Plant Diversity II: Seed Plants, Gymnosperms, and Angiosperms

Chapter 30: Plant Diversity II

Introduction

This chapter explores the evolutionary innovations and diversity of seed plants, focusing on gymnosperms and angiosperms. It covers key events in land plant evolution, the life cycles of major clades, and the ecological and practical significance of plants.

Five Key Events in Land Plant Evolution

Major Evolutionary Milestones

• Multicellularity (1.4 billion years ago): The origin of multicellular organisms set the stage for complex plant life.

• Invasion of Land (470 million years ago): Plants adapted to terrestrial environments, leading to new forms and functions.

• Vascular Tissue (425 million years ago): Development of xylem and phloem enabled efficient transport of water and nutrients.

• Seed Plants (360 million years ago): Seeds provided protection and nourishment for the embryo, enhancing survival.

• Flowers (140 million years ago): The evolution of flowers facilitated efficient reproduction and diversification.

Classification of Extant Plant Phyla

Overview of Plant Diversity

Group	Phylum	Common Name	Number of Known Species
Nonvascular Plants	Hepatophyta	Liverworts	8,000
Nonvascular Plants	Bryophyta	Mosses	15,000
Nonvascular Plants	Anthocerophyta	Hornworts	100
Vascular Plants	Lycophyta	Lycophytes	1,200
Vascular Plants	Monilophyta	Monilophytes	12,000
Seed Plants (Gymnosperms)	Ginkgophyta	Ginkgo	1
Seed Plants (Gymnosperms)	Cycadophyta	Cycads	118
Seed Plants (Gymnosperms)	Gnetophyta	Gnetophytes	75
Seed Plants (Gymnosperms)	Coniferophyta	Conifers	600
Seed Plants (Angiosperms)	Anthophyta	Flowering plants	250,000

Additional info: Angiosperms comprise about 90% of all extant plant species.

Evolutionary Innovations of Seed Plants

Key Adaptations

• Reduction of Gametophyte Stage: Seed plants have a dominant sporophyte stage, with gametophytes reduced and protected within tissues.

• Variation in Spore Size:

  • Most seedless plants are homosporous (one spore type).

  • Seed plants are heterosporous (distinct male and female spores).

  • Megaspores (female) produced by megasporangia.

  • Microspores (male) produced by microsporangia.

• Seeds: Consist of an embryo, food supply, and protective coat.

Male Gametophyte: Pollen

Seedless vs. Seed Plants

• Seedless Plants: Flagellated sperm swim from antheridium (male) to archegonium (female).

• Seed Plants:

  • Male gametophyte is transported as a pollen grain (contains microspore in a sporopollenin coat).

  • Pollination: Transfer of pollen by wind or animals to female reproductive structures.

Major Extant Clades of Seed Plants

Gymnosperms and Angiosperms

• Gymnosperms: "Naked seeds" on cones.

  • Phylum Cycadophyta

  • Phylum Gnetophyta

  • Phylum Ginkgophyta

  • Phylum Coniferophyta

• Angiosperms: Flowering plants, seeds "contained" in fruits.

  • Phylum Anthophyta

  • Basal angiosperms (Amborella, water lilies, etc.)

  • Magnoliids

  • Monocots

  • Eudicots

Gymnosperm Life Cycle

Adaptations to Dry Habitats

• Pine tree is the mature sporophyte.

• Pollen cones (male) with microsporocytes; ovule cones (female) with megasporocytes.

• Microsporocyte develops into pollen (male gametophyte); megasporocyte develops into egg (female gametophyte).

• Pollination leads to fertilization; no need for swimming sperm.

• Seed contains embryo (2n), food reserves (n), and maternal seed coat (2n).

• Seed germinates to become seedling; whole cycle takes about 3 years.

From Ovule to Seed in Gymnosperms

Seed FormationA

• Ovule: Composed of megaspore, megasporangium, and integument (sporophyte tissue, 2n).

• Megaspore develops into female gametophyte (n), which produces egg (n).

• Egg fertilized by sperm (n) from pollen via pollen tube through micropyle.

• Seed: Sporophyte embryo (2n) encased in maternal sporophyte tissue.

Advantages of Seeds

Seed vs. Spore Dispersal

• Seeds are multicellular, have a protective coat, and store food.

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

• Seeds can be dispersed farther and remain viable longer than spores.

Unique Dispersal Methods

Seed Dispersal Adaptations

• Seeds may have wings, hooks, or other structures to aid dispersal by wind, water, or animals.

• Fruits can be fleshy or dry, and may be adapted for animal transport.

Angiosperms: Flowers and Fruits

Specialized Reproductive Structures

• Flowers: Composed of up to four rings of specialized leaves (sporophylls).

  • Sepals: Outermost ring, usually green.

  • Petals: Often brightly colored to attract pollinators.

  • Stamens: Male parts; anthers on filaments produce pollen.

  • Carpels: Female parts; sticky stigma binds pollen, style connects to ovary (with ovules).

• Fruits: Mature ovary; can be fleshy or dry, aid in seed dispersal.

Angiosperm Life Cycle

Highly Specialized Reproduction

• Female gametophyte has large central cell with two nuclei (n+n).

• Pollen releases two sperm:

  • One fertilizes the embryo (2n).

  • One fuses with two nuclei in central cell to form endosperm (3n).

• Endosperm provides food supply for the seed.

Angiosperm Phylogeny

Evolutionary Relationships

• Angiosperms are a monophyletic group (descended from a common ancestor).

• Bennettitales are an extinct group with flower-like reproductive structures.

Ecological and Practical Importance of Plants

Plants and Animals: Interdependence

• Herbivory: Animals feed on plants; plants have chemical (caffeine, nicotine, cyanide) and mechanical (thorns, silica crystals) defenses.

• Many fruits are adapted for animal transport.

• Plant-pollinator relationships may be highly specialized; co-evolution of insects and flowering plants may explain their diversity.

Uses of Plants

• Food: 80% of human calories come from six crops (rice, corn, wheat, etc.).

• Plants feed livestock, provide wood, fibers, herbal medicines, and aid in nutrient cycling.

• Plants fix carbon and use excess CO2.

Summary Table: Seed Plant Innovations

Innovation	Description	Benefit
Reduced Gametophyte	Gametophyte protected within sporophyte tissue	Protection from environmental stress
Heterospory	Separate male and female spores	Specialization of reproductive roles
Pollen	Male gametophyte in protective coat	Enables fertilization without water
Seeds	Embryo, food supply, protective coat	Enhanced dispersal and survival
Flowers/Fruits (Angiosperms)	Specialized reproductive structures	Efficient pollination and seed dispersal

Additional info: The co-evolution of plants and animals has driven the diversification and ecological success of angiosperms.