most multicellular green plants have this haplodiplontic life cycle. Many multicellular green algae and all land plants have haplodiplontic life cycles and undergo mitosis after both gamete fusion and meiosis. The result is a multicellular haploid individual and a multicellular diploid individual–unlike in the human life cycle, in which gamete fusion directly follows meiosis.
Many brown, red, and green algae are also haplodiplontic. Humans produce gametes via meiosis, but land plants actually produce gametes by mitosis in a multicellular, haploid individual. The diploid generation, or sporophyte, alternates with the haploid generation, or gametophyte. Sporophyte means “spore plant,” and gametophyte means “gamete plant.” These terms indicate the kinds of reproductive cells the respective generations produce.
The diploid sporophyte produces haploid spores (not gametes) by meiosis. Meiosis takes place in structures called sporangia, where diploid spore mother cells (sporocytes) undergo meiosis, each producing four haploid spores. Spores are the first cells of the gametophyte generation. Spores divide by mitosis, producing a multicellular, haploid gametophyte.
The haploid gametophyte is the source of gametes. When the gametes fuse, the zygote they form is diploid and is the first cell of the next sporophyte generation. The zygote grows into a diploid sporophyte by mitosis and produces sporangia in which meiosis ultimately occurs.
The relative sizes of haploid and diploid generations vary
All land plants are haplodiplontic; however, the haploid generation consumes a much larger portion of the life cycle in mosses and ferns than it does in the seed plants–the gymnosperms and angiosperms. In mosses, liverworts, and ferns, the gametophyte is photosynthetic and free living. When you look at mosses, what you see is largely gametophyte tissue; the sporophytes are usually smaller, brownish or yellowish structures attached to the tissues of the gametophyte. In other plants, the gametophyte is usually nutritionally dependent on the sporophyte. When you look at a gymnosperm or angiosperm, such as most trees, the largest, most viable portion is a sporophyte.
Although the sporophyte generation can get very large, the size of the gametophyte is limited in all plants. The gametophyte generation of mosses produces gametes at its tips. The egg is stationary, and sperm lands near the egg in a droplet of water. If the moss were the height of a sequoia, not only would vascular tissue be needed for conduction and support, but the sperm would have to swim up the tree! In contrast, the small gametophyte of the fern develops on the forest floor where gametes can meet. Tree ferns are especially abundant in Australia; the haploid spores that the sporophyte trees produce fall to the ground and develop into gametophytes.
Having completed an overview of plant life cycles, we next consider the major groups of seedless land plants. As we proceed, you will see a reduction of the gametophyte from group to group, a loss of multicellular gametangia (structures in which gametes are produced), and increasing specialization for life on land.
All algae acquired chloroplasts necessary for photosynthesis, but green algae diverged from red algae after that event. A single freshwater green alga successfully invaded land; its descendant eventually developed reproductive strategies, conducting systems, stomata, and cuticles as adaptations. Green plants include all green algae and the land plants, whereas the streptophytes include only the land plants and their sister clade, the charophytes. Most plants have a haplodiplontic life cycle, a haploid form alternates with a diploid form in a single organism. Diploid sporophytes produce haploid spores by meiosis. Each spore can develop into a haploid gametophyte by mitosis; the gametophyte form produces haploid gametes, again by mitosis. When the gametes fuse, the diploid sporophyte is formed once more.
Bryophytes: Dominant Gametophyte Generation
Land plants began diverging 450 MYA. Bryophytes are the closest living descendants of these first land plants. Plants in this group are also called nontracheophytes because they lack the derived transport cell called a tracheid.
Fossil evidence and molecular systematics can be used to reconstruct early terrestrial plant life. Water and gas availability were limiting factors. These plants likely had little ability to regulate internal water levels and likely tolerated desiccation, traits found in most extant mosses, although some are aquatic.
Algae, including the Charales, lack roots. Fungi and early land plants cohabited, and the fungi formed close associations with the plants that enhanced water uptake. The tight symbiotic relationship between fungi and plants, called mycorrhizal associations, are also found in many existing bryophytes.
Bryophytes are unspecialized but successful in many environments
The approximately 24,700 species of bryophytes are simple but highly adapted to a diversity of terrestrial environments, even deserts. Most bryophytes are small; few exceed 7 cm in height. Bryophytes have conducting cells other than tracheids for water and nutrients. The tracheid is a derived trait that characterizes the tracheophytes, all land plants but the bryophytes. Bryophytes are sometimes called nonvascular plants, but nontracheophyte is a more accurate term because they do have conducting cells of different types.
