Biology 200: Chapter 22 Gymnosperms

Introduction to Gymnosperms

• Evolutionary History:

  • The oldest known seeds date back to the Late Devonian period, more than 350 million years ago.

  • Seeds represent a significant adaptation for plants residing on land.

• Adaptive Advantages of Seeds:

  • Protective Seed Coat: Provides a barrier against environmental damage.

  • Food Supply: Provides a reservoir of nutrients for the developing embryo.

  • Dormancy: Seeds are capable of remaining dormant during unfavorable environmental conditions, ensuring survival favorable conditions return.

• Ancestry:

  • The first seed plants were fernlike in appearance and were originally known as pteridosperms (seed ferns).

  • These have since been reclassified as gymnosperms.

Characteristics of Gymnosperm Seeds and Structure

• Definition: The term "gymnosperm" refers to the exposed nature of the seeds.

• Seed Production: Seeds are produced on the surface of sporophylls or similar structures. This differs from flowering plants (angiosperms), where seeds are enclosed within a fruit.

• Sporophyll Arrangement: Seed-bearing and pollen-bearing sporophylls are frequently arranged in specialized structures called cones.

Gymnosperm Reproduction and Gametophytes

• Pollen Production: Pollen cones are responsible for producing pollen grains.

• Female Gametophyte Development:

  • The female gametophyte is produced inside an ovule.

  • The ovule contains a tissue called the nucellus.

  • The nucellus is enclosed within an integument.

  • Post-Fertilization: The integument matures and becomes the seed coat following fertilization.

• Reduction and Dependency:

  • The female gametophyte in gymnosperms is even more reduced in cell number than that of ferns and their relatives.

  • It does not grow independently; instead, it develops entirely within the sporophyte structures.

Classification of Gymnosperm Phyla

There are four living phyla of gymnosperms:

• Phylum Pinophyta:

  • Includes pines, firs, spruces, and cedars.

  • Characteristics: Bear distinct strobili (cones) and needlelike leaves arranged in clusters.

  • Approximate diversity: $600\,\text{species}$.

• Phylum Ginkgophyta:

  • Characteristics: Notched, broad, fan-shaped leaves with evenly forked (dichotomous) veins. Plum-like seeds are enclosed in a fleshy covering.

  • Diversity: Contains a single living species, Ginkgo biloba.

• Phylum Cycadophyta:

  • Characteristics: Appear as a cross between a tree fern and a palm; strobili are located in the center of the plant.

  • Approximate diversity: $300\,\text{species}$.

• Phylum Gnetophyta:

  • Characteristics: Unique among gymnosperms for having both vessels and tracheids in the xylem.

  • Approximate diversity: $70\,\text{species}$.

Detailed Study: Phylum Pinophyta – The Conifers

• Genus Pinus (Pines):

  • This is the largest genus within the phylum.

  • Pines are the dominant trees in the coniferous forests of the Northern Hemisphere.

  • Included in this genus are the world’s oldest known living organisms, the Bristlecone pines.

• Structure and Form:

  • Leaves: Needlelike and arranged in clusters.

  • Fascicle: A cluster of leaves (usually two to five).

  • Fascicles are considered short shoots and have restricted growth.

• Adaptations for Harsh Conditions:

  • Hypodermis: Located directly below the epidermis; consists of one to two layers of thick-walled cells.

  • Cuticle: Thick outer layer to prevent water loss.

  • Stomata: Recessed or sunken into the leaf surface.

  • Resin Canals: Produce resin, which is antiseptic and aromatic. Resin prevents the development of fungi and deters insect attacks.

  • Mycorrhizal Fungi: Associated with the roots of most conifers to assist in nutrient uptake.

• Wood Characteristics:

  • Tracheids: Conifer wood consists entirely of tracheids.

  • Softwood: Conifer wood is categorized as softwood because thick-walled cells (vessels and fibers) are absent.

  • Hardwood: Wood from broadleaf trees (dicots) is categorized as hardwood because it contains thick-walled vessels and fibers.

Reproduction in Pinophyta

• Spore Production: Pines are heterosporous, producing two kinds of spores.

• Pollen Cones (Male Strobili):

  • Consist of papery or membranous scales.

  • Microsporangia: Occur in pairs toward the bases of the scales.

  • Development: Meiosis produces microspores, which then develop into pollen grains.

  • Pollen Grain Structure: Consists of four cells and a pair of air sacs. The air sacs provide buoyancy for wind dispersal.

