14.3 Plant Reproduction and Life Cycles

Plants have a unique life cycle that alternates between two generations. This process, called alternation of generations, involves a diploid sporophyte stage and a haploid gametophyte stage. It's a key adaptation that allows plants to thrive on land.

Sexual reproduction in plants involves pollination and fertilization. In flowering plants, pollen carries male gametes to the female parts. This process leads to seed formation, which is crucial for plant dispersal and survival in various environments.

Alternation of Generations

Sporophyte and Gametophyte Generations

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• Plants undergo alternation of generations, a life cycle that alternates between a diploid sporophyte generation and a haploid gametophyte generation
• The sporophyte generation is the dominant stage in vascular plants (ferns, gymnosperms, angiosperms) and produces spores through meiosis
• Spores develop into the gametophyte generation, which is reduced and produces gametes (eggs and sperm) through mitosis
• In non-vascular plants like mosses, the gametophyte generation is dominant and photosynthetic, while the sporophyte is nutritionally dependent on the gametophyte

Alternation and Plant Evolution

• Alternation of generations evolved as plants adapted to life on land, allowing them to disperse and reproduce in the absence of water
• Over the course of plant evolution, the sporophyte generation became increasingly dominant and complex, while the gametophyte generation became reduced
• In angiosperms (flowering plants), the female gametophyte is reduced to just a few cells within the ovule, and the male gametophyte is the pollen grain

Sexual Reproduction

Pollination and Fertilization

• Sexual reproduction in angiosperms involves pollination, the transfer of pollen (male gametophytes) from the anther to the stigma of a flower
• Pollination can occur through various agents like wind, water, animals (bees, birds, bats), or self-pollination within the same flower
• Once pollen lands on the stigma, it germinates and grows a pollen tube that delivers sperm to the ovule
• Angiosperms undergo double fertilization, where one sperm fertilizes the egg to form a zygote, and another fuses with two polar nuclei to form the triploid endosperm

Seed and Fruit Formation

• The fertilized ovule develops into a seed, which consists of an embryo (new sporophyte), endosperm (nutritive tissue), and a seed coat for protection
• As seeds mature, the ovary wall surrounding them develops into a fruit, which aids in seed dispersal and can take various forms (berries, drupes, nuts, legumes)
• Fruits can be fleshy (apples, tomatoes) or dry (nuts, grains) and are often adapted to specific dispersal mechanisms like animal consumption, wind, or explosive dehiscence
• Seeds remain dormant until suitable conditions trigger germination, allowing the embryo to emerge as a seedling and grow into a mature sporophyte

Asexual Reproduction

Vegetative Propagation Methods

• Many plants can reproduce asexually through vegetative propagation, which involves the growth of new individuals from vegetative parts of the parent plant
• Common methods of vegetative propagation include fragmentation (breaking off of stems or leaves), layering (inducing roots to form on stems while still attached), and grafting (joining two plant parts)
• Some plants form specialized structures for vegetative propagation, such as bulbs (onions), corms (gladiolus), tubers (potatoes), and runners (strawberries)
• Advantages of vegetative propagation include faster growth, preservation of desirable traits, and avoidance of the energy costs of sexual reproduction

Asexual Reproduction and Agriculture

• Asexual reproduction is widely used in agriculture and horticulture to propagate crops and ornamental plants with desirable characteristics
• Grafting is commonly used to join disease-resistant rootstocks with high-yielding cultivars in fruit trees (apples, citrus) and grapevines
• Many crops, such as bananas, pineapples, and sugarcane, are propagated almost entirely through vegetative means due to sterility or seedlessness
• However, asexual reproduction can also lead to reduced genetic diversity and increased vulnerability to pests and diseases, as seen in the historical overreliance on a single potato variety (Lumper) that contributed to the Irish Potato Famine