Scientists now agree that bryophytes consist of three quite distinct clades of relatively unspecialized plants: liverworts, mosses, and hornworts. Their gametophytes are photosynthetic and are more conspicuous than the sporophytes. Sporophytes are attached to the gametophytes and depend on them nutritionally in varying degrees. Some of the sporophytes are completely enclosed within gametophyte tissue; others are not and usually turn brownish or straw-colored at maturity. Like ferns and certain other vascular (tracheophyte) plants, bryophytes require water (such as rainwater) to reproduce sexually, tracing back to their aquatic origins. It is not surprising that they are especially common in moist places, both in the tropics and temperate regions.
Liverworts are an ancient phylum
The Old English word wyrt means “plant” or “herb.” Some common liverworts (phylum Hepaticophyta) have flattened gametophytes with lobes resembling those of liver–hence the name “liverwort.” Although the lobed liverworts are the best known representatives of this phylum, they constitute only about 20% of the species. The other 80% are leafy and superficially resemble mosses. The gametophytes are prostrate instead of erect, with single celled rhizoids that aid in absorption like roots but are not organs.
Some liverworts have air chambers containing upright, branching rows of photosynthetic cells, each chamber having a pore at the top to facilitate gas exchange. Unlike stomata, the pores are fixed open and cannot close.
Sexual reproduction in liverworts is similar to that in mosses. Lobed liverworts may form gametangia in umbrella-like structures. Asexual reproduction occurs when lens-shaped pieces of tissue that are released from the gametophyte grow to form new gametophytes.
Mosses have rhizoids and water-conducting tissue
Unlike other bryophytes, the gametophytes of mosses typically consist of small, leaflike structures (not true leaves, which contain vascular tissue) arranged spirally or alternately around a stemlike axis; the axis is anchored to its substrate by means of rhizoids. Each rhizoid consists of several cells that absorb water, but not nearly the volume of water that is absorbed by a vascular plant root.
Moss leaflike structures have little in common with leaves of vascular plants, except for the superficial appearance of the green, flattened blade and slightly thickened midrib that runs lengthwise down the middle. Only one cell layer thick (except at the midrib), they lack vascular strands and stomata, and all the cells are haploid. However, mosses do have stomata on the capsule portion of the sporophyte generation and because of that are the basal land group with stomata.
Water may rise up a strand of specialized cells in the center of a moss gametophyte axis. Some mosses also have specialized food-conducting cells surrounding those that conduct water.
Multicellular gametangia are formed at the tips of the leafy gametophytes. Female gametangia (archegonia) may develop either on the same gametophyte as the male gametangia (antheridia) or on separate plants. A single egg is produced in the swollen lower part of an archegonium, whereas numerous sperm are produced in an antheridium.
When sperm are released from an antheridium, they swim with the aid of flagella through a film of dew or rainwater to the archegonia. One sperm (which is haploid) unites with an egg (also haploid), forming a diploid zygote. The zygote divides by mitosis and develops into the sporophyte, a slender, basal stalk with a swollen capsule, the sporangium, at its tip. As the sporophyte develops, its base is embedded in gametophyte tissue, its nutritional source.
The sporangium is often cylindrical or club-shaped. Spore mother cells within the sporangium undergo meiosis, each producing four haploid spores. In many mosses at maturity, the top of the sporangium pops off, and the spores are released. A spore that lands in a suitable damp location may germinate and grow, using mitosis, into a threadlike structure, which branches to form rhizoids and “buds” that grow upright. Each bud develops into a new gametophyte plant consisting of a leafy axis.
In the Arctic and the Antarctic, mosses are the most abundant plants. The greatest diversity of moss species, however, is found in the tropics. Many mosses are able to withstand prolonged periods of drought, although mosses are not common in deserts.
Most mosses are highly sensitive to air pollution and are rarely found in abundance in or near cities or other areas with high levels of air pollution. Some mosses, such as the peat mosses (Sphagnum), can absorb up to 25 times their weight in water and are valuable commercially as a soil conditioner or as a fuel when dry.
The moss genome
Moss plants can survive extreme water loss–an adaptive trait in the early colonization of land that has been lost from vegetative tissues of tracheophytes. Desiccation tolerance and phylogenetic position were among the traits that led researchers to sequence the genome of the moss Physcomitrella patens as being the first land plant that is not a tracheophytes. Although the moss genome is a single genome bracketed by Chlamydomonas and the tracheophytes, many evolutionary hints are hidden within it. Evidence indicates the loss of genes associated with a watery life, including flagellar arms, were lost in the last common ancestor of the land plants. Genes associated with tolerance of terrestrial stresses, including temperature and water availability, are absent in Chlamydomonas and present in moss. For example, the plant hormone abscisic acid (ABA) is important in stress responses in moss and other land plants and genes needed for ABA signaling are not found in algae.
