Chapter 28 Green Algae and Land Plants Notes

Why Biologists Study Green Algae and Plants

• Green algae and land plants are studied to understand the evolution and significance of photosynthetic organisms.

Introduction

• The Viridiplantae, or green plants, consist of green algae and land plants.
• Green algae are important photosynthetic organisms in freshwater habitats.
• Land plants are the key photosynthesizers in terrestrial environments.
• Green algae are studied alongside land plants because:
  • They are the closest living relatives to land plants.
  • The transition from aquatic to terrestrial life occurred when land plants evolved from green algae.
• Land plants were the first organisms to thrive with their tissues completely exposed to the air.
• Before land plants, terrestrial life was limited to bacteria, archaea, and single-celled protists.
• Plants transformed the nature of life on Earth.

Plants Provide Ecosystem Services

• An ecosystem consists of sunlight, soil, and nutrients.
• Ecosystem services provided by green algae and land plants include:
  • Producing oxygen via oxygenic photosynthesis.
  • Building soil by providing food for decomposers.
  • Holding soil and preventing nutrient loss from wind or water erosion.
  • Holding water in the soil.
  • Moderating the local climate by providing shade and reducing the impact of wind on landscapes.

Plants Are Primary Producers

• Land plants are the dominant primary producers in terrestrial ecosystems.
• They convert energy in sunlight into chemical energy.
• The sugars produced by land plants support virtually all other organisms in terrestrial habitats.
• Land plants are key to the carbon cycle.
  • They take $CO_2$ from the atmosphere and reduce it to make sugars.
  • They fix much more $CO_2$ than they release.
  • The loss of plant-rich prairies has contributed to increased concentrations of $CO_2$ in the atmosphere, contributing to rising temperatures associated with global climate change.

Plants Provide Humans with Food, Fuel, Fiber, Building Materials, and Medicines

• Food:
  • Agricultural research began with the initial domestication of crop plants.
  • Artificial selection has led to dramatic changes in plant characteristics.
• Fiber and Building Materials:
  • Plants provide raw materials for clothing, rope, and household articles.
  • Woody plants provide lumber for houses and furniture, and fibers for paper.
• Medicines:
  • About 25% of prescriptions in the U.S. include at least one molecule derived from plants.
  • Most of these compounds are synthesized by plants to repel herbivores.

How Do Biologists Study Green Algae and Land Plants?

• To understand diversification:
  • Compare morphological traits.
  • Analyze the fossil record.
  • Estimate phylogenetic trees.

Analyzing Morphological Traits

• Green algae:
  • Can be unicellular, colonial, or multicellular.
  • Live in marine, freshwater, or moist terrestrial habitats.
  • Are mostly aquatic.
• Land plants are mostly terrestrial.

Similarities Between Green Algae and Land Plants

• Chloroplast structure is the same.
• Thylakoid arrangements are similar.
• Cell walls, sperm, and peroxisomes are similar in structure and composition.
• Chloroplasts synthesize starch as a storage product.
• Three groups of green algae are most similar to land plants:
  • Zygnematophyceae (conjugating algae).
  • Coleochaetophyceae (coleochaetes).
  • Charophyceae (stoneworts).

Major Morphological Differences Among Land Plants

• Nonvascular plants:
  • Lack vascular tissue.
  • Include mosses.
  • Use spores, not seeds, for reproduction and dispersal.
• Seedless vascular plants:
  • Have well-developed vascular tissue.
  • Do not make seeds; use spores for reproduction.
  • Include ferns.
• Seed plants:
  • Have vascular tissue.
  • Make seeds, which consist of an embryo and a store of nutritive tissue surrounded by a protective layer.
  • Include angiosperms (flowering plants) and gymnosperms.

Using the Fossil Record

• There have been five major events in the diversification of land plants:
  • First evidence of cuticle, spores, and sporangia of land plants.
  • Most major morphological innovations: stomata, vascular tissue, roots, and leaves.
  • Extensive coal-forming swamps.
  • Both wet and dry environments blanketed with green plants for the first time.
  • Diversification of flowering plants.

