Plant Diversity II: The Evolution of Seed Plants (Chapter 30 Study Notes)

Plant Diversity II: The Evolution of Seed Plants

Five Key Events in Land Plant Evolution

The evolution of land plants is marked by several major innovations that enabled their diversification and adaptation to terrestrial environments.

• Multicellularity (1.4 Bya): The origin of multicellular organisms allowed for increased complexity and specialization.

• Invasion of Land (470 Mya): Plants colonized terrestrial habitats, adapting to new environmental challenges.

• Vascular Tissue (425 Mya): The development of xylem and phloem enabled efficient transport of water and nutrients.

• Seed Plants (360 Mya): Seeds provided protection and nourishment for the embryo, enhancing survival and dispersal.

• Flowers (140 Mya): The evolution of flowers facilitated more efficient reproduction and diversification, especially in angiosperms.

Classification of Extant Plant Phyla

Plants are classified into several phyla based on their structural and reproductive features.

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

Additional info: Flowering plants (angiosperms) make up about 90% of all extant plant species.

Three Evolutionary Innovations of Seed Plants

Seed plants exhibit several key evolutionary adaptations that distinguish them from earlier plant groups.

• Reduction of Gametophyte Stage: The gametophyte is reduced and protected within the sporophyte tissue.

• Variation in Spore Size: Seedless plants are typically homosporous (one spore type), while seed plants are heterosporous (distinct male and female spores).

• Seeds: Seeds consist of an embryo, a food supply, and a protective coat, enhancing survival and dispersal.

Megaspores are produced by megasporangia (female), and microspores by microsporangia (male).

Male Gametophyte = Pollen

Pollen represents the male gametophyte in seed plants, enabling fertilization without water.

• Seedless Plants: Flagellated sperm swim from the antheridium to the archegonium.

• Seed Plants: Microspores develop into pollen grains, which are transported by wind or animals to the female reproductive structure.

• Pollination: The process by which pollen is transferred to the female structure, enabling fertilization.

Two Major Extant Clades of Seed Plants

Seed plants are divided into two major clades: gymnosperms and angiosperms.

• Gymnosperms: "Naked seeds" not enclosed in fruits; seeds often develop on cones. Includes cycads, ginkgo, gnetophytes, and conifers.

• Angiosperms: Flowering plants with seeds enclosed in fruits. Includes basal angiosperms, magnoliids, monocots, and eudicots.

Gymnosperm Life Cycle and Adaptations

The gymnosperm life cycle is adapted for survival in drier habitats, with several key features.

• Sporophyte Dominance: The mature pine tree is the sporophyte.

• Pollen Cones: Male cones produce microsporocytes, which develop into pollen.

• Ovule Cones: Female cones produce megasporocytes, which develop into eggs.

• No Need for Swimming: Pollination replaces the need for water in fertilization.

• Seed Structure: Seeds contain an embryo (2n), food reserves (n), and a maternal seed coat (2n).

• Life Cycle Duration: The gymnosperm life cycle can take up to 3 years.

From Ovule to Seed in Gymnosperms

Ovules develop into seeds following fertilization, with distinct tissue layers and developmental steps.

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

• Fertilization: Egg (n) produced by female gametophyte is fertilized by sperm (n) from pollen via a pollen tube.

• Seed Formation: The resulting seed contains a sporophyte embryo (2n) encased in maternal tissue.

Advantages of Seeds

Seeds provide several advantages over spores, contributing to the success of seed plants.

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

• Protection and Nourishment: Seeds have multicellular coats and a store of food for the embryo.

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

Unique Dispersal Methods

Seed plants have evolved a variety of dispersal mechanisms, including wind, water, and animal-mediated dispersal.

• Wind Dispersal: Seeds may have wings or other adaptations for wind transport.

• Animal Dispersal: Fruits and seeds may be adapted for transport by animals.

Angiosperms: Flowers and Fruits

Angiosperms have specialized reproductive structures that enhance their evolutionary success.

• 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 produce pollen.

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

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

Angiosperm Life Cycle

The angiosperm life cycle is highly specialized, involving double fertilization and the formation of endosperm.

• Double Fertilization: One sperm fertilizes the egg (embryo, 2n), another fuses with two nuclei in the central cell to form endosperm (3n).

• Endosperm: Provides food supply for the developing seed.

$\text{Double fertilization:} \quad 1\;\text{sperm} + 1\;\text{egg} \rightarrow \text{embryo} (2n) \quad 1\;\text{sperm} + 2\;\text{central nuclei} \rightarrow \text{endosperm} (3n)$

Angiosperm Phylogeny

Angiosperms are a monophyletic group, meaning they share a common ancestor. Extinct groups like Bennettitales had flower-like reproductive structures.

Plant-Animal Interactions

Plants and animals have co-evolved, leading to specialized relationships and adaptations.

• Herbivory: Plants have chemical (e.g., caffeine, nicotine, cyanide) and mechanical (e.g., thorns, silica crystals) defenses against herbivores.

• Pollination: Many fruits and flowers are adapted for animal transport and pollination.

• Co-evolution: The diversity of flowering plants may be explained by their co-evolution with pollinators.

Uses of Plants

Plants are essential for human life and ecosystem function.

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

• Wood and Fibers: Used for shelter, fuel, clothing, and other purposes.

• Medicines: Many herbal medicines are derived from plants.

• Nutrient Cycling: Plants play a key role in cycling nutrients and fixing carbon dioxide.