LO5

What euk. clade are plants in?

Archaeplastids

What character do all archaeplastids share?

All members of the archaeplastid clade possess chloroplasts derived from primary endosymbiosis

What did land plants evolve from and when?

• 450 mya

• evolved from a multicellular aquatic green algae ancestor

  • DNA and other similarities provide evidence of the close relationship between plants and algae.

what is a similarity between land plants and green algae

both use the same photosynthetic pigments (e.g. chlorophylls a and b, carotenoids)

adaptations for the transition to land

As early plants transitioned to land, they evolved several adaptive traits

• cuticle

• stomata

• spores

• multicellular reproductive organs

cuticle

waxy, covers the above-ground parts to prevent drying out on land

stomata

allow for gas exchange on land. They are microscopic pores on the leavers and stems that allow exchange of CO₂ and O₂

spores

reproductive cells adapted for dispersal on land due to their tough outer coat. They are produced in capsule-like structures called sporangia.

multicellular reproductive organs

• produce gametes and protect the embryo

• Algae have unicellular reproductive organs

• land plants have multi-cellular reproductive organs

archegonia

• female reproductive organs, each produce a single egg

  • the egg is fertilized within the archegonia and develops into an embryo protected within the archegonia

antheridia

male reproductive organs, each produce many sperm.

alternation of generations life cycle

land plants alternate between haploid and diploid generations

gametophyte

• haploid, multicellular generation

• a gametophyte plant produces haploid gametes (sperm or egg) via mitosis inside its reproductive organs (archegonia and/or antheridia)

• gametes fuse (fertilization) to form a diploid zygote, which begins the next generation

sporophyte

• diploid, multicellular generation

• the zygote is the first stage, and develops by mitosis into a multicellular embryo inside the archegonium

• the embryo develops into a mature sporophyte plant that produces haploid spores by meiosis inside sporangia

• the spore, the first stage of the gametophyte generation, develops into a haploid, multicellular gametophyte plant via mitosis

which generation has become increasingly dominant throughout land plant evolution?

sporophyte

how are the four main groups of plants recognized?

the presence or absence of vascular tissue and seeds

vascular tissue

tube cells that transport water and nutrients throughout plants, and provide structural support

seed

a sporophyte embryo with nutrients inside a protective coat

nonvascular plants

(the bryophytes) lack vascular tissue and seeds. They use spores for dispersal

vascular plants

• have vascular tissue

• the vascular seedless plants lack seeds, use spores for dispersal

• seed plants produce spores, but use seeds for dispersal

  • the gymnosperms and angiosperms

nonvascular plants (bryophytes)

• first plants to evolve and colonize the land around 450 million years ago

• because they lack vascular tissue, bryophytes don’t have true leaves, stems or roots, and are small

• they are the only plants to retain the ancestral trait of a dominant gametophyte generation

  • the gametophyte plant is larger, lives longer, and protects and nourishes the sporophyte

• they also retain the ancestral trait of a flagellated sperm that requires external water to swim to the egg

Mosses

• the most common bryophytes

• are ecologically important because they help prevent erosion (their root-like structures hold soil in place) and aid in forming soil (they secrete acids that break up rocks)

vascular plants (make up most plants)

• dominate modern terrestrial ecosystems

• unlike the paraphyletic bryophytes, the vascular plants form a monophyletic group

• vascular tissue forms true leaves, stems, and roots, and allows for larger size

• they have a dominant sporophyte generation

  • the sporophyte plants have the advantage of being diploid, and, only sporophyte plants have vascular tissue (gametophytes never do)

  • the gametophyte may be dependent on the sporophyte plant for nutrition and protection

seedless vascular plants

• the first vascular plants were seedless, and they became the dominant land plants (350 mya) prior to the evolution of seed plants

  • modern seedless vascular plants are typically small

• In seedless vascular plants, the sporophyte is dominant, but the gametophyte is independent

  • ex: a fern is a sporophyte plant, and its gametophyte is a tiny, nonvascular, short-lived plant

• like byrophytes, they are typically restricted to moist terrestrial habitats because their bikont sperm require water to disperse to the egg.

seed plants

• by 360 mya, the first seed plants, gymnosperms, had evolved from ancient seedless vascular plants

