A seed represents a package of life, containing an embryonic plant and a protective food source, all encased in a seed coat. The journey from a flower to a viable seed is the mechanism by which flowering plants reproduce, ensuring the continuation of their species. This guide will cover the core science behind seed development and the hands-on methods used by gardeners to produce and preserve high-quality seeds.
The Biological Foundation of Seed Formation
Seed formation begins with sexual reproduction in the flower, which contains both male (stamen) and female (pistil) structures. The stamen produces pollen grains (male gametes), while the pistil houses the ovules (female gametes) within the ovary. Pollination is the initial step: the transfer of pollen from the anther to the receptive stigma of the pistil.
Once a pollen grain lands on the stigma, it germinates and grows a tube down the style toward the ovule. The generative cell within the pollen grain divides, producing two sperm cells that travel down this tube. Flowering plants undergo a unique process called double fertilization.
In double fertilization, one sperm cell fuses with the egg cell to form the diploid zygote, which develops into the embryo. The second sperm cell fuses with two polar nuclei, forming the triploid endosperm. This endosperm tissue serves as the primary food source for the developing embryo and the seedling. Following fertilization, the ovule matures into the seed, and the surrounding ovary tissue develops into the fruit or seed pod.
Managing the Pollination Process
To save seed that is “true to type” (identical to the parent plant), managing pollination is necessary, especially with cross-pollinating species. Cross-pollination occurs when wind or insects transfer pollen between two different plants, potentially creating an unintended hybrid. Self-pollinating plants, such as tomatoes and beans, naturally pollinate themselves, requiring less intervention.
Controlling parentage involves physical isolation, achieved spatially or mechanically. Spatial isolation means planting different varieties far enough apart to prevent wind or insect pollinators from carrying pollen between them. Mechanical isolation involves covering the flowers to prevent external pollen from reaching them.
Blossom bags or fine netting cages can be used to cover entire plants or individual flowers before they open. For species that require insect-mediated pollination, manual hand-pollination is necessary once the flowers are protected. This involves using a clean paintbrush or cotton swab to gently collect pollen from the male parts of the desired parent plant.
The collected pollen is then carefully transferred and brushed onto the receptive female stigma. This technique is most effective when performed in the morning, as pollen shed is often highest. After hand-pollinating, the flower is immediately re-covered or taped shut to prevent contamination. The stem of the pollinated flower should be marked, ensuring that only the resulting fruit is harvested for seed.
Harvesting and Storing Viable Seeds
The timing of harvest is important because seeds must be allowed to reach physiological maturity, the point when they have achieved their maximum dry weight and vigor. For seeds contained in dry pods or husks, the indicator of maturity is the plant material drying down and turning brown or yellow. Dry seed heads should be left on the plant until they are brittle and hard to the touch, ensuring high viability.
For wet-seeded crops, such as tomatoes, cucumbers, and melons, the fruit must be allowed to ripen fully, often past the point of being desirable for eating. This ensures the seed inside is mature. Seeds from fleshy fruits require a wet cleaning process to remove the pulp, sometimes involving fermentation to dissolve gel coatings and inhibitors.
After cleaning, all seeds must be thoroughly dried to reduce their moisture content significantly. Excess moisture can lead to mold growth or premature germination, which destroys viability. Seeds should be spread in a thin layer on a non-porous surface and dried in a well-ventilated, moderately warm area out of direct sunlight for one to three weeks.
The goal is to achieve a low moisture content, ideally between 6% and 10%, before storage. Seeds should be stored in airtight containers, such as glass jars, in a cool, dark, and dry environment to maintain dormancy and longevity. Optimal storage conditions are met when the temperature (in degrees Fahrenheit) plus the relative humidity percentage totals less than 100.