Plant Diversity I: How Plants Colonized Land (Chapter 29 Study Notes)

Plant Diversity I: How Plants Colonized Land

Introduction

This chapter explores the evolutionary origins of land plants, their adaptations for terrestrial life, and the major groups of spore-bearing plants. Understanding these concepts is fundamental for studying plant biology and the diversity of life on Earth.

Evolutionary Origins of Land Plants

Plants Evolved from Green Algae

Land plants are believed to have evolved from green algae, specifically a group called charophytes. This evolutionary transition required several adaptations to survive outside aquatic environments.

• Charophytes: A group of green algae considered the closest relatives to land plants.

• Shared Ancestral Traits:

  • Chloroplasts containing chlorophyll a and b

  • Rings of cellulose-synthesizing proteins in cell membranes

  • Similar structure of flagellated sperm

Additional info: The evolutionary tree (phylogeny) shows that land plants (embryophytes) are nested within the green algae lineage, with charophytes as their closest living relatives.

Adaptations for Life on Land

Transitioning from water to land posed challenges such as desiccation, structural support, and reproduction. Plants developed several key adaptations:

• Sporopollenin: A durable polymer that covers zygotes and spores, preventing them from drying out.

• Benefits of Moving to Land: Increased sunlight, more carbon dioxide, and initially fewer herbivores and pathogens.

Derived Traits of Land Plants

Key Innovations

Land plants possess several derived traits that distinguish them from their algal ancestors:

• Alternation of Generations: Life cycle alternates between multicellular haploid (gametophyte) and diploid (sporophyte) generations.

• Multicellular, Dependent Embryos: Embryos develop within the tissues of the female parent, receiving nutrients via placental transfer cells.

• Walled Spores Produced in Sporangia: Spores are encased in protective walls and produced in specialized organs called sporangia.

• Multicellular Gametangia: Organs that produce gametes; archegonia (female, produce eggs) and antheridia (male, produce sperm).

• Apical Meristems: Regions of cell division at the tips of roots and shoots, enabling growth and adaptation to terrestrial environments.

Alternation of Generations

Life Cycle Overview

Alternation of generations is a reproductive cycle unique to land plants:

• Gametophyte (n): Haploid generation that produces gametes by mitosis.

• Sporophyte (2n): Diploid generation that produces haploid spores by meiosis.

• Fusion of Gametes: Sperm and egg unite to form a diploid zygote, which develops into the sporophyte.

• Meiosis: Occurs in the sporophyte, resulting in spores that develop into gametophytes.

Equation:

$ext{Sporophyte (2n)} \xrightarrow{meiosis} \text{Spores (n)} \xrightarrow{mitosis} \text{Gametophyte (n)} \xrightarrow{mitosis} \text{Gametes (n)} \xrightarrow{fertilization} \text{Zygote (2n)}$

Additional info: In most nonvascular plants, the gametophyte is the dominant, photosynthetic life stage.

Major Groups of Extant Plants

Classification Overview

Land plants are classified into several major groups based on the presence of vascular tissue and seeds:

• Nonvascular Plants (Bryophytes): Lack vascular tissue; include liverworts, mosses, and hornworts.

• Vascular Plants:

  • Seedless (Spore-bearing): Include lycophytes and monilophytes (ferns and their relatives).

  • Seed-producing: Gymnosperms and angiosperms (covered in Chapter 30).

Table: Ten Phyla of Extant Plants

The following table summarizes the major plant phyla and their approximate number of known species:

Group	Phylum	Common Name	Number of Known Species
Nonvascular Plants	Hepatophyta	Liverworts	9,000
Nonvascular Plants	Bryophyta	Mosses	15,000
Nonvascular Plants	Anthocerophyta	Hornworts	100
Seedless Vascular Plants	Lycophyta	Lycophytes	1,200
Seedless Vascular Plants	Monilophyta	Monilophytes (ferns, horsetails)	12,000
Seed Plants (Gymnosperms)	Ginkgophyta	Ginkgo	1
Seed Plants (Gymnosperms)	Cycadophyta	Cycads	130
Seed Plants (Gymnosperms)	Gnetophyta	Gnetophytes	70
Seed Plants (Gymnosperms)	Coniferophyta	Conifers	500
Seed Plants (Angiosperms)	Anthophyta	Flowering plants	250,000

Nonvascular Plants (Bryophytes)

Features and Life Cycle

Bryophytes are small, herbaceous plants that lack vascular tissue. They include three phyla: liverworts, mosses, and hornworts.

• Not Monophyletic: Bryophytes do not form a single evolutionary lineage.

• Dominant Growth Form: The gametophyte is the dominant, photosynthetic stage.

• Examples:

  • Marchantia (liverwort)

  • Anthoceros (hornwort)

  • Polytrichum commune (hairy-cap moss)

• Life Cycle: Water is required for sperm to swim to the egg; sporophytes are dependent on gametophytes for nutrition.

Additional info: Bryophytes are typically short because they lack lignified vascular tissue for structural support and do not have true roots.

Vascular Seedless Plants

Derived Characteristics

Seedless vascular plants possess several key innovations that allow them to grow larger and inhabit diverse terrestrial environments:

• Sporophyte Dominant Life Cycle: The sporophyte is the main, photosynthetic stage.

• Vascular Tissue:

  • Xylem: Conducts water and minerals from roots to shoots.

  • Phloem: Distributes sugars, amino acids, and other organic products throughout the plant.

• Roots: Anchor the plant and absorb water and nutrients from the soil.

• Leaves: Increase surface area for photosynthesis.

• Sporophylls: Specialized leaves that bear sporangia and produce spores.

Major Groups:

• Phylum Lycophyta: Club mosses, spike mosses, quillworts

• Phylum Monilophyta: Ferns, horsetails, whisk ferns

Additional info: Vascular tissue provides structural support and efficient transport, allowing these plants to grow taller and colonize new habitats.

Summary Table: Comparison of Plant Groups

Feature	Bryophytes (Nonvascular)	Seedless Vascular Plants
Dominant Generation	Gametophyte	Sporophyte
Vascular Tissue	Absent	Present (xylem & phloem)
Roots	Absent (rhizoids only)	Present
Leaves	Absent or simple	Well-developed
Reproduction	Water required for fertilization	Water required for fertilization

Key Terms

• Charophyte: Closest algal relative to land plants

• Sporopollenin: Protective polymer in spores and pollen

• Gametophyte: Haploid, gamete-producing generation

• Sporophyte: Diploid, spore-producing generation

• Sporangium: Structure where spores are produced

• Archegonium: Female gametangium (produces eggs)

• Antheridium: Male gametangium (produces sperm)

• Apical Meristem: Region of growth at plant tips

• Sporophyll: Spore-bearing leaf