Hornworts developed stomata
The origin of hornworts (phylum Anthocerotophyta) is a puzzle. They are most likely among the earliest land plants, yet the earliest hornwort fossil spores date from the Cretaceous period (65-145 MYA), when angiosperms were emerging.
The small hornwort sporophytes resemble tiny green broom handles or horns, rising from filmy gametophytes usually less than 2 cm in diameter. The sporophyte base is embedded in gametophyte tissue, from which it derives some of its nutrition. However, the sporophyte has stomata to regulate gas exchange, is photosynthetic, and provides much of the energy needed for growth and reproduction. Hornwort cells usually have a single large chloroplast.
The bryophytes exhibit adaptations to terrestrial life. Moss adaptations include rhizoids to anchor the moss body and to absorb water, and water-conducting tissues. Mosses are found in a variety of habitats, and some can survive droughts. Hornworts developed stomata that can open and close to regulate gas exchange.
Tracheophyte Plants: Roots, Stems, and Leaves
Tracheophytes, also known as vascular plants, first appeared about 410 MYA. The first tracheophytes with a relatively complete record belonged to the phylum Rhyniophyta. We are not certain what the earliest of these vascular plants looked like, but fossils of Cooksonia provide some insight into their characteristics.
Cooksonia, the first known vascular land plant, appeared in the late Silurian period about 420 MYA, but is now extinct. It was successful partly because it encountered little competition as it spread out over vast tracts of land. The plants were only a few centimeters tall and had no roots or leaves. They consisted of little more than a branching axis, the branches forking evenly and expanding slightly toward the tips. They were homosporous (producing only one type of spore). Sporangia formed at branch tips. Other ancient vascular plants that followed evolved more complex arrangement of sporangia.
Vascular tissue allows for distribution of nutrients
Cooksonia and the other early plants that followed it became successful colonizers of the land by developing efficient water water- and food-conducting systems called vascular tissues. These tissues consist of strands of specialized cylindrical or elongated cells that form a network throughout a plant, extending from near the tips of the roots, through the stems, and into true leaves, defined by the presence of vascular tissue in the blade. One type of vascular tissue, xylem, conducts water and dissolved minerals upward from the roots; another type of tissue, phloem, conducts sucrose and hormones throughout the plant. Tracheids are the cells in the early vascular plants that conducted water in xylem tissue. Vascular tissue enables enhanced height and size in the tracheophytes. It develops in the sporophyte, but (with a few exceptions) not in the gametophyte. A cuticle and stomata are also characteristic of vascular plants.
Tracheophytes are grouped in three clades
Three clades of vascular plants exist today: (1) lycophytes (club mosses), (2) pterophytes (ferns and their relatives), and (3) seed plants. Advances in molecular systematics have changed the way we view the evolutionary history of vascular plants. Whisk ferns and horsetails were long believed to be distinct phyla that were transitional between bryophytes and vascular plants. Phylogenetic evidence now shows they are the closest living relatives to ferns, and they are grouped as pterophytes.
Tracheophytes dominate terrestrial habitats everywhere, except for the highest mountains and the tundra. The haplodiplontic life cycle persists, but the gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes. A similar reduction in multicellular gametangia has occurred as well.
Stems evolved prior to roots
Fossils of early vascular plants reveal stems, but no roots or leaves. The earliest vascular plants, including Cooksonia, had transport cells in their stems, but the lack of roots limited the size of these plants.
Roots provide structural support and transport capability
True roots are found only in the tracheophytes. Other, somewhat similar structures enhance either transport or support in nontracheophytes, but only roots have a dual function: providing both transport and support. Lycophytes diverged from other tracheophytes before roots appeared, based on fossil evidence. It appears that roots evolved at least two separate times.
Leaves evolved more than once
Leaves increase surface area of the sporophyte, enhancing photosynthetic capacity. Lycophytes have single vascular strands supporting relatively small leaves called lycophylls. True leaves, called euphylls, are found only in ferns and seed plants, having distinct origins from lycophylls. Lycophylls may have resulted from vascular tissue penetrating small, leafy protuberances on stems. Euphylls most likely arose from branching stems that became webbed with leaf tissue.
