Plant Diversity II: Seed Plants, Gymnosperms, and Angiosperms

CHAPTER 30: PLANT DIVERSITY II

Introduction to Seed Plants

Seed plants represent a major evolutionary innovation in the plant kingdom, allowing plants to colonize a wide range of terrestrial environments. This chapter focuses on the diversity, life cycles, and adaptations of seed plants, including gymnosperms and angiosperms.

• Seed plants include gymnosperms (such as conifers) and angiosperms (flowering plants).

• Seeds provide protection and nourishment for the developing embryo, facilitating dispersal and survival in diverse habitats.

• Key evolutionary innovations include reduced gametophytes, heterospory, and the development of seeds and pollen.

Major Groups of Extant Plants

Ten Phyla of Extant Plants

Plants are classified into several phyla based on their evolutionary relationships and structural features. The following table summarizes the main phyla and their common names.

Phylum	Common Name	Number of Known Species
Hepatophyta	Liverworts	9,000
Bryophyta	Mosses	15,000
Anthocerophyta	Hornworts	100
Lycophyta	Lycophytes	1,200
Monilophyta	Monilophytes	12,000
Ginkgophyta	Ginkgo	1
Cycadophyta	Cycads	10
Gnetophyta	Gnetophytes	75
Coniferophyta	Conifers	600
Anthophyta	Flowering plants (Angiosperms)	250,000

Additional info: The table above is reconstructed from fragmented notes and standard biology sources.

Key Innovations in Seed Plant Evolution

Three Major Innovations

Seed plants exhibit three major evolutionary innovations that distinguish them from earlier plant groups:

• Further reduction of the gametophyte: The gametophyte generation is greatly reduced and dependent on the sporophyte, providing protection from environmental stress.

• Variation in spore size (heterospory): Seed plants produce two types of spores—megaspores (female) and microspores (male)—leading to the development of separate male and female gametophytes.

• Seeds: A seed consists of an embryo, a food supply, and a protective coat, allowing for enhanced survival and dispersal.

Seed Plant Reproduction

Pollen and Male Gametophyte

Pollen is a key adaptation in seed plants, allowing for fertilization without the need for water.

• Pollen grain: The male gametophyte is encased in a tough sporopollenin coat, protecting it during transport.

• Pollen is transported by wind or animals, facilitating reproduction over long distances.

• Male gametophyte produces sperm, which fertilizes the egg within the female gametophyte.

Seed Structure and Function

Seeds are complex structures that provide protection and nourishment to the developing plant embryo.

• Seed components:

  • Embryo (2n)

  • Food reserves (n)

  • Seed coat (2n)

• Seeds can remain dormant until conditions are favorable for germination.

• Seeds facilitate dispersal and colonization of new habitats.

Gymnosperms

Characteristics and Life Cycle

Gymnosperms are seed plants that produce "naked seeds" not enclosed in fruit. They are well adapted to dry environments.

• Major clades: Conifers, cycads, ginkgo, and gnetophytes.

• Seeds develop on the surface of cone scales rather than within an ovary.

• Life cycle includes production of male and female cones, pollination, fertilization, and seed development.

Example: Pine trees produce male cones (pollen) and female cones (ovules); fertilization leads to seed formation.

Adaptations to Dry Habitats

• Thick seed coats and reduced gametophytes protect against desiccation.

• Pollen eliminates the need for water in fertilization.

• Seeds can remain viable for extended periods, allowing for dispersal and survival in harsh conditions.

Angiosperms

Flowering Plants and Their Innovations

Angiosperms are the most diverse group of plants, characterized by flowers and fruits that aid in reproduction and dispersal.

• Flowers are composed of up to four rings of modified leaves (sporophylls):

  • Sepals (usually green)

  • Petals (often brightly colored to attract pollinators)

  • Stamens (produce pollen)

  • Carpels (contain ovary, style, and stigma)

• Fruits develop from the ovary and aid in seed dispersal; they can be fleshy or dry.

• Pollination is often facilitated by animals, leading to co-evolution and specialization.

Angiosperm Life Cycle

The angiosperm life cycle involves double fertilization, where one sperm fertilizes the egg and another fuses with two nuclei to form endosperm (food supply).

• Ovule contains the female gametophyte.

• Pollen tube delivers sperm to the ovule.

• Double fertilization results in a diploid embryo and triploid endosperm.

Equation:

$\text{Egg} (n) + \text{Sperm} (n) \rightarrow \text{Zygote} (2n)$

$\text{Central cell} (2n) + \text{Sperm} (n) \rightarrow \text{Endosperm} (3n)$

Angiosperm Phylogeny

Angiosperms form a monophyletic group, with fossil evidence indicating their ancient origins and evolutionary relationships.

• Phylogenetic studies use morphological and molecular data to reconstruct evolutionary history.

• Some groups, such as Amborella, are considered basal angiosperms.

Plant-Animal Interactions

Coevolution and Adaptations

Plants and animals have coevolved, leading to specialized relationships and adaptations for defense and dispersal.

• Plants produce chemical defenses (e.g., caffeine, nicotine, morphine, cyanide) and physical defenses (thorns, silica crystals).

• Many plants are adapted for animal-mediated seed dispersal.

• Specialized relationships between pollinators and flowering plants contribute to biodiversity.

Importance of Plants

Ecological and Economic Roles

Plants are essential for life on Earth, providing food, materials, and ecosystem services.

• Plants supply 80% of human calories (mainly rice, corn, and wheat).

• They provide food for livestock, wood for shelter and fuel, fibers for clothing, and medicinal compounds.

• Plants play a key role in nutrient cycling and carbon fixation, helping regulate atmospheric CO2.

Additional info: Some details on plant uses and ecological roles are inferred from standard biology curriculum.