Chpt 28: Green Algae & Land Plants

Individuals Produce Distinctive Male & Female Gametangia

•Antheridium: sperm-producing •Archegonium: egg-producing

Retention of Embryos was a Key Event in Land Plants Evolution

•Embryophyta: the embryo-plant; the retention of the fertilized egg in here is analogous to pregnancy in mammals

Hypothesized that Green Algae are Closely Related to Plants

•Evidence includes: -chloroplast structures is the same -thylakoid arrangements are similar -cell walls, sperm, & peroxisomes are similar in structure & composition -chloroplasts synthesize starch as a storage product

Major Morphological Differences Among Land Plants

•Nonvascular Plants •Seedless Vascular Plants •Seed Plants

Three Groups Most Similar to Land Plants

•Zygnematophyceae -> conjugating algae •Coleochaetophyceae -> coleochaetes •Charophyceae -> stoneworts -largely multicellular & live in freshwater

Green Plants

•aka Viridiplantae •consists of the green algae & land plants

Alternation of Generation

•all land plants undergo this in which individuals have: -a multicellular haploid phase called Gametophyte (are microscopic in gymnosperms & angiosperms) -a multicellular diploid phase called sporophyte •the two phases of the life cycle are connected by distinct types of reproductive cells- gametes & spores •does not occur in algal groups data suggests that it envolved independently of its evolution in other eukaryotes & originated early in their history (land plants)

Homospory/Homoesporous

•all of the nonvascular plants & most of the seedless vascular plants are this •produce a single type of spores •spores develop into bisexual gametophyte that produces both eggs & sperm -can self-fertilize & produce offspring

Double Fertilization

•angiosperms fertilization involves two sperm cells 1. one fuses with egg to form diploid (2n) zygote 2. one fuses with two nuclei in the female gametophyte to form a triploid (3n) nutritive tissue (endosperm) •after they evolved, stamens & carpels later become enclosed by modified leaves called Sepals & Petals -the four structures (stamen, carpel, sepals & petals) then diversified to produce a fantastic array of sizes, shapes & colors

Gametophyte of Seed Plants & Nonvascular & Seedless Vascular Plants

•are either male or female, never both •sperm of the nonvascular & seedless vascular plants have to swim to the egg to fertilize it -water must be available for fertilization to occur

Green Algae

•are important photosynthetic organisms in freshwater habitats •have been considered protists, but are studied along with land plants for two reasons: 1. they are the closest living relative to land plants 2. the transition from aquatic to terrestrial life occurred when land plants evolved from green algae

Gymnosperms

•are prominent in the fossil record from 299-145 mya •some major groups of gymnosperms living today are: -ginkgoes -redwoods, junipers, & yews -pines, spruces & firs •grow readily in dry habitats -both wet & dry environments became blanketed with green plants during this interval

Land Plants

•are the key photosynthesizers in terrestrial environments •were the first organisms that could thrive with their tissues completely exposed to the air -terrestrial life was limited to bacteria, archaea & single-celled protists before •are the dominant primary producers in terrestrial ecosystems -convert energy in sunlight into chemical energy -the sugars produced by land plants support virtually all other organisms in terrestrial habitats •are the key to the carbon cycle -take CO2 from the atmosphere & reduce it to make sugars -fix much more CO2 than they release

Sporophyte-Dominated Life Cycles were Advantageous

•diploids can respond to varying environmental conditions more efficiently than haploid cells can •is especially true if the individual is heterozygous at many genes

The Carboniferous Period

•extensive deposits of coal were found in sediments dated from about 359-299 mya -carbon rich rock packed with fossil spores, branches, leaves & tree trunks -most of the fossils are derived from seedless vascular plants

Pollination

•flowers may be adaptations to increase the probability that an animal will perform this •is the transfer of pollen from one individual's stamen to another individual's carpel

Fossil Record for Green Algae & Land Plants

•green algae begins with 700-725 mya •land plants begins with about 475 mya -supports that land plants are derived from green algae

The Loss of Plant Rich Prairies

•has contributed to increased concentrations of CO2 in the atmosphere -partly responsible for rising temperatures associated with global climate change

Seed Plants

•have vascular tissue & make seeds -seeds consist of an embryo & a store of nutritive tissue, surrounded by a tough protective layer •include angiosperms (encase seeds) or flower plants & gymnosperm (naked seeds)

Seedless Vascular Plants

•have well-developed vascular tissue -do not make seeds -use spores for reproduction •include ferns

