Plant Diversity I: How Plants Colonized Land

Chapter 29: Plant Diversity I – How Plants Colonized Land

Introduction to Plant Diversity

Plants are essential to life on Earth, 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 terrestrial ecosystems.

• Most present-day plants live on land, though some species have returned to aquatic habitats.

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

• Plants evolved from green algae, specifically charophytes, which are their closest relatives.

Evidence for Plant Evolution from Charophytes

Both morphological and molecular evidence support the relationship between plants and charophytes.

• Shared traits with charophytes:

  • Rings of cellulose-synthesizing proteins

  • Structure of flagellated sperm

  • Formation of a phragmoplast (structure involved in cell division)

• DNA comparisons indicate certain charophyte species are the closest living relatives of plants.

• Plants and charophytes share a common ancestor, but plants are not descended from modern charophytes.

Adaptations Enabling the Move to Land

Plants developed several adaptations to survive on land, which provided new opportunities and challenges.

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

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

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

Derived Traits of Land Plants

Key traits that distinguish land plants from their algal relatives:

• Alternation of generations

• Multicellular, dependent embryos

• Walled spores produced in sporangia

• Multicellular gametangia

• Apical meristems

Alternation of Generations

Plants alternate between two multicellular generations in their reproductive cycle.

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

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

• Fusion of gametes gives rise to the sporophyte.

Equation:

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

Multicellular, Dependent Embryos

The diploid embryo is retained within the tissue of the female gametophyte, receiving nutrients via placental transfer cells. This trait is why land plants are called embryophytes.

Walled Spores Produced in Sporangia

Sporophytes produce spores in organs called sporangia. Sporocytes undergo meiosis to generate haploid spores, which have walls containing sporopollenin for resistance to harsh environments.

Multicellular Gametangia

Gametes are produced within multicellular organs called gametangia.

• Archegonia: Female gametangia, produce nonmotile eggs.

• Antheridia: Male gametangia, produce and release sperm.

Apical Meristems

Plants sustain continual growth in length by repeated cell division within apical meristems, which differentiate into various tissues.

Additional Derived Traits

• Cuticle: Waxy covering of the epidermis to prevent water loss.

• Stomata: Specialized cells for gas exchange.

• Mycorrhizae: Symbiotic associations with fungi to aid nutrient absorption.

Origin and Diversification of Plants

Fossil evidence indicates that plants colonized land at least 470 million years ago. The appearance of plant spores and tissues in ancient rocks supports this timeline.

Major Groups 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	500
Angiosperms	Anthophyta	Flowering plants	250,000

Nonvascular Plants (Bryophytes)

Bryophytes include liverworts, mosses, and hornworts. They are thought to represent the earliest lineages to diverge from the common ancestor of land plants.

• Gametophyte-dominated life cycle: Gametophytes are longer-living and more prominent than sporophytes.

• Rhizoids: Anchor gametophytes to substrate; not true roots.

• Flagellated sperm: Require water for fertilization.

• Asexual reproduction: Some mosses produce brood bodies for cloning.

Sporophytes in Bryophytes

• Sporophytes are dependent on gametophytes for nutrition.

• Structure: Consist of foot, seta (stalk), and capsule (sporangium).

• Stomata: Present in moss and hornwort sporophytes, absent in liverworts.

Ecological and Economic Importance of Mosses

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

• Soil stabilization: Mosses help reduce nitrogen loss and harbor nitrogen-fixing cyanobacteria.

• Sphagnum (peat moss): Forms peatlands, stores carbon, and can be used as fuel.

Seedless Vascular Plants

Seedless vascular plants were the first to grow tall due to the development of vascular tissue. They include lycophytes and monilophytes (ferns, horsetails, whisk ferns).

• Dominant sporophyte generation: Sporophytes are larger and more complex than gametophytes.

• Vascular tissues: Xylem (water and minerals) and phloem (organic products).

• Roots and leaves: Well-developed for anchorage and photosynthesis.

• Sporophylls: Modified leaves bearing sporangia.

Vascular Tissue: Xylem and Phloem

• Xylem: Conducts water and minerals; contains tracheids strengthened by lignin.

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

Roots and Leaves

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

• Leaves: Increase surface area for photosynthesis.

• Types of leaves:

  • Microphylls: Small, single-veined leaves.

  • Megaphylls: Larger, highly branched vascular system.

Sporophylls and Spore Variations

• Sporophylls: Modified leaves with sporangia; clusters called sori in ferns.

• Strobili: Cone-like structures formed from groups of sporophylls.

• Homosporous plants: Produce one type of spore (most seedless vascular plants).

• Heterosporous plants: Produce megaspores (female gametophytes) and microspores (male gametophytes).

Classification of Seedless Vascular Plants

Clade	Phylum	Common Name	Key Features
Lycophytes	Lycophyta	Club mosses, spike mosses, quillworts	Small herbaceous plants; ancient trees formed coal deposits
Monilophytes	Monilophyta	Ferns, horsetails, whisk ferns	Most widespread seedless vascular plants; diverse in tropics and temperate forests

Significance of Seedless Vascular Plants

• Ancestors formed the first forests during the Devonian and Carboniferous periods.

• Decaying plants contributed to coal formation over millions of years.

Example: Sphagnum moss forms peatlands that store carbon and preserve ancient remains, such as bog mummies.

Additional info: The study notes above expand on brief points and images from the original materials, providing academic context and definitions for key terms and processes relevant to General Biology students.