About 40 million years separates the appearance of vascular tissue and the wide euphyll leaf–a curiously long time. The current hypothesis is that a 90% drop in atmospheric CO2 360 MYA allowed for the increase in leaf size because of an increase in the number of stomata on a leaf. Large, horizontal leaves capture 200% more radiation than thin, axial leaves. Although beneficial for photosynthesis, larger leaves correspondingly increase leaf temperature, which can be lethal. Stomatal openings in the leaf enhance the movement of water out of the leaf, thereby cooling it. The density of stomata on leaf surfaces correlates with CO2 concentration, as the stomatal openings are essential for gas exchange. As the atmospheric CO2 levels dropped, plants could not obtain sufficient CO2 for photosynthesis. In the low-CO2 atmosphere, natural selection favored plants with higher stomatal densities. Higher stomatal densities favored larger leaves with a photosynthetic advantage that did not overheat. Leaves up to 120 mm wide and 160 mm long have been identified in the fossil record form that time period.
Seeds are another innovation in some tracheophytes phyla
Seeds are highly resistant structures well suited to protecting a plant embryo from drought and to some extent from predators. In addition, almost all seeds contain a supply of food for the young plant. Lycophytes and pterophytes do not have seeds.
Fruits in the flowering plants (angiosperms) add a layer of protection to seeds and have adaptations that assist in seed dispersal, expanding the potential range of the species. Flowers allow plants to secure the benefits of wide out-crossing in promoting genetic diversity, as discussed in chapter 31. Before moving on to the specifics of lycophytes and pterophytes, review the evolutionary history of terrestrial innovations in the land plants.
Most tracheophytes have well-developed vascular tissues, including tracheids, that enable efficient delivery of water and nutrients throughout the organism. They also exhibit specialized roots, stems, leaves, cuticles, and stomata. Many produce seeds, which protect and nourish embryos.
Lycophytes: Dominant Sporophyte Generation and Vascular Tissue
The earliest vascular plants lacked seeds. Members of four phyla of living vascular plants also lack seeds, as do at least three other phyla known only from fossils. As we explore the adaptations of the vascular plants, we focus on both reproductive strategies and the advantages of increasingly complex transport systems.
The lycophytes (club mosses) are relic species of an ancient past when vascular plants first evolved. They are the sister group to all vascular plants. Several genera of club mosses, some of them treelike, became extinct about 270 MYA. Today, club mosses are worldwide in distribution but are most abundant in the tropics and moist temperate regions.
Members of the 12 to 13 genera and about 1150 living species of club mosses superficially resemble true mosses, but once their internal vascular structure and reproductive processes became known, it was clear that they are unrelated to mosses. The sporophyte stage is the dominant (obvious) stage; sporophytes have leafy stems that are seldom more than 30 cm long.
The lycophyte Selaginella moellendorffii is the first seedless vascular plant with a fully sequenced genome. A few clues to the evolution of vascular plants, hidden in the genome, emerged in comparisons with genomes of flowering plants. Genes that play an important role in establishing leaf polarity in flowering plants are not found in Selaginella, indicative of independent origins of leaflike structures in different vascular plant lineages. Genome differences also reflect differences in developmental pathways leading to reproductive maturity in the sporophyte generation in lycophytes and flowering plants.
Comparing predicted proteins in the Chlamydomonas (green alga), Physcomitrella (moss), and Selaginella with 15 angiosperms revealed 3814 gene families that all the green plants share–the essential instructions for building a green plant. About 3000 new genes were acquired in the transition from the single-celled green alga to the multicellular moss, but only 516 genes were added in the transition from nonvascular to vascular plants. This is a first step in sorting out the evolutionary steps that led to the vascular plants.
Lycophytes are basal to all other vascular plants. Although they superficially resemble bryophytes, they contain tracheid-based vascular tissues, and their reproductive cycle is like that of other vascular plants; however, they lack vascularized leaves.
Pterophytes: Ferns and Their Relatives
The phylogenetic relationships among ferns and their near relations is intriguing. A common ancestor gave rise to two clades: One clade diverged to produce a line of ferns and horsetails; the other diverged to yield another line of ferns and whisk ferns–ancient-looking plants.
Whisk ferns and horsetails are close relatives of ferns. Like lycophytes and bryophytes, they all form antheridia and archegonia. Free water is required for the process of fertilization, during which the sperm, which have flagella, swim to and unite with the eggs. In contrast, most seed plants have nonflagellated sperm.
Whisk ferns lost their roots and leaves secondarily
In whisk ferns, which occur in the tropics and subtropics, the sporophytic generation consists merely f evenly forking green stems without roots. The two or three species of the genus Psilotum do, however, have tiny, green, spirally arranged flaps of tissue lacking veins and stomata. Another genus, Tmesipteris, has more leaflike appendages. Currently, systematists believe that whisk ferns lost leaves and roots when they diverged from others in the fern lineage.
