Chapter 26 - The Colonization of Land (copy)

The Greening of Earth

fungi, animals, and plants arose on the land to join the prokaryotes and protists in the last 500 million years ago

• terrestrial surface was lifeless besides prokaryotes and protists for more than the first 3 billion years

Charophytes

green algae

• closest relative of land plants

evidence for algal ancestry

land plants and charophytes share many key traits

Pros and Cons to the move on land for plants

Pros - unfiltered sun, more CO2, nutrient-rich soil, and few herbivores or pathogens

Cons - scarcity of water and lack of structural support

How did charophytes avoid being dried out on the land

formed a durable polymer called spropollenin which prevents exposed zygotes from drying out

Embryophytes

plants that live on land

• defined by their dependency of the embryo on the parent

Derived traits of terrestrial plants

these traits appear in almost all land plants, but absent in charophytes

• alternation of generations (with multicellular, dependent embryos)

• walled spored produced in sporangia

• apical meristems

alternation of generations and stages

reproductive cycle where plants alternate between two multicellular stages

• gametophyte - haploid and produces haploid gametes by mitosis

• sporophyte - fusion of the gametes, which is diploid and produces haploid spores by meiosis

steps of alternation of generations

1. Gametophyte produces haploid gametes by mitosis

2. Gametes unite

3. zygote develops into multicellular diploid sporophyte

4. sporophyte produces haploid spores by meiosis

5. spores develop into multicellular haploid gametophytes

embryos and alternation of generations

• diploid embyro is retained within the tissue of the female gametophyte

• nutrients are transferred from the parent to the embryo through placental transfer cells

sporangia

organs in the sporophyte that produce spores

sporocytes

diploid cells that undergo meiosis to produce haploid spores

sporopollenin

spore walls that makes them resistant to harsh enviornments

gametangia

organs in gametophytes that produce haploid gametes

archegonia

female gametangia that produces eggs and are the site of fertilization

antheridia

male gametangia that produces sperm cells

Role of Fungi in land colonization

• fungi aren’t closely related to plants but have a symbiotic relationship with plants that allows for nutrient uptake by early plants

• fungi were one of the earliest colonizers of land and probably formed mutualistic relationships with early land plants

Fungi

heterotrophs that absorb nutrients from outside their body

they use enzymes to break down a large variety of complex molecules into smaller organic compounds

hyphae

networks of filament that provide a large surface area for absorption

body structures of fungi

most commonly are multicellular filaments and single cells (yeasts)

Micorrhizae

mutually beneficial relationships between fungi and plant roots

• mycorrhizal fungi grow branched extensions on hyphae that exchange nutrients with living plant cells

Origin of fungi

• fungi and animals are more closely related to each other than to plants

• ancestor of fungi was probably unicellular

• fungi and animals separated more than a billion years ago

chitin

strong, flexible polysaccharide that prevents cells from bursting when they absorb nutrients and water from their environment

Mycellium

interwoven mass of hyphae that grows into and absorbs nutrients from material where fungus feeds on

two main types of mycorrizhae

• Arbuscular mycorrhizal fungi grow branched - extend branching hyphae through the root cell walls and into tubes pushing inward of the root cell plasma membrane

• Ectomycorrhyizal fungi - form sheaths of hyphae over the surface of the root and typically grow into root cortex

benefits of mycorrhizae

improve phosphate ion delivery and other minerals to plants because mhycorrhizal fungal networks are more efficient than plant roots

• plants supply fungi with organic nutrients such as carbohydrates in return

The major fungal groups

Chytrids, Zygomycetes, Glomermycetes, Ascomycetes

reproduction of fungi

• can occur sexually or asexually by producing many spores

Sexual reproduction of fungi

fungi nuclei are normally haploid (with the exception of transient diploid stages

• requires fusion of hyphae from different mating strains

Plasmogamy - union of two parent mycelia

• heterokaryon - haploid nuclei from each parent coexist in the mycelium since they don’t fuse right away in most fungi

Karyogamy - haploid nuclei fuse which produces diploid cells

• diploid phase is short-lived and goes through meiosis to produce haploid spores

• hours, days, or even centuries may pass before karyogamy ever occurs

Groupings of Land Plants

• can be informally grouped based the presence or absence of vascular tissue

• Vascular plants - have vascular tissues

• Bryophytes - nonvascular plants

Key evolutionary stages of plants

• Nonvascular plants (bryophytes)

• seedless vascular palnts

• gymnosperms

• angiosperms

3 clades of bryophytes

• liverworts

• mosses

• hornworts

bryophyte sporophytes

• grow out of archegonia and are the smallest and simplest sporophytes of all plant groups

• consists of a foot, seta (stalk), and a sporangium (AKA capsule)

bryophytes

• ground-hugging plants

• don’t have roots and are instead anchored to the ground by rhizoids

• live in moist habitats since they have flagellated sperm that must swim through a film of water to fertilize an egg

• lack vascular tissue

• body parts are too thin so they can’ grow tall

• bryophyte reduction is tied to water

which plants were the first plants to grow tall

seedless vascular plants

vascular plants

• began to diversify during the Devonian and Carboniferous periods

  • but the earliest vascular plants date back to the Silurian period

• had vascular tissue which allowed them to grow tall

• had flagellated sperm and are usually restricted to moist environments

two clades of Seedless vascular plants

• Lycophytes - club mosses and their relatives

• monilophytes - ferns and their relatives

  • previosly known as Pterophytes

significance of seedless vascular plants

ancestors of lycophytes, horsetails, and ferns grew tall during the Devonian and Carboniferous which formed the first forests

• increased photosynthesis which may have helped global cooling at the end of the Carboniferous

