Plant Biology and Reproduction Flashcards

General Characteristics of Plants

• Mobility: Plants are characterized by a lack of mobility; they are anchored in place by roots or other stabilizing structures.

• Cellular Nature: They are eukaryotic organisms, possessing a defined nucleus and membrane-bound organelles.

• Cell Structure: Plant cells are surrounded by rigid cell walls composed of cellulose.

• Multi-cellularity: All plants are multi-cellular organisms.

• Metabolism: Plants are autotrophic, meaning they can manufacture their own food. This is achieved through the presence of chlorophyll, which traps sunlight to facilitate photosynthesis.

• Reproduction: Plants utilize both sexual and asexual methods.

• Sexual Reproduction: Primarily used to produce seeds.

• Asexual Reproduction: Can occur through Various methods, such as taking cuttings from a parent plant.

Classification of the Plant Kingdom

The Plant kingdom is divided into two primary large phyla based on the presence or absence of a vascular system:

• Bryophytes: These are non-vascular plants.

• Tracheophytes: These are vascular plants.

Phylum Bryophyta

• Primary Group: Mosses represent the largest and most familiar group within the Bryophyte phylum.

• Physical Description: Mosses are typically small, low-growing plants. They frequently appear in dense clumps or mats, creating a visual effect similar to a thick green carpet.

• Habitat: They are commonly found on forest floors, rocks, and tree trunks.

• Moisture Requirements: Bryophytes are restricted to very moist environments as they lack a vascular system to transport water over long distances.

• Ecological Role: They are considered pioneer plants, often being the first species to establish themselves in a new or disturbed area.

Phylum Tracheophyta

• Complexity: This phylum consists of more complex plants that possess distinct roots, stems, and leaves.

• The Vascular System: The presence of a vascular system provides two major evolutionary advantages:

  • It allows plants to inhabit a wide range of different habitats.

  • It enables a significant range of sizes, varying from approximately $2\,mm$ to $15\,m$.

• Sub-Groups: Tracheophyta is divided into two major groups:

  1. Pteridophytes: Non-seed producing plants (e.g., ferns).

  2. Spermatophytes: Seed-producing plants.

Pteridophytes: Non-Seed Vascular Plants

• Primary Group: Ferns are the most well-known members of this classification.

• Evolutionary History: Ferns were the first vascular plants to evolve on Earth.

• Habitat: They are primarily located in moist, shady environments.

• Reproductive Features: Ferns do not produce flowers or seeds.

• Physical Identification: They are easily recognized by their large, leafy structures called fronds.

• Reproduction Process: Most ferns reproduce sexually by producing spores. These appear as small brown dots located on the underside of the fronds.

Spermatophytes: Seed-Producing Plants

• Structure: They possess complex systems including roots, stems, leaves, and a vascular system.

• Reproduction: Most species reproduce sexually via seeds.

• Definition of a Seed: A seed is a partially developed plant enclosed within a protective covering that prevents the embryo from drying out.

• Habitat Diversity: While the majority of seed plants live on dry land, certain species, such as the pitcher plant, are adapted to wetlands.

• Growth Potential: These plants can reach massive heights, up to $100\,m$ (e.g., giant redwood trees).

• Structural Support: In trees, the vascular tissue is reinforced with special chemicals like lignin. This chemical strength allows trees to reach great heights, a feat not possible for non-vascular plants.

Gymnosperms: Naked Seed Plants

• Seed Exposure: The term refers to plants where seeds are not enclosed by an ovary or fruit. Instead, seeds are protected by the scales of a cone.

• Common Examples: This group includes conifers such as pine and spruce trees.

• Climate Adaptation: Gymnosperms generally reside in regions with cold winters.

• Needle-like Foliage: They possess needle-like leaves designed to minimize moisture loss.

• Root System: They typically have a wide, spreading root system rather than a deep taproot system.

• Gymnosperm Reproduction Cycle:

  • Cones: They produce two types of cones: male and female.

  • Male Cones: Site where pollen grains (containing sperm) are formed.

  • Female Cones: Site where egg cells are produced within the ovule.

  • Fertilization: Pollen lands on a sticky sap produced by the female cone. Fertilization occurs, creating a zygote which then develops into an embryo.

  • Seed Formation: The embryo remains in the ripened ovule, which is then referred to as the seed.

  • Life Cycle: The seed eventually falls from the female cone. Over a period of one or two years, it develops into a seedling and eventually a mature plant.

Angiosperms: Flowering Plants

• Evolutionary Status: Angiosperms are the most recently evolved plant group and contain more species than all other plant divisions combined.

• Reproductive Center: The flower serves as the reproductive center where both sperm and egg are produced.

• Seed Protection: Seeds are enclosed within a fruit, which is formed by specific parts of the flower.

• Diversity: Flowers appear in various shapes and sizes. While some are vibrant, others (like those in grasses, wheat, and corn) are small and easily overlooked.

