The Evolution Of Early Land Plants
The first organisms to conquer the land were the green algal ancestors of plants. After being water bound for 500 million years, photosynthetic algae slowly crept onto near-shore rocky surfaces, where they began to adapt anatomical strategies for improving their chances of living outside the ocean. The result was myriad evolutionary changes that led to the domination of plants in all known land environments. Today, there are about 300,000 species of land plants found in terrestrial habitats.
The early evolution of plants is one of the most important in the history of life. From humble, single-celled, water-bound beginnings, plants came ashore and transformed dry land in ways that would dramatically influence the evolution of all other organisms and the ecology of the planet.
Major Plant Groups
Plants are multicellular, photosynthesizing members of the Eu-karya—the domain of organisms with a eukaryote cell type that also includes animals and fungi. Most plants are green, although a select few genera have lost their green pigment during the course of their evolution.
During photosynthesis, the green pigment chlorophyll that is found in plant cells uses energy from sunlight to transform carbon dioxide and water into organic compounds, including free oxygen. Land plants are most responsible for altering the face of the Earth into a habitable environment for other organisms. Through the process of converting solar energy, carbon dioxide, and water into other organic compounds, plants are in actuality the key source of stored energy used by humans in such forms as coal and oil.
Most plants are land bound, although some spend a portion of their life in the water. Blue-green algae (cyanobacteria) as well as familiar seaweeds—red and brown algae—are classified as protists rather than plants. This classification is based primarily on the
Plant Cell
Ribosomes-
Rough endoplasmic reticulum -
Nucleus-
Golgi complex
Ribosomes-
Rough endoplasmic reticulum -
Nucleus-
Golgi complex
Lysosome
Vacuole
Ribosomes - Mitochondrion
Plasma membrane
Lysosome
Smooth endoplasmic '■v reticulum
Vacuole
Ribosomes - Mitochondrion
Microfibrils
Plasma membrane
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The eukaryote cell of a plant, one function of which is photosynthesis.
composition of these organisms' genetic relationships. Green algae are genetically linked to the earliest land plants and can arguably be classified as either plants or protists.
Land plants exist in many varieties but are united by several common characteristics. The most fundamental defining trait of plants is the way they handle water and nutrients to nourish themselves. Vascular plants have conducting systems inside their
- Nonvascular plants include mosses and hornworts.
stems and leaves to transport water and food. Nonvascular plants lack vascular tissue and a conducting system; instead, they absorb water and nutrients through a sparse layer of specialized absorbent cells.
These two large categories of land plants can be divided further into four major groups:
• Nonvascular plants, including mosses, liverworts, and hornworts. These plants are found in damp, shady places and cannot survive for long in areas that dry out for long periods. The water-absorbing cells of these plants can be highly efficient. Sphagnum peat moss can absorb up to 25 times its weight in water. There are about 16,600 living species of nonvascular plants. Nonvascular plants were the first land plants and rose during the first half of the Paleozoic Era. The earliest widespread land plants were probably of this variety.
• Seedless vascular plants, including club mosses, ferns, whisk ferns, and horsetails. There are about 12,400 living species of seedless vascular plants. Seedless vascular plants rose to dominate the second half of the Paleozoic Era.
- Seedless vascular plants include ferns and horsetails.
• Gymnosperms. These are seed plants with a protective cone or other body for their seed embryos. Gymnosperms do not produce fruits or flowers. Conifers (evergreen trees), seed ferns, and cycads are found in this group. There are about 600 known living species of gymnosperms. Gymnosperms first appeared in the latter Paleozoic Era but became dominant during the Mesozoic Era.
• Angiosperms, the flowering plants. The angiosperms were the last of the great plant lineages to evolve; but today, they dominate the Earth, with more than 250,000 known living species. These plants utilize flowers to attract pollinators and also encase their seeds in fruits that, when separated from the plant, can aid in the dispersal of seeds. Angiosperms rose during the Late Mesozoic and became the dominant form of land plant during the Cenozoic Era.