Origin of Land Plants

• Most of the earliest plant fossils are microscopic.
  • Some are of thin sheets of waxy material like cuticle which is a watertight barrier that coats aboveground parts of today’s land plants and helps them resist drying.
  • Fossilized spores are surrounded by material almost identical in structure to sporopollenin which is a waxy substance that encases the spores and pollen of modern land plants and helps them resist drying.
  • Fossilized spores are found in association with spore-producing structures (sporangia) that are similar to sporangia in some modern nonvascular plants.

Silurian-Devonian Explosion

• In rocks dated 416 to 359 mya, fossils from most of the major plant lineages are found.
• Adaptations that allow plants to occupy dry, terrestrial habitats are present, including:
  • Water-conducting vascular tissue.
  • Roots.
• Plants colonized land in conjunction with symbiotic fungi.

The Carboniferous Period

• Extensive deposits of coal were found in sediments dated from about 359 to 299 mya.
• Carbon-rich rock packed with fossil spores, branches, leaves, and tree trunks.
• Most of the fossils are derived from seedless vascular plants.

Diversification of Gymnosperms

• Gymnosperms are prominent in the fossil record from 299 mya to 145 mya.
• Major groups of gymnosperms living today include:
  • Ginkgoes.
  • Cycads.
  • Pines, spruces, and firs.
• They grow readily in dry habitats.
• Both wet and dry environments probably became blanketed with green plants for the first time during this interval.

Diversification of Angiosperms

• Angiosperms appear about 150 mya.
• Plants that produced the first flowers are the ancestors of today’s grasses, orchids, daisies, oaks, maples, and roses.
• They produce pollen grains that are transported via wind or insects and that carry the cells that will later develop into sperm.

Evaluating Molecular Phylogenies

• The phylogenetic tree of green plants shows that:
  • The green plants are monophyletic.
  • Zygnematophyceae is the closest living relative to land plants.
• Land plants are monophyletic.
• The nonvascular plants are the earliest-branching groups among land plants.
• The nonvascular plants are paraphyletic.
• The seedless vascular plants are paraphyletic, but the vascular plants as a whole are monophyletic.
• The seed plants—the gymnosperms plus angiosperms—are monophyletic.
• The gymnosperms and angiosperms are monophyletic groups, as are the angiosperms.

Themes in the Diversification of Land Plants

• The evolution of land plants required adaptations that allowed photosynthetic organisms to move from aquatic to terrestrial environments.

How Did Plants Adapt to Dry Conditions with Intense Sunlight?

• Natural selection favored early land plants with three main adaptations that solved the drying problem:
  • Preventing water loss, which kept cells from drying out and dying.
  • Providing protection from harmful ultraviolet (UV) radiation.
  • Moving water from tissues with direct access to water to tissues without direct access.

Providing Protection from UV Irradiation

• Plants out of water are exposed to harmful UV rays of the sun.
• UV light damages DNA by causing thymine dimers.
• Water absorbs UV light, so algae did not face this problem to the same extent.
• Most plants today accumulate UV-absorbing compounds (flavonoids) that protect DNA from damage.

The Importance of Upright Growth

• First land plants were small or had a low, sprawling growth habit.
• They had to grow in a way that kept many or most of their tissues in direct contact with moist soil.
• In a terrestrial environment, individuals that can grow upright have much better access to sunlight than individuals that cannot.

The Origin of Vascular Tissue

• Fossils from the Rhynie Chert formation in Scotland include early land plants that grew upright.
• They contained elongated cells that were organized into tissues along the length of the plant.
  • Simple, cellulose-containing cell walls like water-conducting cells found in today’s mosses.
  • Cell walls with thickened rings containing lignin.
  • Lignin is an extraordinarily strong polymer.

How Do Plants Reproduce in Dry Conditions?

• Three innovations were instrumental for efficient plant reproduction in a dry environment:
  • Spores that resist drying because they are encased in a tough coat of sporopollenin.
  • Gametes that were produced in complex, multicellular structures.
  • Embryos that were retained on and nourished by the parent plant.

Alternation of Generations

• All land plants undergo alternation of generations, in which individuals have:
  • A multicellular haploid phase called the gametophyte.
  • A multicellular diploid phase known as the sporophyte.
• The two phases of the life cycle are connected by distinct types of reproductive cells—gametes and spores.

From Gametophyte-Dominant to Sporophyte-Dominant

• In nonvascular plants, the sporophyte is small and short-lived and is largely dependent on the gametophyte for nutrition (gametophyte-dominant life cycle).
• In ferns and other vascular plants, the sporophyte is much larger and longer-lived than the gametophyte (sporophyte-dominant life cycle).
• Gametophytes of gymnosperms and angiosperms are microscopic.

