Unit 5: Pteridophytes and Gymnosperms

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Pteridophytes: General Account and Classification

Pteridophytes are known as "Vascular Cryptogams" because they are the first terrestrial plants to possess vascular tissues (xylem and phloem) while remaining seedless.

General Characteristics

• Dominant Phase: The sporophyte is the dominant, independent, and vascularized phase of the life cycle.
• Differentiation: The plant body is differentiated into true roots, stem, and leaves.
• Reproduction: They reproduce through spores produced in sporangia, which are often grouped into structures called strobili or sori.
• Habitat: They prefer cool, damp, and shady places, though some are aquatic.

Classification and Economic Importance

• Classification: Classified into four major groups: Psilopsida, Lycopsida, Sphenopsida, and Pteropsida.
• Economic Importance: Used as ornamentals, soil binders, and for medicinal purposes.

Early Land Plants: Rhynia

Rhynia is a fossil genus representing one of the simplest and earliest known land plants.

• Structure: It lacked true roots and leaves. The plant body consisted of a creeping rhizome and dichotomously branched aerial shoots.
• Sporangia: Large, oval sporangia were borne at the tips of the aerial branches.

Heterospory, Seed Habit, and Stelar Organization

Heterospory and Seed Habit

Heterospory is the production of two types of spores: small microspores (male) and large megaspores (female).

• It is considered a significant evolutionary step toward the seed habit found in higher plants.
• The retention of the megaspore within the megasporangium allows for the development of the female gametophyte in a protected environment.

Stelar Organization

The stele is the central vascular cylinder of the sporophyte. Pteridophytes exhibit various stelar types, evolving from simple protostele (solid core of xylem) to complex siphonostele (pith present) and dictyostele.

Morphology and Reproduction: Lycopodium and Selaginella

1. Lycopodium (Club Moss)

• Morphology: Sporophyte with a creeping or erect stem covered with small leaves (microphylls).
• Reproduction: It is homosporous (produces one type of spore). Sporangia are borne on sporophylls which often form terminal strobili.

2. Selaginella (Spike Moss)

• Morphology: Characterized by a creeping stem and the presence of a unique structure called the rhizophore.
• Reproduction: It is heterosporous, producing both microspores and megaspores in the same strobilus.

Gymnosperms: General Account and Classification

Gymnosperms are "Naked Seeded" plants because their ovules are not enclosed by an ovary wall.

General Characteristics

• Wood: Usually perennial, woody plants with well-developed taproot systems.
• Leaves: Often xerophytic, adapted to withstand extreme environmental conditions (e.g., needle-like leaves in Pinus).
• Reproduction: They are heterosporous and produce male and female cones (strobili). Pollination is primarily by wind (anemophily).

Classification and Economic Importance

• Classification: Major groups include Cycadales, Coniferales, Ginkgoales, and Gnetales.
• Economic Importance: Source of timber, resin, turpentine, and edible seeds (e.g., Chilgoza).

Morphology and Reproduction of Gymnosperm Genera

1. Cycas

• Morphology: Palm-like appearance with a stout stem and a crown of pinnately compound leaves. It possesses unique coralloid roots associated with nitrogen-fixing cyanobacteria.
• Reproduction: Dioecious; male plants produce large cones, while female plants produce clusters of megasporophylls (no true female cone).

2. Pinus

• Morphology: Tall, branched tree with a "pyramidal" habit. Leaves are of two types: scale leaves and needle-like foliage leaves borne on dwarf shoots.
• Reproduction: Monoecious; both male and female cones are found on the same plant.

3. Ginkgo (Maidenhair Tree)

• Morphology: Known as a "Living Fossil" with characteristic fan-shaped leaves.
• Reproduction: Dioecious; the seeds are large and fleshy.

4. Gnetum

• Morphology: Often climbers or small trees with broad, angiosperm-like leaves.
• Evolutionary Link: It is considered the most advanced gymnosperm, showing many similarities to angiosperms (e.g., presence of vessels in xylem).

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