Plant Diversity I: How Plants Colonized Land

Plant Diversity I: How Plants Colonized Land

Introduction to Plant Evolution

Plants are a diverse group of organisms that have played a crucial role in shaping Earth's terrestrial environments. The colonization of land by plants was a major evolutionary event, leading to the development of complex terrestrial ecosystems.

• Early Earth: For much of Earth's history, terrestrial surfaces were lifeless. Cyanobacteria and protists likely existed on land about 1.2 billion years ago, but true plants, fungi, and animals emerged within the last 500 million years.

• Plant Diversity: Since colonizing land, plants have diversified into more than 290,000 living species, most of which are terrestrial, though some have returned to aquatic habitats.

• Importance: Plants supply oxygen and are the ultimate source of most food eaten by land animals.

Evolutionary Origins of Plants

Plants and Green Algae

Plants evolved from green algae, specifically a group called charophytes, which are their closest living relatives.

• Morphological and Molecular Evidence: Key traits shared between plants and charophytes include rings of cellulose-synthesizing proteins, structure of flagellated sperm, and formation of a phragmoplast during cell division.

• Genetic Evidence: Comparisons of nuclear, chloroplast, and mitochondrial DNA support the close relationship between plants and certain charophyte genera.

• Common Ancestry: Plants are not descended from modern charophytes but share a common ancestor with them.

Adaptations for Life on Land

Transitioning from water to land required several adaptations to overcome challenges such as desiccation and lack of structural support.

• Sporopollenin: A durable polymer that prevents exposed zygotes from drying out; found in both charophytes and plant spore walls.

• Benefits of Land: Unfiltered sunlight, more plentiful CO2, and nutrient-rich soil.

• Challenges: Scarcity of water and lack of structural support against gravity.

Derived Traits of Plants (Embryophytes)

Several key traits distinguish plants from their algal relatives. These adaptations are found in nearly all plants but are absent in charophytes.

• Alternation of Generations: Plants alternate between two multicellular generations—a haploid gametophyte and a diploid sporophyte.

• Multicellular, Dependent Embryos: The diploid embryo is retained within the tissue of the female gametophyte and receives nutrients via placental transfer cells. This is why plants are called embryophytes.

• Walled Spores Produced in Sporangia: Sporophytes produce spores in organs called sporangia. Sporopollenin in spore walls makes them resistant to harsh environments.

• Multicellular Gametangia: Gametes are produced within multicellular organs—archegonia (female, produce eggs) and antheridia (male, produce sperm).

• Apical Meristems: Regions of cell division at the tips of roots and shoots that allow for continual growth.

Additional Derived Traits

• Cuticle: A waxy covering of the epidermis that prevents water loss.

• Stomata: Specialized pores that allow for gas exchange.

• Mycorrhizae: Symbiotic associations with fungi that help plants obtain nutrients.

Major Groups of Plants

Classification of Extant Plants

Plants are classified into several major groups based on the presence or absence of vascular tissue and seeds.

Group	Phylum	Common Name	Number of Species
Nonvascular Plants (Bryophytes)	Hepatophyta	Liverworts	9,000
	Bryophyta	Mosses	15,000
	Anthocerophyta	Hornworts	100
Seedless Vascular Plants	Lycophyta	Lycophytes	1,200
	Monilophyta	Monilophytes (ferns, horsetails, whisk ferns)	12,000
Seed Plants: Gymnosperms	Ginkgophyta	Ginkgo	1
	Cycadophyta	Cycads	130
	Gnetophyta	Gnetophytes	70
	Coniferophyta	Conifers	600
Seed Plants: Angiosperms	Anthophyta	Flowering plants	250,000

Bryophytes: Nonvascular Plants

Life Cycles Dominated by Gametophytes

Bryophytes (liverworts, mosses, hornworts) are nonvascular plants with life cycles dominated by the gametophyte stage.

• Gametophyte: The dominant, longer-living stage; produces gametes by mitosis.

• Sporophyte: Smaller, dependent on the gametophyte; consists of a foot, seta (stalk), and sporangium (capsule).

• Reproduction: Sperm are flagellated and require water to reach the egg. Some bryophytes can also reproduce asexually via brood bodies.

Ecological and Economic Importance of Mosses

• Habitat: Mosses inhabit diverse environments, especially moist forests and wetlands.

• Soil Health: Some mosses reduce nitrogen loss and harbor nitrogen-fixing cyanobacteria.

• Peat Moss (Sphagnum): Forms peatlands, which store carbon and can be used as fuel. The low temperature, pH, and oxygen levels in peatlands inhibit decay, preserving organic material.

Seedless Vascular Plants

Key Traits of Vascular Plants

Vascular plants have specialized tissues for transporting water and nutrients, allowing them to grow taller and colonize new environments.

• Dominant Sporophyte: The sporophyte is the larger, more complex generation.

• Vascular Tissues: Xylem conducts water and minerals (contains lignin for support); phloem distributes sugars and organic products.

• Roots: Anchor plants and absorb water/nutrients; may have evolved from subterranean stems.

• Leaves: Increase surface area for photosynthesis. Two types: microphylls (small, single vein) and megaphylls (large, branched veins).

• Sporophylls: Modified leaves that bear sporangia; clusters called sori (in ferns) or strobili (cone-like structures).

Spore Variations

• Homosporous: Most seedless vascular plants produce one type of spore, which develops into a bisexual gametophyte.

• Heterosporous: Seed plants and some seedless vascular plants produce two types of spores: megaspores (female gametophytes) and microspores (male gametophytes).

Classification of Seedless Vascular Plants

• Phylum Lycophyta: Includes club mosses, spike mosses, and quillworts. Ancient lycophyte trees were prominent in Carboniferous swamps.

• Phylum Monilophyta: Includes ferns (most widespread), horsetails (now only genus Equisetum), and whisk ferns (resemble ancestral vascular plants).

Significance of Seedless Vascular Plants

• First Forests: Ancestors of modern lycophytes, horsetails, and ferns formed the first forests during the Devonian and Carboniferous periods.

• Coal Formation: Decaying plants from these forests contributed to the formation of coal over millions of years.

Additional info: The above notes synthesize and expand upon the provided lecture slides and textbook excerpts, filling in context for key biological concepts and terminology relevant to General Biology students.