Evolutionary Timeline of Land Plants

• Land Plants (Embryophyta): Emerged approximately $470\,Ma$ (million years ago).

• Vascular System Evolution: Occurred approximately $440\,Ma$.

• Dominant Sporophyte Phase: Established around $400\,Ma$.

• Seed Evolution: Occurred approximately $300\,Ma$.

• Flower Evolution: Emerged between $150\,Ma$ and $200\,Ma$.

• Loss of Motile Gametes: Noted as a significant evolutionary transition for higher land plants.

Detailed Flower Structure

• Non-Reproductive Whorls:

  • Sepals: Found at the base of the flower; protect the budding flower.

  • Petals: Typically colorful to attract pollinators.

• Stamen (Male Reproductive Organ): Consists of the anther and the filament.

  • Anther: The site of male gametophyte (pollen) production.

  • Filament: The stalk that supports the anther.

• Carpels/Pistil (Female Reproductive Organ): Consists of the stigma, style, and ovary.

  • Stigma: The receptive tip that receives pollen.

  • Style: The tube-like structure connecting the stigma to the ovary.

  • Ovary: The base of the carpel where the female gametophyte (embryo sac/egg) is produced.

• Receptacle: The base part of the flower where all four whorls are attached.

Specialized Flower Classifications

• Complete Flowers: Possess all four whorls (sepals, petals, stamens, and carpels).

• Incomplete Flowers: Lack one or more whorls (e.g., flowers with no petals).

• Perfect Flowers: Have both functional stamens and carpels.

• Imperfect Flowers: Missing either stamens or carpels.

Reproductive Variations in Plants

• Monoecious ("One House"): Staminate (male) and carpellate (female) flowers are located on the same plant. Example: Maize.

• Dioecious ("Two Houses"): Staminate and carpellate flowers are located on separate plants. Examples: Sagittaria latifolia (common arrowhead) and Ginkgo biloba.

Development and Classification of Fruits

• Origin: The fruit of a flower develops specifically from the ovary.

• Function: Fruits protect the seeds and facilitate their dispersal.

• Types of Fruits:

  • Simple Fruits: Develop from a single ovary. Examples include peaches, cherries, and apples.

  • Aggregate Fruits: Develop from a single flower that has multiple carpels. Examples include strawberries and raspberries.

  • Multiple Fruits: Develop from a group of tightly clustered flowers. Example: Pineapples.

Seed Growth, Development, and Germination

• Seed Composition:

  1. An embryo.

  2. Nutrient-providing tissue.

  3. A protective coat.

• Embryonic Development: The growing embryo forms a root and a shoot structure.

• Cotyledons: These are the first embryonic leaves to emerge from a seed.

  • Monocots: Develop a single cotyledon.

  • Dicots: Develop a pair of cotyledons.

• Endosperm: This is nutrient-rich material that supports the embryo.

  • Relationship between Cotyledon and Endosperm: Seeds with large cotyledons usually have insignificant endosperm. Conversely, seeds with a large amount of endosperm generally have insignificant cotyledons.

• Dormancy: A state of extremely low biological activity. The embryo is alive but not growing, remaining protected by the seed coat until conditions improve.

• Germination Process: Occurs in optimum moisture and temperature.

  1. The seed absorb water, causing the seed coat to rupture.

  2. The embryo grows rapidly.

  3. The water-absorbing root grows downward (geotropism/chemical response).

  4. The shoot grows upward.

• Nutrient Support: In some plants, cotyledons rise above the soil to photosynthesize temporarily. For others, the cotyledons remain buried, and the plant relies on stored nutrients until it has enough root surface area and chlorophyll to be self-sustaining.

Comparative Table: Monocots vs. Dicots

Seed Adaptations and Dispersal Strategies

• The Dispersal Problem: If all seeds dropped directly beneath the parent plant, they would compete for limited light, water, and minerals, reducing the survival rate.

• Solutions/Adaptations:

  • Fruit Digestion: Animals eat the fruit and disperse the seeds.

    • Case Study: The Dodo bird and the Calvaria tree. The Dodo's digestive tract was required to induce the germination of Calvaria seeds. After the Dodo went extinct in the 1600s, no seeds germinated for $300$ years until researchers successfully used turkeys in the late 1970s to mimic the process.

  • Animal Helpers: Rodents like squirrels and chipmunks bury acorns and seeds. Those left unconsumed grow into new trees.

  • "Flight" (Wind Dispersal):

    • Explosive pods (e.g., poppies or orchids) burst when mature to scatter light seeds.

    • Wing-like structures (e.g., maple trees, dandelions, milkweed) help seeds travel on the wind.

  • Water Dispersal: Plants like water lilies and coconuts create air pockets in seeds to allow them to float and disperse via water currents.

Questions & Discussion

• Question: What is a cotyledon?

• Answer: A cotyledon is an embryonic leaf within the seed of a plant. It is the first leaf, or one of the first pair of leaves, that grows from a seed upon germination.