Vessel Elements

•the most specialized type of water conduction cell, appeared 250-270 mya -are shorted & wider than tracheids -have gaps on both ends where both cells are missing

Coleochates, Stoneworts & Conjugating Algae

•the multicellular form is haplod •only the zygote is diploid

Two Types of Spore-Producing Structures in Heterosporous Species

1. Microsporangia: produce microspores that develop into male gametophytes which produce small gametes called sperm 2. Megasporangia: produce megaspores that develop into female gametophytes which produce large gametes called eggs

Angiosperms Are Divided into Two Major Groups

1. Monocotyledons (monocots) -1 cotyledon -vascular tissue scattered throughout stem -parallel veins in leaves -petals of flowers in multiples of 3 -monophyletic 2. Dicotyledons (dicots) -2 cotyledon -vascular tissue in circular arrangement in stem -branching veins in leaves -petals in multiples of 4 or 5 -paraphyletic -aka eudicots ("true dicots") •are divided based on differences in their cotyledons or first leaves -store nutrients & provide them to the embryo

Five Major Events in the Diversification of Land Plants

1. Origin of Land Plants 2. Silurian-Devonian Explosion 3. Carboniferous: Club Mosses & Horsetails Abundant 4. Gymnosperms Abundant 5. Angiosperms Abundant

Three Innovations were Instrumental for Efficient Plant Reproduction in a Dry Environment

1. spores that resist drying because they are encased in a tough coat of sporopollenin 2. gametes that were produced in complex, multicellular structures 3. embryos that were retained on & nourished by the parent plant

Alternation of Generation Sequence of Events

1. the sporophyte produces haploid spores by meiosis 2. spores germinate & divide by mitosis & develop into multicellular, haploid gametophytes 3. gametophytes produce unicellular haploid gametes by mitosis 4. two gametes unite during fertilization to form a diploid zygote 5. the zygote divides by mitosis & develops into a multicellular, diploid sporophyte

The Diversification of Angiosperms is Associated with Three Key Adaptations

1. water-conducting vessels 2. flowers 3. fruits •these adaptations allow angiosperms to transport water, pollen & seeds efficiently

The Evolution of Embryo Retention had a Downside

•in ferns & horsetails, sporophyte have to live in the same place as their parent gametophyte •seed plants overcame this limitation embryos of seed plants are portable & can disperse to a new location

Pollen Grains

•in land plants allowed plants living in dry habitats to reproduce efficiently -tiny male gametophytes surrounded by a tough coat of sporopollenin -can be exposed to air for long periods of time without drying out

Silurian-Devonian Explosion

•in rocks date 416-359 mya, found fossils from most of the major plant lineages -virtually all of the adaptations that allow plants to occupy dry, terrestrial habitats are present, including water-conducting vascular tissue & roots

Wood

•in stems & branches of some vascular plants, tracheids or a combination of tracheids & vessels form this -strong, support material

Secondary Cell Wall

•increased structural support but water could still move easily through the cells because of the pits -all vascular plants contain tracheids

Seed

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

Fruit

•is a structure that is derived from the ovary & encloses one or more seeds -tissues derived from the ovary are often nutritious & brightly colored; found in angiosperms •made efficient seed dispersal possible

Cuticle

•is a watertight sealant that covers the aboveground parts of the plant & gives them the ability to survive in dry environments -cuticle also keeps CO2 out of plant -most plants solve this problem with stoma -consists of an opening surrounded by specialized guard cells -pores open & closes as guard cells change shape -gas exchange accomplish through pore

Angiosperms

•is the fifth interval in the history of land plants & is still underway •age of flowering plants •appear about 150 mya -plants that produced the first flower are the ancestors of today's grasses, etc. •produce pollen grains that are transported via wind or insects & that carry the cells that will later develop into sperm

Heterospory

•is the production of two distinct types of spores by different structures

Nonvascular Plants

•lacks vascular tissue -specialized groups of cells that conduct water or dissolved nutrients throughout the plant body •include mosses •use spores, not seeds, for reproduction & dispersal

Tracheids

•long, thin, tapering, water-conducting cells evolved about 380 mya •have a thickened, lignin-containing secondary cell wall in addition to a cellulose-based primary cell wall -pits in the sides & ends of the cell which allow water to flow efficiently between tracheids

Medicine

•most of these compounds are synthesized by plants to repel herbivores

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 & color in order to attract specific pollinators •flowers attract pollinators by providing them with food- either protein rich pollen or nectar (sugar-rich fluid)

