Plant Evolution and Anatomy

Plants are often underrated compared to animals, especially mammals, but they are indispensable for our existence.
Plants evolved from aquatic ancestors, specifically algae, which are protists.
Algae are primitive eukaryotic organisms.
Some algae species are multicellular.
Approximately 500 million years ago, plants diverged from algae and moved onto land.
The transition from aquatic to terrestrial life required several evolutionary changes to adapt to the new environment.

Increased Competition: Competition in the oceans drove some organisms to move further inland, seeking new niches in moist, aquatic environments.
Adaptations for Terrestrial Life: Organisms evolved to live closer to the coast and eventually onto land, with selection favoring traits that enabled terrestrial life.
Algae vs. Plants
- Algae: Completely aquatic photosynthesizers.
- Plants: Mostly terrestrial photosynthesizers.
Challenges Presented by Land
- Acquiring and retaining moisture.
- Obtaining resources from the air (carbon dioxide) and soil (nutrients like nitrogen).
- Developing the ability to stand upright.
Algae's Aquatic Support System: Algae are supported by water, with their entire body participating in photosynthesis and absorbing nutrients from the water.
Plant Adaptations to Solve Terrestrial Challenges
- Development of roots to anchor themselves into the soil.
- Evolution of structures to stand upright to compete for sunlight.
- Evolution of leaves for photosynthesis and gas exchange.

The largest single-celled organisms are algae, reaching up to a foot tall without cell walls between them.
Bubble algae are problematic in aquariums because they release numerous baby algae when they burst.
Bubble algae can be up to three and a half inches in diameter.

From Moisture-Rich Aquatic Environments to Air Exposure: Plants needed a way to retain moisture to prevent desiccation.
Evolution of Novel Molecules: Plants evolved new molecules, such as lignin, to provide structure.
- Lignin provides sturdiness to wood and plant stems.
- Lignin is difficult to metabolize, with reindeer being one of the few organisms capable of breaking it down.
Protection of Reproductive Cells: Plants developed methods to protect their gametes in drier environments.
From Holdfasts to Root Systems: Algae use holdfasts to attach to surfaces, while plants developed root systems to anchor more thoroughly in the soil.
Nutrient Uptake: Algae absorb nutrients through their entire body, whereas plants use roots to pull in nutrients and leaves to exchange gases with the air.
Specialized Tissues: Algae's entire body photosynthesizes, and plants have specialized tissues for photosynthesis in specific locations.

Pollen: Protects plant sperm and is carried by wind or animals for sexual reproduction
Egg Protection: The female reproductive cell (egg) stays inside the female reproductive organ of the plant.
Embryo Protection: Embryos (fertilized eggs) must be protected until they develop protective tissues.
Preventing Water Loss:
- Plants in dry environments have a waxy coating to prevent water loss from the leaves. This waxy layer is called the cuticle. The cuticle is secreted by epidermal cells.
Specialization of Tissues:
- Shoot system vs. root system.
- Plant vascular system enables plants to grow taller.

Mycorrhizae:
- Crucial for plants moving onto land.
- Symbiotic relationship with fungi that enhance nutrient acquisition.
- Fungi help plants acquire nutrients and grow more robustly.
Soil Formation:
- Early Earth was bare rock until life moved onto land.
- Soil is dead organic material created by living, dying, and decomposing organisms.
Lichens:
- Symbiotic relationship between a fungus and photosynthetic bacteria or algae.
- They digest rocks and extract minerals to create organic biomass.
- Important in primary succession after a forest fire, breaking down rocks and creating organic material.
Root Nodules:
- Root nodules are protrusions on the roots of legumes (e.g., peanuts, lentils).
- They contain nitrogen-fixing bacteria.
- Nitrogen-fixing bacteria convert atmospheric nitrogen into usable forms for plants.
- Nitrogen is essential for DNA and proteins.

Shoots and Roots:
- Key adaptations for plants moving onto land.
- New growth occurs at the terminal bud of shoots and at the meristem of roots.
Shoot System:
- Includes the leaves and stem.
- Leaves attach at nodes and consist of a blade and petiole.
Root System:
- Can be fibrous or have a taproot. A taproot system has one main root with smaller roots branching off. A fibrous root system has many roots of similar size.
- Growth occurs by adding new cells to the end of the root.
Functions:
- Shoot System:
  - Access to sunlight for photosynthesis.
  - Transport of food and water.
  - Storage of energy.
  - Reproductive structures (in flowering plants).
- Root System:
  - Anchoring to the earth.
  - Water and mineral intake.
  - Asexual reproduction (e.g., runners).

Dermal tissue system:
- Regulates gas exchange.
- Provides an outer protective covering to prevent water loss.
Ground tissue system:
- Contains cells specialized for photosynthesis and food production. These cells are called parenchyma cells.
- Provides structural support and nutrient storage.
- Includes:
  - Parenchyma: Thin-walled cells that perform photosynthesis and store nutrients.
  - Collenchyma: Elongated cells with unevenly thickened walls, providing flexible support.
  - Sclerenchyma: Cells with thick, rigid walls that provide strong support; many sclerenchyma cells are dead at maturity.
Vascular tissue system:
- Transports water and nutrients throughout the plant.