Pollen

• Pollen grains in land plants allowed plants living in dry habitats to reproduce efficiently.
• They are tiny male gametophytes surrounded by a tough coat of sporopollenin.

Seeds

• A seed is a structure that includes an embryo and a store of nutrients provided by the mother, surrounded by a tough protective coat.
• Spores are an effective dispersal stage for nonvascular plants and seedless vascular plants, but they lack the stored nutrients found in seeds.

Pollination by Insects and Other Animals

• After they evolved, stamens and carpels later became enclosed by modified leaves called sepals and petals.
• Flowers may be adaptations to increase the probability that an animal will perform pollination, which is the transfer of pollen from one individual’s stamen to another individual’s carpel.
• Biologists proposed the directed-pollination hypothesis:
  • Natural selection favored structures that reward an animal for carrying pollen directly from one flower to another.
  • Flowers vary in size, structure, scent, and color in order to attract specific pollinators.
• Flowers attract pollinators by providing them with food—either protein-rich pollen or nectar, a sugar-rich fluid.
• The relationship between flowering plants and their pollinators is mutually beneficial.
  • The pollinator gets food, and the plant gets fertilized.
• The spectacular diversity of angiosperms resulted from coevolution with animal pollinators.
  • Evolutionary changes in angiosperms and corresponding changes in their pollinators were highly dependent on each other.
  • Many animals and the flowering plants they pollinate depend on each other for their survival.

Fruits

• A fruit is a structure that is derived from the ovary and encloses one or more seeds.

Key Lineages of Green Algae and Land Plants

• The green algae are a paraphyletic group that totals about 8000 species.
• Green algae are important primary producers in:
  • All types of freshwater habitats.
  • Unusual environments such as snowfields and ice floes.
• Green algae live in close association with an array of other organisms:
  • Unicellular green algae are common endosymbionts in planktonic protists that live in lakes and ponds.
  • Lichens are stable associations between green algae or cyanobacteria and fungi, often found in terrestrial environments that lack soil; approximately 85% of the 17,000 species of lichens involve green algae.

Key Lineages of Green Algae

• Ulvophyceae (Ulvophytes):
  • Known Species: 4000
  • Habitat: Marine, Freshwater
  • Structure: Unicellular/multicellular
  • Reproduction: Sexual and asexual; spores and gametes are motile; male and female gametes look identical; external fertilization.
  • Life Cycle: Zygotes are the only diploid stage in unicellular species; alternation of generations only in multicellular species; gametophytes and sporophytes may look identical or different.
  • Relevance: Important primary producers in freshwater environments and in coastal areas of oceans.
• Charophyceae (Stoneworts):
  • Known Species: 6000
  • Habitat: Freshwater (lakes)
  • Structure: Multicellular (some species over a meter in length)
  • Reproduction: Sexual and asexual; spores and sperm are motile; eggs are retained on the parent and nourished after fertilization.
  • Life Cycle: Zygotes are the only diploid stage (no alternation of generations).
  • Relevance: Form extensive beds on lake bottoms or ponds and provide food for waterfowl and shelter for fish; heavily studied given their close relationship to land plants.
• Coleochaetophyceae (Coleochaetes):
  • Known Species: 19
  • Habitat: Freshwater
  • Structure: Multicellular
  • Reproduction: Sexual and asexual; spores and sperm are motile; eggs are retained on the parent and nourished after fertilization.
  • Life Cycle: Zygotes are the only diploid stage (no alternation of generations).
  • Relevance: Heavily studied given their close relationship to land plants.
• Zygnematophyceae (Conjugating algae):
  • Known Species: 2700
  • Habitat: Freshwater
  • Structure: Unicellular/multicellular (filamentous)
  • Reproduction: Sexual and asexual; conjugation occurs between cells of adjacent filaments; gametes are of equal size and pass from one cell to another through conjugation tubes.
  • Life Cycle: Zygotes are the only diploid stage (no alternation of generations).
  • Relevance: Heavily studied given recent evidence indicating their close relationship to land plants.

