Green Algae and Seedless Plant Diversity

Green Algae

group of photosynthetic organisms that are not land plants

Features of green algae

• Eukaryotic

• most are protists

• contain chlorophyll for photosynthesis

• can be unicellular or multicellular

• primarily aquatic

Green algae fall into two major groups:

chlorophytes and Charophytes

plant

(kingdom: Plantae) are photosynthetic organisms that have complex tissue structures that evolved for living on land

features of plants

• eukaryotic

• photosynthetic (contain chlorophyll and chloroplasts)

• multicellular

• primarily terrestrial (live on land)

plant groups

• non-vascular plants

• seedless vascular plants

• seed plants

plants and green algae are primary producers.

sun is the primary source of energy; autotrophs convert solar energy to be stored in organic matter; primary producers are responsible for generating organic matter; heterotrophs use the organic matter for fuel for their own cells.

challenge: preventing desiccation

adaptation for land: protective cuticle (waxy covering)

challenge: support for the body

adaptation for land: fibrous stems provide stability

challenge: obtaining resources for growth

adaptation for land: vascular tissues (roots absorb H2O and minerals, while green shoots conduct photosynthesis)

challenge: gas exchange

adaptation for land: stomata (pores in the cuticle)

challenge: fertilization of gametes with limited water

adaptation for land: protective housing for gametes called gametangia (antheridia for sperm and archegonia for eggs)

plant life cycle

alternation of generations: sporophyte (diploid (2n)) →← gametophyte (haploid (n))

sporophyte (2n)

multicellular diploid stage

sporangium (2n)

location of spore production by meiosis

spore (n)

a haploid cell for asexual reproduction

gametophyte (n)

multicellular haploid stage

gametangia (n)

location for gamete production and mitosis

Antheridium (n)

location of sperm production

archegonium (n)

location of egg production

Alternation of Generation

• Plants have two multicellular phases;

  • sporophyte (diploid, 2n)

  • gametophyte (haploid, n)

• sporophyte (2n) produces sporangium (2n)

  • sporangium (2n) produces spores (n) by meiosis

  • Spores (n) develop into a new gametophyte (n) by mitosis

• Gametophyte (n) produces gametangia (n)

  • Antheridium - produces sperms (n) by mitosis

  • Archegonium - produces eggs (n) by mitosis

  • Egg + sperm fuse (fertilization) into zygote (2n) which develops into a new sporophyte (2n) by mitosis

Charophytes (green algae)

Evidence indicates that charophytes are the closest relatives of terrestrial plants. similar mechanism for forming a cell plate in cell division; similar nuclear and chloroplast genes; cell walls with a high concentration of cellulose; flagellated sperm similar to some land plants’ sperm; genus Chara produces gametes within gametangia - a key feature that allowed plants to colonize dry land.

Genus spirogyra

genus of filamentous charophyte named for the spiral arrangement of its chloroplast

asexual reproduction: spirogyra

haploid under normal conditions and usually reproduce asexually by mitosis

sexual reproduction: spirogyra

when conditions deteriorate, spirogyra undergoes sexual reproduction (conjugation). resulting in diploid zygospore (2n) with a thick coat that can withstand harsh conditions. the zygospore remains dormant until conditions improve.

Spirogyra conjugation

• A: vegetative filament

• B: formation fo conjugation tubes between two filaments

• C: gamete (n) from one filament migrates to another filament

• D: fusion of two gametes (n) forms a zygospore (2n)

Genus chara

genus of branching charophyte that shares many features with land plants (gametangia)

Seedless plants reproduce with spores

• seedless plants (mosses and ferns) were the earliest land plants

  • most abundant plants on earth until - 285 MYA

  • reproduce using spores, not seeds

  • many species are adapted to grow directly on rocks

bryophytes: non-vascular land plants

• Don’t have vascular tissue to transport water

  • Relies on diffusion and osmosis for the transport of water and minerals

  • anchored to rocks/soil by rhizoids - different from roots

  • The gametophyte(n) is the dominant generation

  • The sporophyte is dependent on the gametophyte for energy and nutrition

moss gametophytes are either male or female

Moss life cycle

moss antheridium

located at the top of the male gametophyte (n) and contain sperm inside

moss archegonium

located at the top of the female gametophyte and contains eggs inside

Moss capsule

Alternation of generations (moss)

Ferns

• have vascular tissue including roots, leaves, and stems

  • dominant generation is the sporophyte (2n)

  • both generations are free living (neither depends on the other for nutrition) - different from bryophytes

Dominant generation

ferns and seed plants evolved with the sporophyte (2n) as the dominant generation.

fern sporophyte (2n)

contains structures called sori under the fronds

sori

contain the sporangia, where meiosis takes place to produce spores

fern gametophyte

contains both male and female gametangia