Plant Diversity II: The Evolution of Seed Plants

Plant Diversity II: The Evolution of Seed Plants

Introduction

This chapter explores the evolutionary innovations that allowed seed plants to become the dominant producers in most terrestrial ecosystems. It covers the origin, structure, and diversity of seed plants, focusing on gymnosperms and angiosperms, and their reproductive adaptations.

Seed Plants: Key Innovations

Seeds and Adaptations for Life on Land

• Seed: A structure containing an embryo and a food supply, surrounded by a protective coat. Seeds can disperse over long distances and remain dormant until conditions are favorable for germination.

• Other Adaptations: Reduced gametophytes, heterospory, ovules, and pollen are common features of seed plants.

Reduced Gametophytes

• Gametophytes of seed plants are microscopic and develop within the walls of spores retained within tissues of the parent sporophyte.

• This arrangement protects gametophytes from environmental stress and allows them to obtain nutrients from the sporophyte.

Heterospory

• Homosporous plants produce one type of spore, usually giving rise to a bisexual gametophyte.

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

• Seed plants are heterosporous.

Ovules and Production of Eggs

• An ovule consists of a megasporangium, a megaspore, and one or more protective integuments.

• Gymnosperm ovules have one integument; angiosperm ovules usually have two.

Pollen and Production of Sperm

• A pollen grain consists of a male gametophyte enclosed within a pollen wall.

• Pollination is the transfer of pollen to the part of a seed plant containing the ovules.

• Pollen eliminates the need for water in fertilization and can be dispersed by air or animals.

Evolutionary Advantages of Seeds

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

• They contain a stored food supply for the developing embryo.

• Seeds are more resistant to harsh environments compared to spores.

Gymnosperms: "Naked Seed" Plants

Characteristics of Gymnosperms

• Gymnosperms bear "naked" seeds, typically on cones (strobili).

• Seeds are exposed on sporophylls; they are not enclosed in ovaries (unlike angiosperms).

• Most gymnosperms are cone-bearing plants called conifers.

Life Cycle of a Pine (Gymnosperm Example)

• The pine tree is the sporophyte and produces sporangia in male and female cones.

• Male cones produce microspores that develop into pollen grains (male gametophytes).

• Female cones produce megaspores that develop into female gametophytes within ovules.

• Pollination occurs when pollen is transferred to the ovule; fertilization leads to seed development.

• It takes nearly three years from cone formation to mature seed production.

Gymnosperm Diversity

• Four phyla of gymnosperms:

  • Cycadophyta (cycads): Large cones, palmlike leaves, flagellated sperm.

  • Ginkgophyta: One living species (Ginkgo biloba), flagellated sperm, tolerant to pollution.

  • Gnetophyta: Three genera (Gnetum, Ephedra, Welwitschia), diverse habitats.

  • Coniferophyta (conifers): Largest phylum, mostly evergreens, woody or fleshy cones.

Angiosperms: Flowering Plants

Characteristics of Angiosperms

• Angiosperms are seed plants with reproductive structures called flowers and fruits.

• They are the most widespread and diverse group of plants.

• Classified in a single phylum: Anthophyta.

Flowers

• 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, where pollen is produced).

  • Carpels: Female organs (ovary, style, stigma).

• Flowers can have radial or bilateral symmetry.

Fruits

• A fruit is formed when the ovary wall thickens and matures after fertilization.

• Fruits protect seeds and aid in their dispersal (by wind, water, or animals).

• Fruits can be fleshy (e.g., tomato, nectarine) or dry (e.g., milkweed, hazelnut).

Angiosperm Life Cycle

• Flowers contain both male and female structures.

• Male gametophytes are in pollen grains; female gametophytes (embryo sacs) develop in ovules within the ovary.

• Pollination leads to fertilization, which involves double fertilization:

  • One sperm fertilizes the egg (forms zygote).

  • Other sperm combines with two nuclei to form triploid endosperm (food for embryo).

• Embryo develops within a seed, which contains one or two seed leaves (cotyledons).

Evolutionary Links with Animals

• Animals influence plant evolution (e.g., herbivory selects for plant defenses).

• Pollinator-plant interactions drive coevolution and can increase speciation rates.

• Flower symmetry can affect pollinator behavior and pollen transfer specificity.

Angiosperm Diversity

Major Groups of Angiosperms

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

• Magnoliids: Woody and herbaceous plants (e.g., magnolias).

• Monocots: One cotyledon, parallel leaf veins, scattered vascular tissue, fibrous roots, floral organs in multiples of three (e.g., orchids, grasses, palms).

• Eudicots: Two cotyledons, netlike leaf veins, vascular tissue in a ring, taproot, floral organs in multiples of four or five (e.g., legumes, roses, oaks).

Feature	Monocots	Eudicots
Embryos	One cotyledon	Two cotyledons
Leaf venation	Parallel veins	Netlike veins
Stems	Vascular tissue scattered	Vascular tissue in ring
Roots	Fibrous (no main root)	Taproot (main root) usually present
Pollen	One opening	Three openings
Floral organs	Multiples of three	Multiples of four or five

Human Uses and Conservation

• Plants provide medicines, food, and ecosystem support for animals.

• Loss of plant species threatens biodiversity and the discovery of new medicinal compounds.

Examples of Plant-Derived Medicines

Compound	Source	Use
Atropine	Belladonna plant	Eye pupil dilator
Digitalin	Foxglove	Heart medication
Menthol	Eucalyptus tree	Throat soother
Quinine	Cinchona tree	Malaria preventive
Taxol	Pacific yew	Ovarian cancer drug
Vinblastine	Periwinkle	Leukemia drug

Additional info: The images provided (textbook cover and volcanic landscape) are likely used to introduce the topic and illustrate plant colonization of new environments, such as after volcanic eruptions.