Plant Diversity II: The Evolution of Seed Plants – Study Notes

Chapter 30: Plant Diversity II – The Evolution of Seed Plants

Introduction to Seed Plants

Seed plants represent a major evolutionary advancement in the plant kingdom, allowing their bearers to become dominant producers in most terrestrial ecosystems. The origin of seed plants dates back approximately 360 million years, and their success is attributed to several key adaptations.

• Seed: A structure consisting of an embryo and nutrients surrounded by a protective coat. Seeds can disperse over long distances by wind or other means.

• Pollen: Enables fertilization without the need for water, facilitating reproduction in diverse environments.

• Reduced Gametophytes: Gametophytes are microscopic and develop within the tissues of the parent sporophyte, providing protection and nutrition.

Adaptations for Life on Land

Seed plants possess several adaptations that enhance survival and reproduction on land.

• Reduced Gametophytes: Develop within spores retained in the parent sporophyte, protecting them from environmental stress.

• Heterospory: Seed plants produce two types of spores: megaspores (female gametophytes) and microspores (male gametophytes).

• Ovules: Structures containing the female gametophyte, megasporangium, and protective integuments.

• Pollen: Male gametophyte enclosed within a pollen wall, allowing for dispersal by air or animals.

Gametophyte-Sporophyte Relationships

Seed plants exhibit a dominant sporophyte generation and highly reduced, dependent gametophytes.

• Sporophyte: The main, visible plant body.

• Gametophyte: Microscopic, dependent on the sporophyte for nutrition and protection.

Heterospory in Seed Plants

Heterospory is a defining feature of seed plants, involving the production of two distinct types of spores.

• Megasporangia: Produce megaspores → female gametophytes.

• Microsporangia: Produce microspores → male gametophytes.

• Megasporophylls: Modified leaves bearing megasporangia.

• Microsporophylls: Modified leaves bearing microsporangia.

Ovules and Egg Production

The ovule is a key reproductive structure in seed plants.

• Components: Megasporangium, megaspore, and one or more integuments.

• Gymnosperms: Typically have one integument.

• Angiosperms: Usually have two integuments.

Pollen and Sperm Production

Pollen grains are the male gametophytes of seed plants, enabling fertilization without water.

• Pollen Grain: Contains the male gametophyte, enclosed in a protective wall.

• Pollination: Transfer of pollen to the ovule-containing part of the plant.

• Pollen Tube: Grows from the pollen grain to deliver sperm to the female gametophyte.

Evolutionary Advantages of Seeds

Seeds offer several evolutionary benefits over spores.

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

• Stored Food: Seeds contain a supply of nutrients for the developing embryo.

• Protection: Seeds are encased in a protective coat.

Gymnosperms: "Naked Seeds"

Gymnosperms are seed plants whose seeds are not enclosed in ovaries; instead, they are exposed on sporophylls, typically forming cones.

• Conifers: The largest group of gymnosperms, including pines, firs, and redwoods.

• Other Gymnosperm Phyla: Cycadophyta, Ginkgophyta, Gnetophyta.

Table: Major Gymnosperm Phyla

Phylum	Key Features	Examples
Cycadophyta	Large cones, palmlike leaves, flagellated sperm	Cycas revoluta
Ginkgophyta	Single living species, flagellated sperm, pollution tolerant	Ginkgo biloba
Gnetophyta	Three genera, varied appearance, tropical/desert habitats	Gnetum, Ephedra, Welwitschia
Coniferophyta	Woody cones, mostly evergreen, photosynthesis year-round	Pine, fir, redwood

Gymnosperm Life Cycle (Pine Example)

The pine life cycle illustrates key gymnosperm features: reduced gametophytes, seed production, and pollen-mediated fertilization.

• Sporophyte: The pine tree, produces male and female cones.

• Male Cones: Bear microsporangia → microspores → pollen grains.

• Female Cones: Bear megasporangia → megaspores → female gametophytes.

• Fertilization: Takes about three years from cone appearance to mature seed dispersal.

Angiosperms: Flowering Plants

Angiosperms are seed plants with reproductive structures called flowers and fruits. They are the most widespread and diverse group of plants.

• Phylum: Anthophyta

• Key Adaptations: Flowers and fruits

Flower Structure and Function

Flowers are specialized shoots with up to four types of modified leaves (floral organs):

• Sepals: Enclose the flower

• Petals: Often brightly colored to attract pollinators

• Stamens: Male organs (filament + anther)

• Carpels: Female organs (ovary + style + stigma)

Ovules within the ovary develop into seeds after fertilization. The term pistil refers to one or more fused carpels.

Flower Symmetry

• Radial Symmetry: Multiple lines divide the flower into equal parts (e.g., daffodil).

• Bilateral Symmetry: Only one line divides the flower into equal parts (e.g., orchid).

Fruit Structure and Seed Dispersal

Fruits develop from the ovary wall and aid in seed protection and dispersal. Fruits can be fleshy (e.g., tomato, grapefruit, nectarine) or dry (e.g., milkweed, hazelnut).

• Dispersal Mechanisms: Wind, water, animals, explosive action, barbs, edible fruits.

Angiosperm Life Cycle

The angiosperm life cycle involves double fertilization and the formation of seeds and fruit.

• Male Gametophyte: Pollen grains produced in anthers.

• Female Gametophyte: Embryo sac develops in ovule within ovary.

• Double Fertilization: One sperm fertilizes the egg; another combines with two nuclei to form triploid endosperm.

• Seed Structure: Embryo (root + cotyledons), endosperm (nutrient tissue).

Evolutionary Links with Animals

Animals have influenced plant evolution through herbivory and pollination. Bilateral flower symmetry can restrict pollinator movement, increasing pollination specificity and potentially leading to speciation.

Angiosperm Diversity

Angiosperms comprise over 250,000 living species, divided into several major groups:

• Monocots: One cotyledon, parallel leaf veins, scattered vascular tissue, fibrous roots, floral organs in multiples of three.

• Eudicots: Two cotyledons, netlike leaf veins, vascular tissue in a ring, taproot system, floral organs in multiples of four or five.

• Basal Angiosperms: Oldest lineages (e.g., Amborella trichopoda, water lilies, star anise).

• Magnoliids: Woody and herbaceous plants, closely related to monocots and eudicots.

Table: Comparison of Monocots and Eudicots

Feature	Monocots	Eudicots
Embryos	One cotyledon	Two cotyledons
Leaf Venation	Parallel veins	Netlike veins
Vascular Tissue	Scattered	Arranged in ring
Root System	Fibrous (no main root)	Taproot (main root)
Pollen	One opening	Three openings
Floral Organs	Multiples of three	Multiples of four or five

Plant-Derived Medicines

Many important medicines are derived from plants, including compounds used for heart medication, cancer treatment, and muscle relaxation.

Threats to Plant Diversity

Loss of plant species threatens the animal species they support. Rapid habitat loss may result in the extinction of up to 50% of Earth's species within the next few centuries. Tropical rain forests may contain undiscovered medicinal compounds.

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