Plant Diversity Notes

Some algae are prokaryotes (e.g., cyanobacteria) and some are protists (e.g., dinoflagellates).

Fossil evidence suggests green algae appeared before plants emerged on land.
Green algae are primarily aquatic, found in fresh and salt water, and moist land areas.
Most green algae are single cells or branching filaments, enabling direct absorption of moisture and nutrients.
Life cycles of many green algae switch between haploid and diploid phases.
Under unfavorable conditions, Chlamydomonas can switch to sexual reproduction.

Many green algae form colonies, giving insights into the evolution of multicellular plants.

Mosses are bryophytes, differing from algae by having specialized reproductive organs and growing from embryos.
Bryophytes are small and live in damp soil due to the lack of water-conducting vascular tissue.
Vascular tissue allows other plants to draw water against gravity.
The absence of vascular tissue limits bryophyte height to a few centimeters.
Like all plants, bryophytes show alternation of generations, with the gametophyte as the dominant stage.
Gametes form in reproductive structures at gametophyte tips.
Eggs are produced in archegonia, and sperm in antheridia, needing water to swim to the egg cells.
Sperm and egg fuse to form a diploid zygote, which grows into a sporophyte with a sporangium (spore capsule).
Inside the sporangium, haploid spores are produced by meiosis.

Vascular plants, like horsetails, are also called tracheophytes, named after tracheids.
Tracheids are hollow, tube-like cells with thick cell walls strengthened by lignin.
Tracheids are in xylem, which carries water up from the roots.
Vascular plants also have phloem, another transport tissue with long, specialized cells for fluid movement.
Xylem and phloem enable vascular plants to move fluids against gravity.

Three groups of seedless vascular plants exist today: club mosses, horsetails, and ferns.
The large, recognizable ferns are the diploid sporophyte phase of the fern life cycle.

A seed contains a plant embryo and its food supply, encased in a protective covering.
Each seed has a living plant ready to grow under suitable conditions.

Adaptations for seed plants to reproduce without standing water:
- Reproduction in cones or flowers.
- Sperm transfer by pollination.
- Embryo protection in seeds.

In seed plants, male and female gametophytes grow and mature within the sporophyte.
- Gymnosperms: Bear seeds directly on cone scales.
- Angiosperms: Bear seeds in flowers inside a protective tissue layer.
The entire male gametophyte is in a pollen grain.
Pollen transfer from male to female reproductive structures is called pollination.
After fertilization, the zygote in a seed grows into a sporophyte embryo.
A tough seed coat protects the embryo and prevents drying out.

Conifers produce pollen cones (male) and seed cones (female).
Meiosis in pollen cones produces pollen grains.
Seed cones are larger and produce female gametophytes.
Each scale of the seed cones has two ovules.
Pollination and fertilization in conifers take about two years.
The cycle starts in spring when male cones release pollen grains carried by the wind.