Chapter 29: Plants – Structure, Evolution, and Diversity

Plants: Adaptations and Basic Characteristics

Introduction to Plant Life on Land

Plants are multicellular, primarily photosynthetic organisms that have evolved a variety of adaptations to thrive on land. Their structural and physiological features enable them to survive in terrestrial environments.

• Cellulose and Lignin: Plants use tough materials like cellulose and lignin in their cell walls to provide structural support.

• Stomata: Specialized pores in leaves called stomata regulate water loss and gas exchange.

• Cuticle: Leaves are covered with a waxy cuticle to minimize water loss.

Primary and Secondary Plant Products

Plants produce a wide range of organic compounds through metabolic processes, which can be classified as primary or secondary products.

• Primary Products: Essential for plant growth and development, including amino acids, nucleic acids, and cellulose.

• Secondary Products: Often unique to specific species and serve ecological functions such as defense or attraction. Examples include toxins and waxes.

Plant Reproduction and Life Cycles

Multicellular Reproductive Organs

Plants have evolved complex reproductive structures to facilitate sexual reproduction.

• Gametangia: Multicellular organs that produce gametes (reproductive cells).

• Alternation of Generations: Plants exhibit a life cycle with two distinct multicellular forms:

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

  • Sporophyte: Multicellular, diploid (2n) generation that produces spores by meiosis.

Key Point: In plants and some algae, both haploid and diploid stages are multicellular, unlike animals, which lack a multicellular haploid stage.

Life Cycle Processes

• Meiosis: The sporophyte undergoes meiosis to produce haploid spores.

• Mitosis: The gametophyte produces gametes via mitosis.

• Fertilization: Male and female gametes fuse to form a diploid zygote.

Relatedness: Land plants are closely related to streptophyte green algae.

Plant Phylogeny and Major Groups

Streptophyte Green Algae

These are the closest algal relatives to land plants, sharing several key features such as cellulose synthesis and certain reproductive traits.

Bryophytes: Nonvascular Plants

Bryophytes are the earliest diverging group of land plants and include mosses, liverworts, and hornworts. They lack specialized vascular tissues, which limits their size and ability to transport water and nutrients.

• Examples: Mosses (Phylum Bryophyta), Liverworts (Phylum Hepatophyta), Hornworts (Phylum Anthocerophyta).

• Dominant Generation: The gametophyte is the dominant, longer-lived form.

• Key Structures:

  • Antheridium: Male gametangium that produces sperm.

  • Archegonium: Female gametangium that produces eggs.

  • Sporangium: Structure that produces spores.

Phylum Bryophyta (Mosses)

• Grow in mats in damp environments.

• Sphagnum moss is especially abundant and ecologically important.

Phylum Hepatophyta (Liverworts)

• Have lobed, liver-like structures.

• Can reproduce both sexually and asexually.

Phylum Anthocerophyta (Hornworts)

• Characterized by elongated, horn-shaped sporophytes.

Vascular Plants

Introduction to Vascular Tissue

Vascular plants possess specialized tissues for the transport of water, minerals, and nutrients, allowing them to grow larger and inhabit a wider range of environments.

• Xylem: Conducts water and minerals from roots to other parts of the plant; composed of dead cells.

• Phloem: Transports sugars, amino acids, and other organic nutrients; composed of living cells.

• Lignin and Cellulose: Provide structural support; lignified cells form fibers.

Dominant Generation: In all vascular plants, the sporophyte is the dominant form.

Seedless Vascular Plants

These plants reproduce via spores rather than seeds and include two major phyla.

• Phylum Lycophyta: Club mosses, many of which are epiphytes (grow on trees but are not parasitic).

• Phylum Pteridophyta: Includes ferns, horsetails, and whisk ferns.

Phylum Lycophyta (Club Mosses)

• Many are tropical epiphytes.

• Produce spores in structures called sporangia.

Phylum Pteridophyta

• Ferns: The most common seedless vascular plants, with about 12,000 species.

• Most are herbaceous, but some are tree ferns in the tropics.

• Leaves are called fronds.

• Sporangia are often clustered in groups called sori on the underside of fronds.

• Horsetails: Contain silica, giving them a rough texture ("scouring rushes").

• Whisk ferns: Lack true leaves and roots.

Comparison Table: Bryophytes vs. Vascular Plants

Feature	Bryophytes	Vascular Plants
Dominant Generation	Gametophyte	Sporophyte
Vascular Tissue	Absent	Present (xylem & phloem)
Size	Small, low-growing	Can be large
Examples	Mosses, liverworts, hornworts	Ferns, club mosses, horsetails

Summary of Plant Life Cycles

• All plants exhibit alternation of generations with multicellular haploid and diploid stages.

• Bryophytes: Dominant gametophyte, dependent sporophyte.

• Vascular plants: Dominant sporophyte, reduced gametophyte.

Key Equations

• Meiosis (sporophyte to spores):

• $2n \xrightarrow{\text{meiosis}} n$

• Fertilization (gametes to zygote):

• $n + n \xrightarrow{\text{fertilization}} 2n$

Example: Life Cycle of a Fern

• Sporophyte (2n) produces spores (n) by meiosis.

• Each spore grows into a gametophyte (n).

• Gametophyte produces gametes (n) by mitosis.

• Fertilization produces a new sporophyte (2n).

Additional info: Some details, such as the specific structure of the fern life cycle and the ecological roles of bryophytes, were inferred and expanded for academic completeness.