Plant Adaptations for Living on Land
Plants were pioneers—the first organisms to colonize dry land.
Adapting for life on land required several key modifications.
- Typical Gymnosperm Cones
Pine tree
Gymnosperms are seed plants with a protected cone or other body for their seed embryos, such as conifers (evergreen trees), seed ferns, and cycads.
Pine tree
Gymnosperms are seed plants with a protected cone or other body for their seed embryos, such as conifers (evergreen trees), seed ferns, and cycads.
Plants originated in the nourishing environment of the water. Life on land required plants to develop a way to reduce water loss and the drying effects of desiccation. This protection came in the form of a waxy outer covering called a cuticle. The cuticle is a thin, impermeable covering that grows on the outside surface of the exposed parts of a plant. In addition to slowing water loss, the cuticle may sometimes protect a plant from the harmful effects of ultraviolet solar radiation—a danger that was more acute for the first land organisms than it is for today's because in the Early Paleozoic Era, Earth's atmosphere was still developing its protective ozone shield.
Typical Angiosperm
Plant body
Vertical section through flower Stamen
Angiosperms, the flowering plants, utilize flowers to attract pollinators, and some encase their seeds in fruits to aid in their dispersal.
Plants need to breathe; this, too, posed a challenge for the first inhabitants of the land. Now that they no longer were immersed in water, plants on land needed to develop a new physiological technique: a way to grab carbon dioxide molecules from the air. Plants evolved a network of tiny pores on their outer surfaces for this purpose. Called stomata, these pores enable an exchange of gases between the plant and the outside air, making photosynthesis possible.
Plants living in the water are held up or suspended by the buoyancy of the marine environment. On land, larger plants must lift themselves from the ground so that they do not collapse under their own weight. This is accomplished by a skeletonlike structure of stems, branches, and trunks that gives strength and shape to land plants. Early land plants evolved such structures and, as a result, expanded their habitable environment in a vertical direction. This dramatically—and literally—increased the range of terrestrial plants over and above the flat surface of the ground, making possible taller plants including trees.
The vascular systems of land plants were another key evolutionary innovation that enabled such plants to thrive. These systems improved the plants' ability to conduct water and nourishing minerals to different parts of their structures. Roots evolved as a specialized means to absorb water. These increasingly effective methods of providing food and energy led to the growth and diversity of all kinds of plants.
One final challenge for plants living on land was to find a means to reproduce effectively. In the marine environment, plants passed sperm to egg through the medium of water. Plants in a terrestrial habitat evolved many different solutions to the challenge of achieving the union of sperm and egg. Most of these solutions depend on reproductive cells called spores. Spores can be blown through the air, transported by available surface water, and transported by pollinating insects to make plant reproduction possible.
Many of these plant adaptations were mirrored by the evolution of invertebrate and vertebrate animals for life on the land. Plants
Photosynthesis
Water + light = chemical energy
Light energy
- Sugar leaves leaf
Chemical energy + carbon dioxide = sugar
Chemical energy + carbon dioxide = sugar
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Plants nourish themselves through photosynthesis. Using a network of tiny pores, or stomata, on their outer surface enables plants to exchange gases with the outside air, allowing them to breathe.
share with some animal groups the internalization of vital body systems such as sexual organs, the development of a protective outer skin, and functions such as gas exchange.
The Origin and Evolution of Land Plants
Photosynthesizing algae first arose in the oceans. As improbable as it may seem, a single species of green algae, living in the water, was likely responsible for giving rise to every form of land plant that followed. This was a monumental step in the evolution of life on Earth, yet it was also a kind of accident—an accident repeated over and over until life stuck to dry land once and for all. It began when near-coastal colonies of algae mats—stromatolites—became exposed to the air when the tides waned. The slow adaptation of green algae to dry land was pushed further along by the action of waves that left bits of algae stranded on exposed dry surfaces on the shore. Over time, through the process of natural selection, some of these algae became hearty enough to exist and reproduce outside of the water as a biotic smear on an exposed rock surface.
