Life Sciences Grade 12: Biodiversity of Plants and Reproduction

Significance of Seeds

• All seed-bearing plants belong to the phylum Spermatophyta, including Gymnosperms (naked-seed bearing plants) and Angiosperms (flowering plants).

• Seeds are adapted for dispersal, leading to well-distributed species. This dispersal is crucial for colonization of new habitats and escaping unfavorable conditions.

• Seeds store reserve food, enabling survival in harsh conditions. This stored food, mainly in the form of starch, oils, and proteins, supports the seedling during germination until it becomes self-sufficient.

• The testa (seed cover) protects the embryo against unfavorable conditions, such as physical damage, desiccation, and pathogen attacks.

• Cotyledons provide food for developing seedlings, acting as the primary source of nutrients before true leaves develop.

Seed Structure

• The seed is surrounded by a seed coat or testa, which is derived from the integuments of the ovule.

• The testa protects the seed's contents from drying out, mechanical damage, and attack by pathogens.

• The cotyledon stores food for the embryo, providing it with the energy needed to germinate and establish itself.

• The embryo is found within the seed and consists of the radicle (the embryonic root), plumule (the embryonic shoot), and axis (the connection between the radicle and plumule).

Advantages of Seeds for Plants

• Seeds possess a dispersal mechanism, like the parachute on pine seeds for wind dispersal, burrs for animal dispersal, or fleshy fruits for animal consumption and dispersal.

• Dispersal mechanisms allow seeds to be widely distributed, reducing competition among plants and enabling colonization of new areas.

• The testa prevents the embryo from drying out, and the cotyledons provide food, ensuring the embryo remains viable until conditions are favorable for germination.

• The testa and cotyledons protect the embryo in early development from physical damage, temperature extremes, and microbial attacks.

• The cotyledons supply energy for seedling growth until leaves develop and photosynthesis begins, providing essential nutrients for initial growth.

Seeds as a Source of Food

• Plants with edible seeds are a major food source, categorized into grains, legumes, and nuts, providing essential nutrients for human and animal diets.

• Grains (rice, wheat, maize): Important source of energy-rich carbohydrates (starch). The testa of whole grains provides fiber, which is beneficial for digestive health.

• Legumes: Important source of protein, low in fat, high in fiber, and easily cultivated, transported, and stored, making them a staple food in many cultures.

• Nuts: High in protein, fiber, antioxidants, and mono-unsaturated fatty acids, contributing to heart health and overall well-being.

Seed Banks

• Seed banks are facilities used to store seeds of various plants and crops to maintain biodiversity, acting as a safeguard against species extinction.

• Many countries have seed banks; notable ones include Kew’s Millennium Seed Bank Project (UK) and the International Seed Vault (Sweden), which aim to conserve plant genetic resources globally.

• Seed banks maintain specific conditions: very cold temperatures (-20^{\circ}C) and a dry environment to prolong seed viability and reduce metabolic activity.

Maintaining Biodiversity with Seed Banks

• Seed banks protect against the loss of species due to habitat destruction, climate change, and overuse of species, preserving genetic diversity for future generations.

• Uses for seed banks:

  • Re-establish endangered or extinct plants, restoring lost species to their natural habitats.

  • Rehabilitate damaged or destroyed plants, preserving their genetic material for future restoration efforts.

  • Cultivate overexploited plants, reducing pressure on wild populations by providing cultivated alternatives.

  • Cultivate new, hardier, disease-resistant hybrids, improving crop resilience and productivity.

  • Conserve endemic species, protecting unique plants found only in specific regions.

  • Produce plant material for research, facilitating studies on plant genetics, evolution, and conservation.

Terminology

• Spermatophyta: Phylum of seed-bearing plants, encompassing all plants that reproduce via seeds.

• Endemic: Species occurring only in a specific area, often vulnerable to extinction due to limited distribution.

• Mitochondrial DNA: DNA found in the mitochondria, used to study genetic relationships and evolutionary history.

• Plastid DNA: DNA found in the chloroplast, providing insights into plant evolution and adaptation.

• Nuclear DNA: DNA found in the nucleus, containing the majority of the plant's genetic information.

• Indigenous species: Species that occur naturally in an area, representing a region's native flora.

Review Questions

• Question 1: The radicle grows into the first root