Diversity of Plants

General Characteristics of Plants:
- Plants are multicellular, photosynthetic eukaryotes.
- They likely evolved from freshwater algae approximately $500 \times 10^6$ years ago ( $500 \text{ million years ago}$ ).
Shared Traits with Freshwater Algae:
- Both contain Chlorophylls $a$ and $b$ .
- Both store excess carbohydrates in the form of starch.
- Both possess cellulose within their cell walls.
The Evolutionary Timeline:
- The transition of plants from water to land was made possible after the development of the ozone layer, which provided protection from ultraviolet radiation.

Vascular Tissue:
- First seen in Lycophytes.
- Function: To transport water and nutrients throughout the plant body.
- Anatomy: Evolution of true roots, stems, and leaves.
Megaphylls:
- Characteristic of Ferns.
- Function: Increase the surface area available for photosynthesis, thereby increasing carbohydrate production.
Seeds:
- Characteristic of Gymnosperms.
- Structure: Contains the embryo and stored nutrients inside a protective seed coat.
- Advantage: Allows the embryo to survive unfavorable environmental conditions.
Flowers:
- Characteristic of Angiosperms.
- Function: Specialized reproductive structures designed to attract pollinators.
- Outcome: Give rise to fruits that protect and help disperse seeds.

Life Cycle Overview:
- Plants alternate between two distinct forms throughout their life cycles.
Sporophyte Phase ( $2n$ ):
- The diploid stage of the plant.
- Produces spores via the process of meiosis.
- Spores are haploid cells that can develop into a new organism without fusing with another cell.
Gametophyte Phase ( $n$ ):
- The haploid stage of the plant.
- Produces gametes (sperm and eggs) via the process of mitosis.
- Gametes fuse (fertilization) to form a diploid zygote, which develops into the sporophyte.
Evolutionary Trends in Life Cycles:
- Green Algae: The zygote is the only diploid phase.
- Bryophytes (Nonvascular): The gametophyte is the dominant generation; the sporophyte is physically dependent on the gametophyte.
- Ferns and Seed Plants: Movement to land saw a shift in importance toward the sporophyte. In these groups, the sporophyte is the dominant generation.

Classification:
- Includes Hornworts, Liverworts, and Mosses.
General Characteristics:
- They lack vascular tissue (no true roots, stems, or leaves).
- Reproduction: They require water for fertilization because sperm must travel through a film of water to reach the egg.
Moss Life Cycle (Example: Polytrichum):
- A. Mature Sporophyte: Consists of a capsule sitting atop a stalk. It remains attached to and dependent on the gametophyte.
- B. Spore Production: Meiosis occurs inside the capsule to form spores. The capsule pops open to release them.
- C. Germination: Spores germinate and grow into immature gametophytes.
- D. Male Gametophyte: Features sperm-producing structures called antheridia at the tips.
- E. Female Gametophyte: Features egg-producing structures called archegonia at the tips.
- F. Fertilization: Raindrops disperse sperm, which swim to the eggs in the archegonia.
- G. Zygote Development: The resulting diploid zygote ( $2n$ ) grows into a new sporophyte while still attached to the female gametophyte.

Main Groups:
- Lycophytes: (e.g., Lycopodium).
- Ferns.
Key Characteristics:
- They possess a dominant sporophyte generation.
- They produce wind-blown spores for dispersal.
- They still require water for reproduction (sperm must swim to the egg).
Lycophytes:
- Characterized by small leaves known as microphylls.
Ferns:
- Characterized by large leaves known as megaphylls (fronds), which allow for more efficient photosynthesis.
- Spores are dispersed from structures located on the lower surfaces of the frond leaves.

General Features:
- These are the most plentiful plants on land.
- The seed coat and stored food protect the sporophyte embryo, allowing it to remain dormant until environmental conditions are favorable.
- Independence from Water: Water is no longer required for reproduction.
Reproductive Biology:
- Seeded plants produce two types of spores that develop into two different kinds of gametophytes.
- Pollen Grains: These are the male gametophytes. They are dispersed by wind or animals rather than water.
- Female Gametophytes: These develop inside the ovule.
- Definition of a Seed: A "mature ovule."

Characteristics:
- Vascular seeded plants with "naked" seeds, meaning the seeds are not enclosed within a fruit.
- Reproductive structures are typically found in cones.
Types of Gymnosperms:
- Cycads: Ancient gymnosperms that have existed since the Carboniferous period.
- Conifers: Group includes pines, firs, spruces, cedars, and redwoods.
  - Coastal Redwood: Known as the tallest living vascular plant.
Gymnosperm Life Cycle:
- 1. Male and female cones produce spores.
- 2. Male cones release pollen grains.
- 3. Wind carries the pollen to the female cone.
- 4. Fertilization occurs.
- 5. A seed forms and eventually grows into a new sporophyte plant.

Overview:
- The most diverse plant lineage.
- The only plants that produce flowers and fruits.
- Possess "covered seeds" (seeds enclosed in fruit).
Reproductive Structures:
- Flowers: A specialized reproductive shoot.
- Ovaries: Chambers that enclose the ovules within the flower. After fertilization, the ovary matures into a fruit.
Dispersal Strategies:
- Pollination: Uses wind or animal pollinators.
- Fruit Dispersal: Fruits may float on water, stick to animal fur, or contain seeds that survive passage through animal digestive tracts.

Pollinators:
- Animals, such as insects, that move pollen grains from the male parts of one flower to the female parts of another while feeding on pollen or nectar.
Coevolution:
- A process where two species (plants and their animal pollinators) jointly evolve over time.
- Changes in the morphology or behavior of one species exert selection pressure on the other, leading to highly specialized relationships.