Lichens were one of the earliest land organisms to prosper. Not classified as plants, lichens are a form of fungi that live in symbiosis with photosynthetic algae or cyanobacteria. Even today, lichens can be found in some of Earth's harshest environments. Lichens were probably capable of surviving the extreme conditions of the Early Paleozoic Earth, where harsh changes in temperature and long periods of drought were common. Fossil evidence shows that lichens were probably widespread by the Early Devonian Epoch, and some scientists place the origin of lichens to the very beginning of the Cambrian Period.
By the Late Cambrian Period, oxygen had probably risen to levels adequate to sustain a viable ozone layer in the atmosphere—an essential shield to protect organisms directly exposed to lethal ultraviolet radiation from the Sun. The next requirement for sustaining life on land was the development of a ground-covering soil. Although fossil evidence of microbiotic crusts or mats dating from the Precambrian suggests that near-shore soils were beginning to take shape even in the Early Cambrian, the appearance of true soils underlying these crustal mats was slow in coming. It took 100 million years or more for the thin bacterial and fungal carpets of the Early Cambrian to thicken into soil covers capable of supporting the first traces of plants and small animals such as arthropods and ancestral worms. Spore traces of early plants show that by the Late Ordovician Epoch, the terrestrial ecosystem had become fixed and stable enough to support the relatively sudden and explosive expansion of land plants during the Silurian and Devonian Periods.
Early Nonvascular Plants
Fossil evidence of ground cover other than lichens is found in deposits from the Middle Ordovician Epoch and consists of isolated spores that resemble those of modern liverworts. These spores have what is called a tetrad design and consist of a four-part membrane with a decay-resistant wall for housing the spores. One might say that these liverwort-type spores came in four-packs. Liverworts are good candidates for one of the earliest land plants because they can been seen today living symbiotically with algae, sometimes on microbiotic crusts that are thought to be similar to the earliest kinds of soils.
Mosses and hornworts make up two other families of likely early nonvascular plants. Like liverworts, mosses and hornworts do not have roots, and most varieties reproduced by dispersing spores. Mosses can resemble liverworts but sometimes have twisting branches with leaflike structures, though these are all nonvas-cular features. Mosses commonly hug the surface to which they are attached and can be found thriving in moist environments.
Hornworts also grow in damp, humid places. They have large, horn-shaped leaves clustered around the flattened body of the plant. Like mosses, some varieties of living hornworts are commonly found growing on the bark of trees.
Seedless Vascular Plants
Fossil spores from the Early Silurian Period to the Middle Devonian Epoch show a decline in the diversity of tetrad-type spores in favor of simpler, single plant spores that were dispersed individually. Although evidence associating these single spores with specific kinds of plants is rare in the fossil record from that time, spores such as this are used by seedless vascular plants and by some extant species of nonvascular plants such as mosses and hornworts. The protective outer walls of these single spores were improved over the protective sheathes of the tetrad spores of nonvascular plants; this improvement made the single spores more likely to spread and prosper than their predecessors. This is exactly what happened. As seedless nonvascular plants arose, they swarmed into niches once occupied by nonvascular plants. This effectively restricted the liverworts, mosses, and hornworts to the habitats they now hold.
Thus began an extraordinary second phase in the evolution of land plants. Between 480 million and 360 million years ago (Middle Ordovician Epoch to the Early Carboniferous Period), land plants made a transition from their humble beginnings as mere rock-covering smears and low-lying ground cover. They developed a variety of new anatomical structures, reproductive schemes for the survival of their species, and adaptations for many varied habitats. It was a green revolution on land that transformed rocky, barren continents into life-supporting terrestrial habitats.
The first definitive fossils of land plants come from Middle Silurian deposits of Northern Europe and Late Silurian deposits of Australia, Bolivia, and northwestern China. These fossils include specimens of seedless vascular plants, such as Rhynia and Cook-sonia, with internal channels for conducting water and nutrients. Club mosses such as Baragwanathia also are found from this time, as are several plants such as Salopella whose connection to modern families of plants is not yet understood.