Nonvascular Plants

• Nonvascular plants:
  • First lineages to branch off phylogeny of land plants.
  • Gametophyte is dominant and longer-lived phase of life cycle.
  • Individuals anchored to soil, rocks, or tree bark by rhizoids.
  • Lack vascular tissue with lignin-reinforced cell walls.
  • Flagellated sperm that swim to eggs.
  • Spores dispersed by wind.

Key Lineages of Nonvascular Plants:

• Hepaticophyta (Liverworts):
  • Known Species: 8500
  • Features: Some have liver-shaped leaves; many have pores similar to stomata.
  • Reproduction: Sexual and asexual by dispersal of small fragments called gemmae; motile sperm; spores dispersed by wind or rain.
  • Life Cycle: Gametophyte dominant; sporophyte is small and depends on gametophyte for nutrition.
  • Relevance: Thought to resemble first land plants; their decaying tissues contribute to initial stages of soil formation.
• Bryophyta (Mosses):
  • Known Species: 12,000
  • Features: Some able to withstand extreme drying; some have rudimentary conducting tissues.
  • Reproduction: Sexual and asexual by clonal propagation of gametophyte; motile sperm; a sporophyte can produce 50 million spores; spores usually dispersed by wind.
  • Life Cycle: Gametophyte dominant; sporophyte is small and depends on gametophyte for nutrition.
  • Relevance: Partially decayed mosses form large patches of peat, which is used as fuel for cooking and heating; Sphagnum can hold up to 20 times its weight in water and is a common soil additive.
• Anthocerophyta (Hornworts):
  • Known Species: 19
  • Features: Sporophytes have stomata.
  • Reproduction: Sexual and asexual by fragmentation of gametophyte; motile sperm; gametophytes are either unisexual or bisexual; spores dispersed by wind or rain.
  • Life Cycle: Gametophyte dominant; sporophyte is small and obtains some nutrition from gametophyte, but also performs photosynthesis.
  • Relevance: Some have symbiotic cyanobacteria that fix nitrogen.

Seedless Vascular Plants

• Paraphyletic group
• Vascular tissue comprised of lignin-reinforced cells
• Sporophyte dominant, longer-lived phase of life cycle
• Gametophyte physically independent of sporophyte
• Eggs retained on gametophyte, and sperm swim to egg with flagella
• Sporophytes develop on gametophyte and are nourished by gametophyte when small

Key Lineages of Seedless Vascular Plants

• Lycophyta (Lycophytes):
  • Known Species: 1000
  • Features: Most ancient lineage with roots.
  • Reproduction: Sexual and asexual; in some species by dispersal of small fragments called gemmae; motile sperm; some species show heterospory.
  • Life Cycle: Sporophyte dominant; gametophyte of some lives with symbiotic fungi; gametophyte may live up to 15 years.
  • Relevance: Tree-sized lycophytes were abundant during the Carboniferous period; spores of some are flammable and were used as flash powder for early photography.
• Psilophyta (Whisk ferns)
  • Known Species: 6
  • Features: Body consists of only branching stems.
  • Reproduction: Sexual and asexual by extension of underground stems; spores dispersed by wind; bisexual gametophytes.
  • Life Cycle: Sporophyte dominant; gametophyte is only about 2 cm long; sporophyte may be up to 30 cm tall and grows directly on the gametophyte.
  • Relevance: Some derive nutrition from symbiotic fungi; used in landscaping, especially in Japan; often grows as an unwanted weed in greenhouses.
• Pteridophyta (Ferns):
  • Known Species: 12,000
  • Features: Common in moist, humid habitats; have large leaves called fronds.
  • Reproduction: In bisexual gametophytes, sperm and eggs mature at different times, minimizing self-fertilization; motile sperm.
  • Life Cycle: Sporophyte dominant; gametophyte is photosynthetic; sporangium pops open in dry conditions, releasing spores.
  • Relevance: Young fronds (fiddleheads) are a culinary delicacy; widely used as ornamental plants.
• Equisetophyta (Horsetails):
  • Known Species: 25
  • Features: Hollow stems allow oxygen to move down to roots that often grow in oxygen-poor soil.
  • Reproduction: Sexual and asexual; reproductive stems produce clusters of sporangia; spore dispersal in wind is facilitated by wing-like structures called elaters; sperm and eggs mature at different times, minimizing self-fertilization.
  • Life Cycle: Sporophyte dominant; gametophytes are small and short-lived.
  • Relevance: Stems are rich in abrasive silica granules and used to scour pots and pans.

