8.4 Plant Reproduction

Anthers contain pollen sacs where meiosis ultimately produces haploid pollen grains, the male gametophytes. Ovules inside an ovary contain the female gametophyte and egg. A stigma receives pollen, a style separates stigma from ovary, and the ovary encloses ovules. Petals, scent and nectar can attract animal pollinators, while sepals protect the developing bud. Structure varies widely because pollen transfer can involve insects, birds, mammals, wind or water.

Insect-pollinated flowers often have conspicuous petals, scent or nectar, sticky or sculptured pollen and anthers positioned to contact visitors. Wind-pollinated flowers tend to expose dangling anthers, produce abundant light pollen and possess large or feathery stigmas. These are probabilistic adaptations rather than rigid definitions. The central test is whether a feature increases the chance that pollen reaches a compatible stigma by the relevant vector.

Pollination is transfer of pollen to a stigma; fertilization is fusion of gametic nuclei and happens later. If compatible pollen hydrates, it germinates and a pollen tube grows through the style. Chemical guidance directs the tube toward an ovule. Two sperm nuclei travel through it: one fuses with the egg to form the diploid zygote, while the other fuses with central nuclei to form nutritive endosperm, usually triploid. This double fertilization is characteristic of angiosperms.

Self-fertilization can preserve a genotype and guarantee reproduction when partners are scarce, but repeated selfing increases homozygosity and can expose harmful recessive alleles. Plants promote crossing by separating anther and stigma in space, maturing them at different times, producing separate male and female flowers, or using molecular self-incompatibility. In self-incompatibility, recognition between pollen and stigma alleles blocks pollen germination or tube growth when the match indicates self-origin.

After fertilization, the zygote becomes an embryo, the ovule becomes a seed and the ovary commonly develops into a fruit. The seed coat protects the embryo, and cotyledons or endosperm provide reserves. Fruit can protect seeds and recruit dispersal agents. Wind dispersal favors wings, plumes and low mass; animal dispersal can involve edible flesh or hooks; water dispersal requires buoyancy and resistance to soaking; explosive fruits launch seeds mechanically.

A mature seed often enters dormancy, surviving unfavorable conditions with very low metabolism. Germination requires water for rehydration and enzyme activity, oxygen for aerobic respiration and a suitable temperature for enzyme-controlled reactions. Some species also respond to light, fire, scarification or a period of cold. Water uptake causes imbibition, the seed swells, hydrolytic enzymes mobilize stored reserves and the radicle emerges before the shoot.

Early growth depends on stored molecules until leaves can photosynthesize enough to become a source. Starch may be hydrolyzed to sugars for respiration and synthesis, while proteins and lipids provide additional substrates. Germination percentage and rate should be distinguished in experiments: the same final percentage can be reached at different speeds. Replication, controlled conditions and a justified criterion for germination are needed for reliable comparisons.