The earliest vascular land plants were morphologically simple and small. None had roots; they existed as creeping plants that spread across the ground. Cooksonia—found in Middle Silurian deposits of central Asia, Europe, eastern North America, and Brazil—consisted of upright stalks only about one inch (2.5 cm) tall. Steganotheca (Late Silurian, Great Britain) was a little more bushlike and stood two inches (5 cm) tall.
Going into the Devonian Period, there was a sharp difference in plant biology. The short, barely rooted, and flimsy experiments in vascularity of the Silurian gave way to stronger, more robust
- Ferns are seedless, vascular, spore-bearing ground cover. They are noted for their creeping stems, photosynthetic featherlike leaves or fronds, and fibrous roots similar to those of seed plants.
vascular plants. The stalks of these plants were woodier; this allowed them to grow taller and distribute their spores more widely. By the Middle Devonian Epoch, the rise of vascular plants was leading to great diversity in plant design and size. Psilophyton was an early vascular plant reinforced by robust bundles of conducting channels. Some specimens measure more than three feet (1 m) tall. Many times taller than Cooksonia and Steganotheca, Psilophyton was a stepping-stone to the largest and most successful seedless vascular plants—the lycopods, ferns, and sphenopsids (horsetails).
One of the most spectacular groups of early seedless vascular plants were the lycopods. The lycopods were the first major group of vascular plants; they arose during the Silurian Period and became widespread on a worldwide basis by the Carboniferous. The evolutionary origins of the lycopods are unclear, but they may be related to the earlier Cooksonia. Lycopods developed strong root structures and hard stalks to resist desiccation in hot, tropical climates. The largest members of this group grew to be an astounding 100 feet (30 m) tall and formed huge forests. Unlike in modern trees, the leaves of lycopods were attached directly to the stalk of the main trunk. As a lycopod grew taller, it shed its leaves, leaving only a cluster of leaves at the top. This gave the tree a distinctive, umbrellalike appearance. As the leaves were shed, they left a diamond-shaped pattern of scars on the trunk, a visually distinctive feature for which these fossils are known. The spores of lycopods were borne on fertile leaves. Modern remnants of lycopods are not nearly as spectacular as their ancestors; they include the branching club mosses and "ground pine."
Ferns arose during the latter part of the Devonian as seedless, spore-bearing ground cover. By the Carboniferous Period, many fern varieties had become widespread, and some species towered over the coal forest as large trees. There currently are more than 10,000 species of true ferns that trace their lineage back to the Paleozoic; this makes ferns the most prevalent type of modern seedless vascular plant. Ferns are noted for their creeping stems; their photosynthetic featherlike leaves or fronds; and their fibrous roots, similar to those of seed plants. Ferns were one of the first groups of plants with significant root structures. These structures further strengthened the plants' physical stature, improved their intake of nutrients from the soil, and encouraged ferns to grow larger and taller. Fern spores are found as small dots on the underside of the leaves.
Sphenopsids were seedless vascular plants that evolved during the Devonian Period, at about the same time as ferns. The name sphenopsid means "joint-stemmed"; the plants owe their name to the structure of their long, hollow, jointed stems. The only modern-day representatives of the sphenopsids are 15 species of the horsetail.
- This image shows Lepidodendron, an extinct lycopod (or giant club moss), which is a spectacular early seedless vascular plant.
The living sphenopsids include perennial species that are most at home when rooted in sandy soil along a riverbank or stream. Leaves of modern varieties may be broomlike or may resemble a horse's tail, with the thin, needlelike leaves attached at the jointed points in the hollow stalk. The spores of sphenopsids cluster with the leaves. One variety of living sphenopsid—the scouring rush-has a fibrous texture that made it a popular tool in the early American kitchen for scouring pots and pans. The heyday of the sphenopsids spanned the Carboniferous and Permian Periods, when some varieties towered as tall as 100 feet (30 m). The extinct genus Calamites had a bamboolike stalk with vertical ribbing and needlelike leaves. Calamites grew to more than 31 feet (9.5 m) tall, and its fossils are often found in deposits of Carboniferous Period coal-bearing formations.
