Seed Plants: Gymnosperms

Introduction to Seed Plants

Seed plants are a major evolutionary group within the plant kingdom, characterized by the production of seeds. This adaptation has allowed them to dominate terrestrial ecosystems. Seed plants are divided into two main groups: gymnosperms (naked seeds) and angiosperms (flowering plants).

• Seed: A structure containing a sporophyte embryo, food supply, and a protective coat.

• Gymnosperms: Seed plants with seeds not enclosed in fruits; includes conifers, cycads, ginkgos, and gnetophytes.

• Angiosperms: Seed plants with seeds enclosed in fruits (developed from flowers).

Evolutionary Trends in Seed Plants

Seed plants evolved several key adaptations that contributed to their success:

• Reduced gametophyte generation: The gametophyte is microscopic and protected within the sporophyte.

• Heterospory: Production of two types of spores: megaspores (female) and microspores (male).

• Seeds: Enable long-distance dispersal and survival in harsh conditions.

• Pollen: Allows fertilization without water, making reproduction independent of moist environments.

Homospory vs. Heterospory

Seed plants are heterosporous, a key evolutionary step from their ancestors.

• Homospory: One type of spore produces a bisexual gametophyte (seen in most ferns and mosses).

• Heterospory: Two types of spores: megaspores (female gametophyte) and microspores (male gametophyte).

Seed Structure and Function

A typical seed consists of:

• Embryo: The young sporophyte plant.

• Food supply: Nutrient tissue to support early growth.

• Seed coat: Protective outer covering.

Seeds can remain dormant for extended periods, allowing survival through unfavorable conditions and facilitating dispersal.

Key Features of Gymnosperms

• No flowers or fruits; seeds are exposed ("naked").

• Seeds develop on the surfaces of sporophylls, usually in cones (strobili).

• Separate male (pollen) and female (ovulate) cones.

• Mostly wind pollinated.

• Vascular tissue present for water and nutrient transport.

• Dominant sporophyte generation.

Secondary Growth in Seed Plants

Secondary growth allows plants to increase in thickness (girth), supporting taller and larger structures.

• Primary growth: Increases length (height) via apical meristems.

• Secondary growth: Increases girth via lateral meristems (vascular cambium and cork cambium).

Gymnosperm Life Cycle

The gymnosperm life cycle is characterized by:

• Reduction of gametophytes (microscopic, dependent on sporophyte).

• Production of seeds as the dispersal stage.

• Transfer of sperm to ovules by pollen (no need for water for fertilization).

Classification of Gymnosperms

There are four major phyla of gymnosperms:

• Cycadophyta (cycads): Mostly tropical, large cones, palm-like leaves, flagellated sperm, dioecious (separate sexes).

• Ginkgophyta (ginkgo): Only one living species (Ginkgo biloba), fan-shaped leaves, dioecious, flagellated sperm.

• Gnetophyta: Three genera (Gnetum, Ephedra, Welwitschia), diverse habitats, some angiosperm-like features (e.g., vessel elements, double fertilization in Ephedra).

• Coniferophyta (conifers): Largest group, includes pines, firs, spruces; mostly evergreen, needle-like leaves, adapted to cold and dry environments.

Ecological and Economic Importance of Gymnosperms

• Dominate boreal (northern) forests and many temperate regions.

• Provide timber, paper, resins, and other products.

• Important for carbon sequestration and as habitats for wildlife.

Summary Table: Major Gymnosperm Groups

Key Terms

• Sporophyte: Diploid, spore-producing phase of the plant life cycle.

• Gametophyte: Haploid, gamete-producing phase.

• Ovule: Structure that develops into a seed after fertilization.

• Pollen: Male gametophyte, delivers sperm to ovule.

• Strobilus (cone): Structure bearing sporangia (spore-producing organs).

Formulas and Equations

• Alternation of generations: