Plant Diversity and Evolution: A Comprehensive Study Guide
Exam Information and Fundamental Plant Concepts
Upcoming Assessments:
Quiz 2: Scheduled for Thursday, covering all of Unit 2.
Next Exam: One week from today.
Understanding Ploidy: Diploid and Haploid Stages
Ploidy Definition: Refers to the number of complete sets of chromosomes in a cell.
Diploid (2n):
Cells contain two sets of chromosomes. Chromosomes occur in distinct pairs (homologous chromosomes).
Represented in diagrams by the bottom half, often in a bluish color.
Processes involved: Both mitosis (for growth and repair) and meiosis (for sexual reproduction) can occur in diploid organisms.
Mitosis in Diploid Stage: From a zygote to a spore mother cell, growth involves numerous mitotic divisions.
Meiosis in Diploid Stage: The spore mother cell undergoes meiosis to produce haploid spores.
Crucially, when you think "diploid," automatically associate it with "pairs of each kind of chromosome."
Haploid (n):
Cells contain only one copy of each gene/chromosome (one set of chromosomes).
Represented in diagrams by the top half, often in an orangish/beige color.
Haploid Spores: These cells, produced via meiosis, are functional as they contain all necessary genes, albeit in single copies.
Spore Germination: Haploid spores develop into a multicellular haploid organism (gametophyte) solely through mitosis.
Why only Mitosis in Haploid Stage? Mitosis is the only process that can occur because there are no pairs of chromosomes to unpair or reduce. The purpose of meiosis is to reduce chromosome number and create genetic variation by unpairing homologous chromosomes.
Alternation of Generations in Plants
Definition: Plants exhibit a life cycle demonstrating an alternation between a multicellular diploid stage (the sporophyte) and a multicellular haploid stage (the gametophyte).
Isomorphic Alternation of Generations:
Found in most green algae (e.g., Ulva).
The sporophyte and gametophyte generations are visually (morphologically) identical.
To distinguish between them, one would need to take cell samples and examine their ploidy level (diploid or haploid).
"Morph" means shape or form, so "isomorphic" means "same shape."
Heteromorphic Alternation of Generations:
Characteristic of most land plants.
The sporophyte and gametophyte generations are distinctly different in appearance and size.
It is visually obvious which is the diploid and which is the haploid generation, even without a DNA sample.
Example: A large tree is an obvious diploid sporophyte. Inside its flowers (not the petals/sepals themselves, but the reproductive structures within), tiny gametophytes (male and female) develop, which bear no resemblance to the tree.
This is the general rule in plants, with isomorphic being the exception.
Most plants observed today are diploid sporophytes.
Homosporous vs. Heterosporous Life Cycles
This concept adds a layer of complexity to alternation of generations.
Homosporous Plants:
The sporophyte produces only one kind of spore.
This spore germinates into a gametophyte individual that produces both egg and sperm (a bisexual gametophyte).
The provided diagram for alternation of generations only illustrates homospory, showing one kind of sporangia, one kind of spore mother cell, and one kind of spore.
"Homo" means "same."
Heterosporous Plants:
The sporophyte produces two distinct kinds of spores:
Megaspore: Large spore that germinates into a gametophyte individual that produces only eggs (female gametophyte).
Microspore: Small spore that germinates into a gametophyte individual that produces only sperm (male gametophyte).
Terminology: The prefixes "mega-" and "micro-" are used consistently:
Sporangium: A spore-producing structure.
Megasporangium: A structure producing megaspores.
Microsporangium: A structure producing microspores.
Spore Mother Cell (Sporocyte): Produces spores via meiosis.
Megaspore Mother Cell: Produces megaspores via meiosis.
Microspore Mother Cell: Produces microspores via meiosis.
Example: Pollen is an example of a microspore.
Prevalence: Most plants are heterosporous. Modern trees (sporophytes) are typically both megasporophytes (bearing megasporangia, which house megaspore mother cells that produce megaspores) and microsporophytes (bearing microsporangia, which house microspore mother cells that produce microspores).
"Hetero" means "different" or "other."
Evolution of Gametes
Isogamy:
The most primitive form of gametes (reproductive cells).
Gametes are identical in size and motility; there is no distinction between sperm and egg.
Found in some ancestral algae species.
Anisogamy:
An evolutionary intermediate stage.
Gametes differ in size, but both may still be motile. The smaller gamete typically moves faster.
This appears later in plant evolution.
Oogamy:
The most evolved and common form of gametes in most plants and animals.
Involves a large, non-motile ovum (egg) that waits for a smaller, motile sperm to find it.
This strategy maximizes the chances of fertilization while conserving energy for the large egg.
Colonization of Land by Plants: Evolutionary Timeline
Ancestry: Land plants are thought to have evolved from green algae.
Timeline:
Late Paleozoic Era: Initial colonization of land occurred approximately 400 million years ago. Early land plants were short and limited.
Mesozoic Era: The peak period for many plant groups.
Early land plants diversified into larger forms.
Seed plants, particularly gymnosperms, became dominant during this era.
Cenozoic Era: Beginning 65 million years ago.
Angiosperms (flowering plants) became overwhelmingly dominant, largely outcompeting gymnosperms.
Angiosperms thrive in warm, wet regions, especially tropical rainforests, which boast the highest plant diversity near the equator.
Gymnosperm Niche: While losing overall dominance to angiosperms, gymnosperms successfully adapted and thrive in regions where angiosperms struggle, such as:
Cold climates (e.g., northern latitudes like Canada).
High-altitude mountainous regions.
They are often called evergreens (e.g., pine trees) because they can remain green and photosynthesize even in cold temperatures.
Challenges and Adaptations for Land Plant Life
Transitioning from an aquatic to a terrestrial environment presented significant challenges:
1. Obtaining and Transporting Water & Nutrients:
Problem: Algae were surrounded by water and dissolved nutrients. On land, water and nutrients are in the soil and must be actively acquired and distributed throughout the plant.
Solution: Evolution of vascular tissues: xylem (transports water and minerals from roots) and phloem (transports sugars from leaves). These tissues act like a plant's "blood vessels," allowing for larger growth and efficient distribution. Early nonvascular plants remained tiny.
2. Water Loss (Desiccation):
Problem: Air is much drier than water, leading to rapid evaporation from plant surfaces. This is most evident in arid environments like deserts, which have scarce plant life.
Solution: Development of a waxy cuticle (waxy surface) on stems and leaves to retard water evaporation. This cuticle can also help shed excess water in very wet, tropical environments.
3. Gas Exchange:
Problem: Necessary for photosynthesis (absorbing CO2) and respiration (absorbing O2), but surfaces must remain moist. Protecting against water loss can hinder gas exchange.
Solution: Evolution of stomata (plural; singular: stoma) – small pores on leaves, regulated by guard cells, allowing for controlled gas exchange while minimizing water loss. Plants produce oxygen as a byproduct of photosynthesis, essential for most life forms.
4. Gravity:
Problem: Terrestrial organisms must actively support their own weight against gravity, unlike aquatic organisms whose weight is supported by water (e.g., blue whales are the largest animals but are aquatic). Tall plants need structural integrity not to topple over.
Solution: Development of a robust root system for anchoring the plant securely in the soil. Strong, rigid support tissues composed of cellulose and eventually lignin (forming wood, primarily xylem) allow plants to grow impressively tall (e.g., 400 feet or 122 meters).
5. Reproduction:
Problem: Plants are sessile (cannot move) and lack the ability to actively seek out mates. Gametes, if unprotected, would dry out in the air. Water was previously a medium for gamete dispersal.
Solution: Evolution of mechanisms for gamete dispersal (e.g., wind, animals for pollen/seeds). Crucially, the gametophyte (which produces gametes) became progressively smaller and was protected by surrounding sporophytic tissue. This minimizes the vulnerability of the haploid stage, which only has one set of genes, making it susceptible to harmful mutations.
6. Temperature Fluctuation:
Problem: Terrestrial environments experience far greater temperature swings than aquatic ones. Seasonal changes (e.g., equinoxes and solstices due to Earth's tilt) lead to varying light levels and extreme cold in some regions.
Solution: Adaptations like dormancy (e.g., dropping leaves in fall/autumn, hence the name) and starch storage (in roots, stems, tubers like carrots or potatoes) allow plants to survive harsh cold periods and rapidly resume growth when conditions improve. Humans often exploit these storage structures for food.
Key Land Plant Features Summary
Chlorophylls a and b: Essential photosynthetic pigments. Green algae also use the full spectrum of visible light (ROY G BIV), which is readily available in shallow water and in air, supporting their ancestral link to land plants.
Note on Non-Green Plants: While most plants are green, some display other colors due to additional pigments (e.g., carotenoids in carrots). These variations are often adaptations, such as shade tolerance in rainforest understories where less direct sunlight (ROY G BIV) penetrates the canopy.
Starch Storage: A common strategy for energy reserve, allowing plants to survive unfavorable conditions (cold, dry) by entering dormancy.
Protected Gametophyte: The small, vulnerable haploid gametophyte is encased and supported within diploid sporophytic tissue, especially crucial due to its single gene set.
Stomata: For regulated gas exchange.
Waxy Surface (Cuticle): Reduces water loss or aids water shedding.
Root System: For anchoring and absorption of water and nutrients.
Conduction System: Specialized vascular tissues (xylem and phloem) for long-distance transport.
Support Tissues: Strong structural components like cellulose and lignin (forming wood) provide rigidity and allow for upright growth against gravity.
Plant Classification Overview
Botanical Terminology: Historically, botanists used "division" instead of "phylum." For this course, they are interchangeable.
Total Plant Species: There are approximately 300,000 known plant species globally.
Dominance of Angiosperms: A staggering 250,000 of these species are angiosperms (flowering plants), highlighting their current evolutionary success and dominance in the Cenozoic era. They have largely outcompeted other plant groups in diverse environments.
Nonvascular (Bryophyte) Plants
Characteristics:
Lack true vascular tissues (xylem and phloem), limiting their size and growth.
Gametophyte is the dominant generation: It is the larger, more visible, and longer-lived stage.
Sporophyte is tiny, ephemeral, and grows directly out of (and is dependent on) the gametophyte.
Require moist environments for reproduction and survival.
Typically small in stature.
Major Groups (Divisions/Phyla):
Bryophyta: Mosses.
Hepatophyta: Liverworts.
Anthocerophyta: Hornworts.
Note: Visual identification of these groups will not be required for the exam.
Seedless Vascular Plants
Characteristics:
The first plants to evolve true vascular tissues (xylem and phloem), allowing for efficient transport of water, nutrients, and sugars.
Sporophyte is the dominant generation: This marks a significant evolutionary shift from the nonvascular plants. The sporophyte is now the larger, independent, and more visible stage.
Can grow taller and larger due to vascular support.
Possess true leaves, which evolved from small, scale-like structures called microphylls.
Later, larger, more complex leaves known as megaphylls evolved (most modern plants have megaphylls).
Major Groups (Divisions/Phyla):
Lycophyta: Club mosses.
Possess true roots and microphylls.
Psilophyta: Whisk ferns.
Unique in lacking true roots and leaves.
Photosynthesis occurs primarily in their green stems.
Sphenophyta: Horsetails.
Genus Equisetum.
Named for their resemblance to a horse's tail (hence the connection to Equus, the genus for horses).