• decaying plants of these carboniferous forests eventually became coal

Rise of vascular plants

• lacked seeds

• but had well developed roots and leaves

• well developed vascular systems which allowed for long distance transport of water and nutrients

what is the dominant stage in the life cycle of bryophytes (and mosses)

gametophyte

what is the dominant stage in the life cycle of ferns and other vascular plants

sporophyte

types of vascular tissue

• Xylem - conducts most of the water and minerals and includes dead cells called tracheids

• Phloem - consists of living cells and distributes sugars, amino acids, and other organic products

lignin

provides structural support to water-conducting cells (like in the xylem and phloem)

Roots

organs that anchor vascular plants

• enable vascular plants to absorb water and nutrients from the soil

• may have evolved from subterranean stems

leaves

organs that increase the surface area of vascular plants which allows for more solar energy captured that is used for photosynthesis

2 types of leaves

• microphylls - leaves with a single vein

• megaphylls - leaves with a highly branched vascular system

microphylls evolved first as a growth of stems

what are key adaptations for life on land

seeds and pollen grains

seed

embryo and nutrients surrounded by a protective coat

• originated about 360 million years ago

2 major clades of seed plants

• gymnosperms - the naked seed plants including conifers

• angiosperms - flowering plants

terrestrial adaptations of seed plants

• seeds

• greatly reduced gametophytes - gametophytes develop within the walls of spores and are contained within tissues of the parent sporophyte

• ovules

• pollen

allow for new ways for plants to cope with terrestrial conditions such as drought and exposure to UV radation

ovule

consists of megasporangium, megaspore, and one or more protective integument

how many integuments do the megaspores of gymnosperms and angiosperm have

gymnosperms have one integument, angiosperms usually have two integuments

what do microspores develop into?

• microspores develop into pollen grains which contain the male gametophytes

pollination

transfer of pollen to the part of a seed plants containing ovule

• eliminates the need for water being used and can be dispersed greater distances by air or animals

• if a pollen grains germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

evolutionary advantage of seeds

• a seed develops from the whole ovule

• evolutionary advantages seeds have over spores:

  • may remain dormant for days to years until conditions are favourable for germination

  • may be transported long distances by wind or animals

Gymnosperm evolution

• appeared early in the fossil record and dominated the Mesozoic terrestrial ecosystems

• were better suited to drier conditions than nonvascular plants

• conifers (cone-bearing gymnosperms) dominate the northern latitudes

some examples of gymnosperms

• cycads - have large cones and palmlike leaves

  • thrived in the Mesozoic, but very few species live today

• conifers - largest group of gymnosperms

  • are evergreens and can carry out photosynthesis year round

Angiosperms

• seed plants with reproductive structures called structures called flowers and fruits

• most widespread and diverse of all plants

flower

angiosperm structure specialized for sexual reproduction

• specialized shoot with up to 4 types of modified leaves

• many species are pollinated by insects or animals while some are wind-pollinated

parts of the flower

4 types of modified leaves:

• Sepals - enclose the flower

• petals - brightly coloured and attract pollinators

• stamens - produce pollen on their terminal anthers

• carpels consists of an ovary at the base, and a style leading up to a stigma where pollen is received

fruits

• protects seeds and aid in dispersal

• consist of mature ovary but can also include other flower parts

• mature fruits can either be fleshy or dry

• seeds of fruits can be carried by wind water, or animals to new locations

• ex. tomatoes, ruby grapefruits, nectarine, hazelnuts, milkweeds

angiosperm evolution

• originated in the early Cretaceous

• major branches of the clade diverged from their common ancestor during the late Mesozoic

• by the mid-Cretacous, they began to dominate terrestrial ecosystems

Fossil angiosperms

have derived and primitive traits

• ex. archaefructus sinensis has anthers and seeds but lacks petals and sepals

2 main groups of angiosperms

• monocots - have one cotyledon

  • make up more than ¼ of angiosperm species

  • ex. corn, wheat, rice, lilies, and palms, etc.

• eudicots - true dicots

  • make up more than ¾ of angiosperm species

  • ex. zucchini, beans, roses, daises, etc.

lichen

symbiotic relationship between a photosynthetic organism and a fungus where millions of photosynthetic cells are held in a mass of fungal hyphae

• fungal component of a lichen is usually an ascomycete

• algae or cyanobacteria make up an inner layer below the lichen surface

• algae provide carbon compounds, cyanobacteria provide organic nitrogen, and fungi provide the environment for growth

benefits of lichens

pros:

• important pioneers on new rock and soil surfaces

• important in soil formation

• sensitive to sollution, so their death can be a warning that air quality is deteriorating

plants and carbon cycling

• carbon forms the basis of organic compounds that are essential to life

• during photosynthesis, plants remove atmospheric carbon dioxide, and action that can affect global climate

• in the Carboniferous, the grwoth of forests contributed to a large drop in global CO2, which caused global cooling and widespread glacier formation

Fungi as mutualists

• mycorrhizae form beneficial relationships with plants

• endophytes - fungi or bacteria that live inside plant leaves or other parts

  • some endophytes benefit their hosts by producing toxins to keep herbivores away

fungi as pathogens

• about 30% of known fungal species are parasites or pathogens (mostly to plants)

• some fungi that attack food crops are also toxic to humans

mycosis

fungal infection in animals

interactions between plants and animals in the ecosystem

• pollination of flowers and transport of seeds by animals are two important relationships in terrestrial ecosystems

  • clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers

  • this is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations

products from seed plants

• wheat, rice, maize, potatoes, cassava, and sweet potatoes yield 80% of the calories consumed by humans

• modern crops are products of recent genetic change caused by artificial selection

• many plant seeds provide wood

• secondary compounds of seed plants are used in medicines