• Seed Cones (Female Strobili):

  • Larger than pollen cones.

  • Consist of woody scales with inconspicuous bracts between them.

  • Ovule Placement: A pair of ovules is located at the base of each seed cone scale.

  • Megaspores: Produced in megasporangia within the ovules.

Production and Development of Megaspores

• Ovule Anatomy:

  • Contains a megasporangium which houses the nucellus and a single megasporocyte.

  • The megasporangium is surrounded by the integument.

  • Micropyle: A small pore in the integument through which pollen enters.

• Megasporogenesis:

  1. The megasporocyte undergoes meiosis, producing four megaspores.

  2. Three of the megaspores degenerate.

  3. The remaining megaspore develops into the female gametophyte.

  4. The female gametophyte produces archegonia at the micropyle end.

Maturation of Seed Cones: A Two-Year Process

• First Year Events:

  1. Pollen grains are captured by sticky pollen drops oozing out of the micropyle.

  2. The pollen grain produces a pollen tube that grows through the nucellus.

  3. Two sperm cells are produced within the pollen tube.

  • Mature Male Gametophyte: Defined as the germinated pollen grain with a pollen tube and two sperm.

  • Note: The sperm have no flagella, and no antheridium is formed.

  1. The megaspore begins its development.

• Second Year Events:

  1. The female gametophyte and archegonium reach maturity.

  2. The pollen tube arrives at the archegonium.

  3. One sperm unites with the egg to form a zygote ($2n$).

  4. The other sperm degenerates.

  5. The resulting embryo is nourished by the female gametophyte.

  6. The integument hardens to become the seed coat.

Diversity of Other Gymnosperms

• Other Conifers:

  • Yew (Taxus) and California Nutmeg (Torreya): Produce ovules singly at the tips of shoots. Each ovule is at least partially surrounded by a fleshy, cuplike structure called an aril.

  • Podocarpus: A Southern Hemisphere conifer. It produces fleshy-coated seeds with a large appendage at the base.

  • Junipers: Feature fleshy seed cones.

• Phylum Ginkgophyta (Ginkgos/Maidenhair Trees):

  • Only one living species (Ginkgo biloba); exists primarily in cultivation.

  • Leaves: Fan-shaped and notched; borne on short, slow-growing spurs. They lack a midrib or prominent veins; instead, hair-like veins branch dichotomously. They are deciduous.

  • Reproduction: Dioecious (male and female structures are on separate trees). Seeds are enclosed in a fleshy seed coat that produces a nauseating odor.

• Phylum Cycadophyta (Cycads):

  • Slow-growing plants found in tropical and subtropical regions.

  • Appearance: Tall, unbranched trunks with a crown of large pinnately divided leaves.

  • Pollination: Sometimes carried out by beetles.

  • Reproduction: Dioecious; possess both pollen and seed strobili.

• Phylum Gnetophyta (Gnetophytes):

  • Unique for having vessels in the xylem.

  • Genera:

    • Ephedra (Joint Firs): Comprise more than half of gnetophyte species. Shrubby plants found in dry regions of southwestern North America, South America, Asia, and Europe.

    • Gnetum: Contains most remaining species. Vine-like plants with broad leaves; found in the tropics of South America, Africa, and Southeast Asia.

    • Welwitschia: Only one species. Confined to the deserts of southwestern Africa. Features a short stem, long taproot, and only two straplike leaves that become tattered and split over time. It is dioecious with male and female strobili.

Human Relevance of Gymnosperms

• Uses of Conifers:

  • Food: Edible inner bark and needles of white pine; seeds of nearly all pines.

  • Maritime: Used for masts in sailing vessels.

  • Construction and Industry: Crates, boxes, matchsticks, furniture, telephone poles, railroad ties, and mine timbers.

  • Chemical Extracts: Turpentine and rosin are derived from resin.

  • Amber: Processed from the fossilized resin of extinct conifers.

  • Paper: Pulpwood for newsprint.

  • Crafts: Lashing for canoes and basketry.

  • Arts: Wood used for the soundboards of musical instruments.

  • Medicine: Production of taxol from yew trees, used in the treatment of ovarian cancer.

• Uses of Other Gymnosperms:

  • Ginkgo: Seeds used for food (after the seed coat is removed); extracts are used to increase blood circulation.

  • Ephedra: Source of "Mormon tea." The drug ephedrine is derived from a Chinese species of Ephedra to treat respiratory problems.