• the sporophyte is even more dominant in seed plants

  • the microscopic gametophyte is dependent on the sporophyte for nutrition and protection

    • the gametophyte is on cones in most gymnosperms and within flowers in most angiosperms

• seed plants produce spores, but use seeds for dispersal

  • seeds have a tougher outer coat, a food supply, and are multicellular (versus a single egg)

heterosporous (seed plants only)

• plants produce two types of spores, megaspores and microspores

• spores are not released from the sporophyte parent, as they are in homosporous plants

megaspores

female spores produced in megasporangia. Develop into female gametophytes

microspores

male spores, produced in microsporangia, develop into male gametophytes

how does heterospory contribute to the evolution of seeds?

• heterospory was a necessary precursor for the evolution of seeds

  • in heterosporous plants, a megaspore remains in its megasporangium (collectively called the ovule) as it develops into a female gametophyte

    • the female gametophyte produces an egg (while still protected within the megasporangium), that if fertilized will develop into a new sporophyte embryo

Thus, the entire female gametophyte generation develops within the parent sporophyte plant, and fertilization occurs within the sporophyte.

what makes up a seed?

The new sporophyte embryo, the remaining female gametophyte tissue (i.e. the food supply), the outer layer of the megasporangium (i.e. the seed coat)

pollen grains

• microscopic, reduced male gametophytes

• microspores develop into pollen grains inside of microsporangia

• pollen grains are dispersed (by wind or animals) to ovules during pollination

pollen tube

• each pollen grain produces a nonflagellate sperm that is transported to an egg for fertilization by a pollen tube

• the pollen tube delivers the sperm to the egg, eliminating the need for flagella and external water

gymnosperms

• ‘naked seeds’: many gymnosperm seeds develop on cones on the sporophyte plant

• As the climate became drier during the Mesozoic (250 mya), gymnosperms became the dominant plants on land

how did gymnosperms outcompete seedless vascular plants?

they did not have to rely on external water to disperse their sperm, and because they disperse seeds.

conifers

• the most abundant gymnosperms, are still the dominant plants in many forests

  • conifers (e.g. junipers and pine trees) are incredibly ecologically important as habitat for many organisms, food, and for the prevention of soil erosion (their roots)

angiosperms

• flowering plants

• evolved from ancient gymnosperms, and have been the dominant plants for the past 90 my

  • the vast majority of plants are angiosperms

angiosperm flowers

• evolved from leaves to better facilitate reproduction

  • within a flower, angiosperm ovules are protected inside of an ovary

    • upon fertilization, an ovule becomes a seed and the ovary becomes a fruit

    • not only does the fruit protect the seed but it also aids in seed dispersal

Why did angiosperm flowers evolve?

for reproduction

outer parts of the flower

• non-reproductive

• protect the flower and attract pollinators (the petals)

the inner parts of the flower

• include the male and/or female reproductive organs

  • the ovary contains one or more ovules

  • the anthers contain microsporangia that produce pollen grains

  • the angiosperm pollen grains contain two sperm

double fertilization

• one sperm fertilizes the egg in an ovule, forming a diploid zygote that develops into a sporophyte embryo inside the ovary

• the other fuses with two haploid nuclei (also in an ovule), forming a triploid (3n) food supply that grows with the embryo

flowering plants and pollinator coevolution

• flowering plants and their animal pollinators have affected one another’s evolution, becoming mutually adapted to each other

  • animal pollinated flowers have evolved brightly-colored petals, scent, and nectar to attract specific animal pollinators

  • ex: the hawk moth has a 10-inch long proboscis to obtain nectar from the long floral tube of specific plants

how have flowered traits evolved in response to abiotic pollen vectors?

wind-pollinated flowers lack obvious petals, scent, and nectar, and produce large amounts of pollen

asexual reproduction in angiosperms

• axesual reproduction often involves part of a parent plant splitting off to become a genetically identical daughter plant

benefits of asexual reproduction

can result in rapid increase in offspring, and offspring are initially more hardy than seeds

disadvantages of asexual reproduction

plants lose the dispersal benefit of seeds, and lack the genetic diversity that results from sexual reproduction, which is especially critical if the environment changes.