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 for greater adaptation to terrestrial environments. This chapter explores the origin, structure, and diversity of seed plants, focusing on gymnosperms and angiosperms.

• Seed plants originated about 360 million years ago.

• A seed consists of an embryo and nutrients surrounded by a protective coat.

• Seeds can disperse over long distances by wind or other means.

• Key adaptations for life on land include seeds and pollen grains.

Key Features of Seed Plants

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

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

• Ovules: Structure consisting of the megasporangium, megaspore, and protective integuments.

• Pollen: Microspores develop into pollen grains, which contain the male gametophyte and can be dispersed by wind or animals.

Advantages of Seeds

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

• They contain stored food to nourish the developing embryo.

• Seeds provide protection and facilitate dispersal.

Gymnosperms

Characteristics of Gymnosperms

Gymnosperms are seed plants that bear "naked" seeds, typically on cones. Their seeds are not enclosed in ovaries, unlike angiosperms.

• Most gymnosperms are cone-bearing plants called conifers.

• They dominate many terrestrial ecosystems, especially in northern latitudes.

Gymnosperm Life Cycle

The life cycle of gymnosperms, such as pine trees, demonstrates key adaptations for terrestrial life.

• The sporophyte is the dominant generation and produces male and female cones.

• Male cones produce microspores that develop into pollen grains.

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

• Pollination occurs when pollen is transferred to ovules, leading to fertilization and seed development.

• Seed maturation can take several years, and seeds are dispersed by wind.

Diversity of Gymnosperms

Gymnosperms are divided into four major phyla, each with unique characteristics.

Phylum	Key Features	Examples
Cycadophyta	Large cones, palmlike leaves, flagellated sperm	Cycas revoluta, Encephalartos woodii
Ginkgophyta	Single living species, fan-shaped leaves, flagellated sperm	Ginkgo biloba
Gnetophyta	Three genera, varied appearance, some tropical/desert	Gnetum, Ephedra, Welwitschia
Coniferophyta	Largest phylum, mostly woody cones, evergreen	Pine, fir, Douglas fir, bristlecone pine

Angiosperms

Characteristics of Angiosperms

Angiosperms, or flowering plants, are the most diverse and widespread group of seed plants. They are classified in the phylum Anthophyta and have two key adaptations: flowers and fruits.

• Flowers: Specialized structures for sexual reproduction, often attracting pollinators.

• Fruits: Mature ovaries that protect seeds and aid in their dispersal.

Structure of Flowers

Flowers are composed of up to four types of modified leaves called floral organs.

• Sepals: Enclose and protect the flower before it opens.

• Petals: Often brightly colored to attract pollinators.

• Stamens: Male reproductive organs, consisting of a filament and anther (where pollen is produced).

• Carpels: Female reproductive organs, consisting of ovary, style, and stigma.

Flower Symmetry

• Radial symmetry: Multiple lines can 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 Adaptations

Fruits develop from the ovary wall and can be fleshy or dry, aiding in seed protection and dispersal.

Fruit Type	Description	Example
Fleshy, soft pericarp	Soft outer and inner layers	Tomato
Fleshy, firm outer layer	Firm outer, soft inner layer	Grapefruit
Fleshy, hard inner layer	Soft outer, hard inner (pit)	Nectarine
Dry, splits open	Opens at maturity	Milkweed
Dry, remains closed	Does not open at maturity	Hazelnut

• Fruit adaptations include mechanisms for wind, water, or animal dispersal.

Angiosperm Life Cycle

The angiosperm life cycle involves double fertilization and the development of seeds within fruits.

• Flowers contain both male and female structures.

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

• Pollination leads to fertilization, where one sperm fertilizes the egg and another forms endosperm ($3n$) for nourishment.

• Seeds contain the embryo (with cotyledons) and stored food.

Evolutionary Links with Animals

Animals have influenced plant evolution through herbivory and pollination, leading to co-evolution and specialized adaptations.

• Pollinator specificity can increase speciation rates in plants.

• Flowers with bilateral symmetry can restrict pollinator movement, ensuring effective pollen transfer.

Angiosperm Diversity

Major Groups of Angiosperms

Angiosperms are divided into several clades, with monocots and eudicots being the largest groups.

Group	Key Features	Examples
Basal Angiosperms	Oldest lineages	Amborella trichopoda, water lilies, star anise
Magnoliids	Woody and herbaceous plants	Southern magnolia
Monocots	One cotyledon, parallel leaf veins, scattered vascular tissue	Orchids, grasses, palms
Eudicots	Two cotyledons, netlike leaf veins, vascular tissue in ring	Legumes, roses, oaks

Comparison of Monocots and Eudicots

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)
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, highlighting the significance of plant diversity for human health.

Compound	Source	Use
Atropine	Atropa belladonna	Eye pupil dilator
Digitalin	Digitalis	Heart medication
Menthol	Mentha	Throat soother
Quinine	Cinchona	Malaria preventive
Taxol	Taxus	Ovarian cancer drug
Tubocurarine	Chondrodendron	Muscle relaxant
Vinblastine	Catharanthus	Leukemia drug

Threats to Plant Diversity

Loss of plant species threatens the animal species that depend on them and may result in the extinction of up to 50% of Earth's species in the coming centuries. Tropical rain forests may contain undiscovered medicinal compounds.

Additional info:

• Equations relevant to plant reproduction include the genetic composition of endosperm: $n + 2n = 3n$ (double fertilization in angiosperms).

• Seed plants are classified as heterosporous, producing both microspores and megaspores.