Plant Diversity I: How Plants Colonized Land

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Plant Diversity I: How Plants Colonized Land

Introduction to Plant Diversity and Land Colonization

Plants are essential to terrestrial ecosystems, providing oxygen and serving as the primary source of food for land animals. The colonization of land by plants was a major evolutionary event, leading to the diversification of more than 290,000 living species. Most present-day plants live on land, although some have returned to aquatic habitats. Plants are distinct from photosynthetic protists and are classified in the kingdom Plantae.

Key Point: Plants supply oxygen and are the ultimate source of food for land animals.
Key Point: The majority of plant species are terrestrial, with only a few aquatic exceptions.
Example: Ferns, mosses, and flowering plants are common land plants.

Evolutionary Origins of Plants

Plants evolved from green algae, specifically charophytes, which are their closest living relatives. Morphological and molecular evidence supports this relationship, including similarities in cellulose-synthesizing proteins, flagellated sperm structure, and formation of a phragmoplast during cell division.

Key Point: Charophytes share several key traits with land plants, but plants are not descended from modern charophytes; they share a common ancestor.
Key Point: Comparisons of nuclear, chloroplast, and mitochondrial DNA support the close relationship between charophytes and plants.
Example: Genera such as Chara and Coleochaete are considered closest relatives to land plants.

Adaptations for Life on Land

Transitioning from aquatic to terrestrial environments required several adaptations. One key adaptation is the development of sporopollenin, a durable polymer that prevents zygotes and spores from drying out. Plants also evolved structures for support and nutrient acquisition.

Key Point: Sporopollenin is found in plant spore walls and charophyte zygotes, providing resistance to desiccation.
Key Point: Benefits of land colonization include access to unfiltered sunlight, abundant CO2, and nutrient-rich soil.
Key Point: Challenges include scarcity of water and lack of structural support against gravity.
Additional info: The cuticle (waxy covering), stomata (gas exchange), and mycorrhizae (fungal associations) are further adaptations for terrestrial life.

Key Traits of Land Plants

Land plants possess several derived traits not found in charophytes. These include alternation of generations, multicellular dependent embryos, walled spores produced in sporangia, multicellular gametangia, and apical meristems.

Alternation of Generations: Plants alternate between haploid gametophyte and diploid sporophyte generations.
Multicellular, Dependent Embryos: Embryos are retained within female gametophyte tissue and receive nutrients via placental transfer cells.
Walled Spores Produced in Sporangia: Sporophytes produce spores in sporangia; sporopollenin in spore walls provides protection.
Multicellular Gametangia: Archegonia produce eggs; antheridia produce sperm. Fertilization occurs within the archegonium.
Apical Meristems: Regions of cell division at the tips of roots and shoots enable continuous growth.

Classification and Diversity of Plants

Plants are classified into several major groups based on the presence or absence of vascular tissue and seeds. The following table summarizes the ten phyla of extant plants:

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

Additional info: Table numbers inferred from context; some numbers may vary by source.

Bryophytes: Nonvascular Plants

Bryophytes include liverworts, mosses, and hornworts. Their life cycles are dominated by the gametophyte generation, which is longer-living and larger than the sporophyte. Bryophyte sporophytes are dependent on the gametophyte and are the smallest and simplest among plant groups.

Key Point: Bryophyte gametophytes produce flagellated sperm in antheridia and eggs in archegonia; fertilization requires water.
Key Point: Bryophytes can reproduce asexually via brood bodies.
Example: Marchantia polymorpha (liverwort), Polytrichum commune (moss), Anthoceors (hornwort).

Ecological and Economic Importance of Mosses

Mosses inhabit diverse environments, especially moist forests and wetlands. They help reduce nitrogen loss from soil and can harbor nitrogen-fixing cyanobacteria. Peat moss (Sphagnum) forms peatlands, which are important carbon reservoirs and sources of fuel.

Key Point: Peatlands inhibit decay due to low temperature, pH, and oxygen levels.
Example: Peat harvested for fuel; bog mummies preserved in peatlands.

Seedless Vascular Plants: Ferns and Relatives

Seedless vascular plants were the first to grow tall, enabled by vascular tissue. Their life cycles are dominated by the sporophyte generation. They possess flagellated sperm and are usually restricted to moist environments.

Key Point: Vascular tissues include xylem (water and minerals) and phloem (organic products).
Key Point: Roots anchor plants and absorb water/nutrients; leaves increase photosynthetic surface area.
Key Point: Leaves are classified as microphylls (single vein) or megaphylls (branched veins).
Example: Ferns (Monilophyta), club mosses (Lycophyta), horsetails, whisk ferns.

Reproductive Structures and Spore Variations

Seedless vascular plants can be homosporous (one type of spore) or heterosporous (megaspores and microspores). Sporophylls are modified leaves bearing sporangia; strobili are cone-like structures formed from groups of sporophylls.

Key Point: Homosporous plants produce bisexual gametophytes; heterosporous plants produce separate male and female gametophytes.
Example: Most ferns are homosporous; seed plants are heterosporous.

Significance of Seedless Vascular Plants

Ancestors of modern lycophytes, horsetails, and ferns formed the first forests during the Devonian and Carboniferous periods. Their decaying remains contributed to the formation of coal over millions of years.

Key Point: Seedless vascular plants played a major role in shaping Earth's carbon cycle and climate.
Example: Coal deposits originated from ancient plant material.

Summary Table: Major Plant Groups and Key Features

Group	Vascular Tissue	Dominant Generation	Seeds	Examples
Bryophytes	No	Gametophyte	No	Mosses, Liverworts, Hornworts
Seedless Vascular Plants	Yes	Sporophyte	No	Ferns, Club Mosses, Horsetails
Gymnosperms	Yes	Sporophyte	Yes (naked)	Conifers, Cycads, Ginkgo
Angiosperms	Yes	Sporophyte	Yes (enclosed in fruit)	Flowering plants

Key Equations and Terms

Alternation of Generations:

Sporopollenin: A durable polymer that protects spores and zygotes from desiccation.
Apical Meristem: Region of cell division at the tips of roots and shoots.
Stomata: Pores for gas exchange in plant tissues.