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THINK ABOUT IT
The Early Evolution of Leaves
Leaves are the primary factories of photosynthesis in vascular plants. Photosynthesis occurs when light energy from the Sun, carbon dioxide in the air, and water are chemically combined to create food energy for the plant inside the leaves. As a byproduct of this process, leaves release oxygen into the air and enable plants to maintain an atmospheric balance of carbon dioxide and oxygen that sustains the life of invertebrate and vertebrate animals.
The size, shape, and arrangement of leaves are important to their function and affect the amount of sunlight that the plant can absorb to fuel photosynthesis. The earliest vascular plants did not have leaves as are commonly seen today. Several environmental factors, including changes in the makeup of Earth's atmosphere, influenced the evolution of the first leaves.
The first vascular plants that colonized the land during the Middle Silurian Period were leafless. They absorbed sunlight and carried out photosynthesis through their stems. Early vascular plants such as the extinct Cooksonia and other related rhyniophids consisted only of branching stems that grew low to the ground. Fossil evidence of the first leaves does not appear in the fossil record until about 40 million years later, near the end of the Devonian Period.
The appearance of plants with leaves was a monumental event in the natural history of the Earth. Plants with leaves form the foundation of a worldwide ecosystem that to this day is vital to the survival of all other animals. Why it took so long for leaves to evolve from the first vascular plants has been a source of puzzlement to plant paleontologists. After all, leaves greatly improve the ability of plants to photosynthesize, furthering their survival and distribution.
In 2001, a team of British paleobotanists that included David J. Beerling, Colin P. Osborne, and William G. Chaloner tackled the mystery of the
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slow emergence of the first leaves. The team noted a correlation between a tenfold drop in the concentration of carbon dioxide in the atmosphere, the rise of atmospheric oxygen, and the appearance and diversification of plant leaves. This change in the composition of the atmosphere occurred between 410 million and 370 million years ago, during the Devonian Period, and corresponds well with the appearance and design of leaves. For proof of the concept, the scientific team turned to the fossil record, where they focused on finding fossil evidence. What they found dramatically illustrated their point. As the world became more habitable for oxygen-breathing organisms, so, too, did the size, shape, and effectiveness of leaf blades improve. From the simple branching stems of Rhynia, early leaf structures emerged in the form of stem branches in Psi-lophyton; in the form of side branches in Actinoxylon, to further improve the number of photosynthesizing elements; and in the development of fuller leaves with infilling of spaces between the branching stems, as in Archaeopteris.
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The boom in seedless vascular plants during the Devonian led to the spectacular spread of vast tropical swamps and forests. Spore-bearing ferns and seed ferns formed a dense layer of mid-height vegetation under a canopy of lycopsids, sphenopsids, and the towering progymnosperm, Archaeoptreris. Growing big and tall was the rule of the day as branching plants of all types competed for exposure to sunlight.
The seedless vascular plants dominated the Paleozoic terrestrial ecosystem; however, as the world of the Permian Period began to cool and dry out, land plants began another important transition in their evolution. In the shadow of the seedless vascular plants were the first seeded plants, or gymnosperms, that would come to dominate the next important phase of plant evolution.
Early Gymnosperms
Gymnosperms—the "naked seed" plants—were the focus of the next great evolutionary triumph for land vegetation. Seed plants did not dominate terrestrial environments until the Mesozoic Era, but their appearance during the Late Paleozoic in the form of seed ferns, towering trees such as Cordaites, and shrublike plants such as Glossopteris was an auspicious marker between the old tropical world of the Paleozoic and the drier, more temperate habitats of the Mesozoic.