Given the simple structure of whisk ferns, it was particularly surprising to discover that they are monophyletic with ferns. The gametophytes of whisk ferns are essentially colorless and are less than 2 mm in diameter, but they can be up to 18 mm long. They form symbiotic associations with fungi, which furnish their nutrients. Some develop elements of vascular tissue and have the distinction of being the only gametophytes known to do so.
Horsetails have jointed stems with brushlike leaves
The 15 living species of horsetails are all homosporous. They constitute a single genus, Equisetum. Fossil forms of Equisetum extend back 300 million years to an era when some of their relatives were treelike. Today, they are widely scattered around the world, mostly in damp places. Some that grow among the coastal redwoods of California may reach a height of 3 m, but most are less than a meter tall.
Horsetails sporophytes consist of ribbed, jointed, photosynthetic stems that arise form branching underground rhizomes with roots at their nodes. A whorl of nonphotosynthetic, scalelike leaves emerges at each node. The hollow stems have silica deposits in the epidermal cells of the ribs, and the interior parts of the stems have two sets of vertical, tubular canals. The larger outer canals, which alternate with the ribs, contain air, and the smaller inner canals opposite the ribs contain water. Horsetails are also called scouring rushes because pioneers of the American West used them to scrub pans.
Ferns have fronds that bear sori
Ferns are the most abundant group of seedless vascular plants, with about 11,000 living species. Recent research indicates that they may be the closest relatives to the seed plants.
The fossil record indicates that ferns originated during the Devonian period about 350 MYA and became abundant and varied in form during the next 50 million years. Their apparent ancestors were established on land as much as 375 MYA. Rainforests and swamps of lycopsid and fern trees growing in the Eastern United States and Europe over 300 MYA formed the coal currently being mined. Today, ferns flourish in a wide range of habitats throughout the world; however, about 75% of the species occur in the tropics.
The conspicuous sporophytes may be less than a centimeter in diameter (as in small aquatic ferns such as Azolla), or more than 24 m tall, with leaves up to 5 m or longer in the tree ferns. The sporophytes and the much smaller gametophytes, which rarely reach 6 mm in diameter, are both photosynthetic.
The fern lifecycle differs from that of a moss primarily in the much greater development, independence, and dominance of the fern’s sporophyte. The fern sporophyte is structurally more complex than the moss sporophyte, having vascular tissue and well-differentiated roots, stems, and leaves. The gametophyte, however, lacks the vascular tissue found in the sporophyte.
Fern sporophytes, like horsetails, have rhizomes. Leaves, referred to as fronds, usually develop at the tip of the rhizome as tightly rolled-up coils (“fiddleheads”) that unroll and expand. Fiddleheads are considered a delicacy in several cuisines, but some species contain secondary compounds linked to stomach cancer.
Many fronds are highly dissected and feathery, making the ferns that produce them prized as ornamental garden plants. Some ferns, such as Marsilea, have fronds that resemble a four-leaf clover, but Marsilea fronds still begin as coiled fiddleheads. Other ferns produce a mixture of photosynthetic fronds and nonphotosynthetic reproductive fronds that tend to be brownish in color.
Ferns produce distinctive sporangia, usually in clusters called sori (singular, sorus), typically on the underside of the fronds. Sori are often protected during their development by a transparent, umbrella-like covering. (At first glance, one might mistake the sori for an infection on the plant.) Diploid spore mother cells in each sporangium undergo meiosis, producing haploid spores.
At maturity, the spores are catapulted form the sporangium by a snapping action, and those that land in suitable damp locations may germinate, producing gametophytes that are often heart-shaped, are only one cell layer thick (except in the center), and have rhizoids that anchor them to their substrate. These rhizoids are not true roots because they lack vascular tissue, but they do aid in transporting water and nutrients from the soil. Flask-shaped archegonia and globular antheridia are produced on either the same or a different gametophyte.
The multicellular archegonia provide some protection for the developing embryo.
The sperm formed in the antheridia have flagella, with which they swim toward the archegonia when water is present, often in response to a chemical signal secreted by the archegonia. One sperm unites with the single egg toward the base of an archegonium, forming a zygote. The zygote then develops into a new sporophyte, completing the lifecycle.
The developing fern embryo has substantially more protection from the environment than a charophyte zygote, but it cannot enter a dormant phase to survive a harsh winter the way a seed plant embryo can. Although extant ferns do not produce seeds, seed fern fossils have been found that date back 365 million years. The seed ferns are not actually pterophytes, but gymnosperms.
Ferns and their relatives have a large and conspicuous sporophyte with vascular tissue. Many have well-differentiated roots, stems, and leaves (fronds). The gametophyte generation is small and lacks vascular tissue.