Seed Plants: Gymnosperms and Angiosperms

• Seed plants:
  • Are a monophyletic group.
  • Include gymnosperms and the angiosperms.
  • Are defined by two key synapomorphies: production of seeds and production of pollen grains.
  • Angiosperms produce seeds in ovaries, gymnosperms do not.

Key Lineages of Seed Plants: Gymnosperms

• Ginkgophyta (Ginkgoes):
  • Known Species: 1
  • Features: Loses leaves in winter (is deciduous); leaves are virtually identical to 150-million-year-old fossils.
  • Life Cycle: Separate male and female plants; pollen carried by wind; motile sperm released into female gametophyte swim to egg.
  • Relevance: Widely planted as ornamentals; resistant to air pollution; leaf extracts used as dietary supplement.
• Cycadophyta (Cycads):
  • Known Species: 140
  • Features: Have been around for approximately 300 million years; resemble palms; unique among gymnosperms in having compound leaves.
  • Life Cycle: Separate male and female plants; pollen carried by wind; motile sperm released into female gametophyte swim to egg.
  • Relevance: Roots contain symbiotic cyanobacteria that fix nitrogen; popular landscaping plants.
• Cupressophyta (Redwoods, junipers, yews):
  • Known Species: 150
  • Features: Size ranges from small shrubs to giant redwood trees; small leaf surface area reduces water loss.
  • Life Cycle: Wind pollinated; separate male and female cones; seeds dispersed by wind, birds, or mammals.
  • Relevance: Cedar wood is commonly used for furniture, decks, or roofing; juniper "berries" are used to flavor gin; chemotherapy drug taxol was originally obtained from bark of yew trees.
• Pinophyta (Pines, spruces, firs):
  • Known Species: 240
  • Features: Needle-like leaves have small surface area; often found growing in dry areas.
  • Life Cycle: Wind pollinated; separate male and female cones; female cones may take two years to mature.
  • Relevance: Dominant plants at high latitudes and high elevations; seeds are source of food for mice, squirrels, and other animals; wood is used for building and paper industry.
• Gnetophyta (Gnetophytes):
  • Known Species: 70
  • Features: Grow as vines, trees, or shrubs; closely related to angiosperms; display several angiosperm features, including wood that contains vessel elements.
  • Life Cycle: Pollen is transferred by wind or insects; double fertilization occurs, but results in the formation of two embryos (endosperm is not formed).
  • Relevance: The drug ephedrine was originally isolated from Ephedra; Welwitschia lives only in the Namib desert and may live up to 1500 years.

Key Lineages of Seed Plants: Angiosperms

• Basal angiosperms:
  • Known Species: 200
  • Features: Oldest living angiosperm lineages; share some features with monocots and eudicots; numerous flattened stamens; some lack vessel elements.
  • Phylogenic Relationships: A paraphyletic assemblage of species at the base of the angiosperm lineage tree.
  • Relevance: Evolved prior to the vast majority of other angiosperms; Amborella is considered the sister taxon to all other flowering plants.
• Monocotyledons (monocots):
  • Known Species: 60,000
  • Features: Embryo has a single seed leaf (cotyledon); flower parts typically in multiples of three; lack secondary growth; pollen grains with a single groove; leaf veins run parallel.
  • Phylogenic Relationships: A monophyletic group closely related to the magnoliids.
  • Relevance: Some of the major crops worldwide are monocots (e.g., corn, wheat, rice, sugarcane).
• Magnoliids:
  • Known Species: 9000
  • Features: Large trees, shrubs, or vines; large flowers with numerous petals and sepals; large net-veined leaves.
  • Phylogenic Relationships: A monophyletic group that includes many species traditionally categorized as dicots
  • Relevance: Many economically important species used for food, medicines, perfumes, and timber.
• Eudicotyledons:
  • Known Species: 200,000
  • Features: Embryo has two seed leaves (cotyledons); distinguished from other groups by producing pollen grains with three grooves.
  • Phylogenic Relationships: A monophyletic group that is sister to a group including the monocots and magnoliids.
  • Relevance: By far the largest group of angiosperms; many economically important species; used for food, medicines, perfumes, and timber; most trees in deciduous forests are eudicots.