Convergent Evolution

•occurred as vessels evolved independently in angiosperms, gnetophytes & several species of seedless vascular plants

Adaptive Radiation

•occurs when a single lineage produces a large number of descendant species that are adapted to a wide variety of habitats -angiosperms represent one of the great adaptive radiation in the history of life

Ecosystem Services

•provided by green algae & land plants include: -producing oxygen via oxygenic photosynthesis -building soil by providing food for decomposers -holding soil & preventing nutrients from being lost to wind or water erosion -holding water to soil -moderating the local climate by providing shade reducing the impact of wind on landscapes

Evolution of Land Plants

•required adaptations that allowed photosynthetic organisms to move from aquatic to terrestrial environments •plants had to adapt to living & reproducing in a dry environment

Life Cycles of Sexually Reproducing Eukaryotes

•serve several functions -increasing genetic variability due to meiosis fertilization -increasing the number of individuals -dispersing individuals to new habitats

Most of the Earliest Plant Fossils are Microscopic

•some are thin sheets of waxy material like cuticle -weightight barriers that coats aboveground parts of today's land plants & helps them resist drying •fossilized spores surrounded by material almost identical in structure to sporopollenin -wacy substance that encases the spores & pollen of modern land plants & helps them resist drying •fossilized spores found in association with spore-producing structures (sporangia; sporangium) that are similar to sporangia in some modern nonvascular plants

In Ferns & Other Vascular Plants

•sporophyte is much larger & longer lived than gametophyte -sporophyte-dominatned life => evolved later

In Nonvascular Plants

•sporophyte is small & short lived & is largely dependent on gametophyte for nutrition -gametophyte-dominanted life cycle=>evolved early

Angiosperms: The Flowering Plant

•the flower is the reproductive organ •Stamen contains Anther, where microsporangia develop (sperm) •Carpel contains Ovary, where ovules are found -ovules contain the megasporangia

Gametangia

•the fossilized structures of early land plants contained specialized reproductive organs known as this -charophy (stoneworts) developed gametangia but gametangia found in land plants are larger & more complex •protected the gametes from drying & from physical damage -presented in all land plants living today except angiosperms where structures inside the flower perform the same functions

Directed-Pollination Hypothesis has Strong Experimental Support

•the spectacular diversity of angiosperms resulted from coevolution with animal pollinators •many animals & the flowering plants they pollinate depend on each other for survival

Natural Selection Favoring Early Land Plants

•there were three main adaptations that solved the drying problem by: 1. preventing water loss, which kept cells from drying out & dying 2. providing protection from harmful UV radiation 3. moving water from tissues with direct access to water to tissues without direct access

Life Cycle of a Pine Tree

•typically heterospory in gymnosperms 1. starting with the sporophyte called cons, where microsporangia & megasporangia develop 2. microsporangia contain microspore mother cells that divide by meiosis to form microspores which then divide by mitosis to form pollen grains 3. megasporangia are found inside protective structures called ovules. Each megasporangium contains a megaspore mother cell that divides by meiosis to form a megaspore 4. megaspore undergoes mitosis to form the female gametophyte, which then produces egg cells through further rounds of mitosis 5. the female gametophyte stays attached to the sporophyte as pollen grains arrive & produce sperm that fertilize the eggs 6. seeds mature as the embryo develops. Inside the seed, cells derived from the female gametophyte become packed with nutrients provided by the sporophyte

Types of Green Algae Species

•unicellular •colonial •multicellular •live in marine, freshwater, or moist terrestrial habitats -vast majority of green algae are aquatic -vast majority of land plants are terrestrial

First Land Plants

•were small or had a low, sprawling growth habit •had to grow in a way that kept many or mot of their tissues in direct contact with moist soil -competition for space & light would have become intense soon after the first plants began growing on land •cuticle, stomata, & vascular tissues were key adaptations that allowed early land plants to colonize land -evolved just once n(cuticle, pores, stomata, vascular tissue & tracheids)

Compare & Contrast Zygotes, Spores & Gametes

•zygotes & spores are both single cells that divide by mitosis to form a multicellular individual ` -zygotes develop into sporophytes; spores develop into gametophytes •zygotes are diplod & spores & gametes are haploid •zygotes result from the fusion of two haploid cells, such as sperm & egg, but spores are not formed by the fusion of gametes •spores are produced by meiosis inside structures called sporangia; gametes are produced by mitosis inside gametangia