10/17 Evolution of Land Plants

What are the pros of an aquatic vs terrestrial lifestyle?

Pros:

Bathed in nutrients

Supported against gravity (buoyancy)

Extensive transport not necessary

Gametes and offspring can be dispersed by water

No problem of desiccation (dying out)

What are the cons of an aquatic vs terrestrial lifestyle?

Cons:

Limited visibility in deeper regions

Lack of abundance of CO2 for photosynthesis

Why leave the water?

More direct sun

Untapped nutrients and minerals on land

Abundant CO2 for photosynthesis

Initially, absence of herbivores

What features did land plants inherit from their algal ancestors?

Cell Walls

Flagellated haploid cells/sperm

Chlorophyll (pigment that allows plants to do photosynthesis)

What are the new features that define land plants and separate them from algae?

Protected embryos stay on plant (fertilization in place as opposed to gametes dumped in water)

Cuticle (waxy layer) that coats outside of plant tissue to prevent water loss

Alternation of Generations (sporophytes and gametophyte)

What are the four main plant groups?

• Nonvascular Bryophytes

  • Mosses

  • Liverworts

  • Hornworts 

• Ferns

• Gymnosperms

• Angiosperms (flowers and fruits)

1. Nonvascular Bryophytes

Earliest land plants; mosses, liverworts, hornworts.

Can be abundant (moist environments, tundra)

Limitations of Nonvascular plants

Lack of vascular tissues and structural support; must stay short and small

Reliance on water for reproduction

Lack of durable dispersal phase (swimming sperm)

Gametophyte dominates life-cycle so not much dispersal of spores since sporophyte must depend upon the gametophyte for life and sporophyte is smaller and short lived.

2. Ferns

First vascular plants; can grow large.

Seedless Vascular Plants

Principle breakthrough was vascular tissue and this allows for large size and efficient distribution of water and nutrents by xylem and phloem

Limitations of Ferns

Requirement of water for reproduction and the presence of a fragile gametophyte stage

Gymnosperms (the origin of seed plants)

Less than 1200 species remain

Gymnosperms: 2 Main Breakthroughs

1. Evolution of seeds: embryo (develops into a sporophyte) protected inside the ovule (seed). Can wait for favorable conditions before growing. Can be carried by wind or animals = dispersal.

Gymnosperms: 2 Main Breakthroughs

2. Reduction of the gametophyte + use of pollen. Male gametophyte can be carried by wind to the female gametophyte . Now free from water for reproduction.

Wind Pollination Drawbacks

Inefficient: must produce lots of pollen

Angiosperms

Flowering plants; extremely successful, most of our major crops

Angiosperms 2 Main Breakthroughs

1. Flowers: use animals to pollinate. Don’t need to rely on wind, easier to avoid inbreeding, can reproduce even when in low abundance

Angiosperms 2 Main Breakthroughs

2. Fruits: derived from the ovary of the flower; great for dispersal (attract pollinators)

Gametophyte vs Sporophyte

A gametophyte is the haploid stage in the life cycle, responsible for producing gametes through mitosis, which then fuse to form a diploid zygote, starting the sporophyte generation. This "alternation of generations" involves distinct haploid (gametophyte) and diploid (sporophyte) phases, with the gametophyte being free-living and dominant in some plants (like mosses) or highly reduced and dependent (like pollen in flowering plants).