Seedless plants thrived in the swamps and forests of the Middle Paleozoic because they required moisture to reproduce. Liquid water was needed for the sperm and egg components of these spore-bearing plants to join and fertilize. Early seedless plants were at first restricted to ground-hugging forms that lived near bodies of water. As the world became hotter and wetter, and vascular plants grew taller, they extended their range to the humid interiors of expansive tropical forests. In the Devonian and Carboniferous Periods, the water that was needed to spread and fertilize spores came in the form of rain and humidity that frequently drenched the leaves of plants living in the densely vegetative habitats.
The early gymnosperms adapted to cooler, drier, and more elevated environments by developing a way to protect their seeds from desiccation. Spore-bearing plants before them produced tiny plants called gametophytes that contained both eggs and sperm. Water was needed to join these reproductive cells together so that fertilization could take place. The tiny, unprotected gametophytes were highly susceptible to drying out and would not survive outside of the moist environment of the swamp. Gametophytes also were not very mobile; new plants grew close to the parent plant. This hindered the ability of a plant to distribute its offspring over a wider geographic area.
Gymnosperms found a way to protect their reproductive cells from desiccation. Each plant produced male and female cones.
The male cones produced pollen grains, and the female cones produced the beginnings of a true seed. The wind carried the male pollen to the seed-bearing female cone, and in that cone the pollen fertilized the egg. The fertilized egg then grew within the protective shell of the seed covering, where it fed on nutrients found within the seed itself. Not only did this advance protect the seed from drying out, it also provided a mobile seed that could be spread by the wind, further extending the range of such plants.
Seed plants first appeared in the Carboniferous Period and were probably related to a group of seedless vascular plants known as progymnosperms. Archaeopteris is one such ancestor whose fossils are found in Late Devonian forests around the world. Carboniferous Period Archaeopteris reproduced by spores like a seedless fern, but it had a woody bark—a protective covering like that found in a typical conifer from the Mesozoic. As such, Archaeopteris represents an intermediate stage in the adaptation of plants from the moist, humid habitats of the Middle Paleozoic to the drier, cooler environments leading into the Mesozoic.
Significant climate shifts during the Permian Period dried the once fertile coal swamps and encouraged the spread of hearty gymnosperms. With the gymnosperms came a great expansion of terrestrial habitats into higher elevations and floodplains. Although the most diverse period of gymnosperm evolution was still ahead, several varieties of these seed-bearing plants became plentiful during the Late Paleozoic Era.
Seed ferns were the earliest gymnosperms. Their leaves resembled seedless ferns, but these plants reproduced by way of seeds. Elkinsia, from the Late Devonian Epoch, is the earliest of these seed ferns. The first seed ferns did not have cones but produced seeds along their branches in protective, cuplike capsules.
Cordaites, which are now extinct, were some of the tallest common gymnosperms of the Late Paleozoic. Growing up to 100 feet (30 m), Cordaites had tall, branchless trunks with a tuft of long, strap-shaped leaves bunched at the top. Seed-bearing cones were clustered among the leaves. Found primarily in North America and Europe, the Cordaites disappeared at the end of the Permian Period.
Glossopteris is another notable gymnosperm from the end of the Paleozoic. Found in terrestrial environments from the southern
Paleozoic supercontinent of Gondwana—India, Australia, Africa, South America, and Antarctica—Glossopteris first appeared in the Permian Period and was extinct by the Late Triassic Epoch. The broad, flat, tongue-shaped leaves of this tree ranged from four inches (10 cm) to more than three feet (0.9 m) long and may have been shed on a seasonal basis. Male and female reproductive cells appear to have been produced on different leaves, but the way they were contained—e.g., on a pollen stalk or in a sack—has not yet been determined with certainty by examining their fossils. Although many fossil leaves of Glossopteris have been discovered, the same cannot be said for the trunk or body of this genus. It may have been a large shrub or small tree similar to a cycad and measuring 13 to 20 feet (4-6 m) tall.
Following closely on the migration of plants to dry land were the invertebrate organisms that provided the ancestral stock of today's land arthropods. These included insects, spiderlike creatures, mites, ancestral scorpions, worms, millipedes, centipedes, and their kin.
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