Seeds are the product of sexual reproduction in the plant. Pollination occurs when pollen grains are transferred to the stigmatic surface of the flower. Fertilization of the female gamete by the male gamete occurs, and the seed develops to maturity.
Characteristics in any plant are determined by genetic makeup. Genes occur in pairs, with each chromosome coming from either the male or female gamete. Plants are described as homozygous when both genes of a pair are the same. For example, a dwarf pea would contain genes for dwarfness (tt) only. When a plant is homozygous and self-pollinating, then seeds that are produced will come true-to-type.
A plant that is heterozygous for a particular trait would contain different genes – a tall pea might contain a gene for tallness (T) and a gene for dwarfness (t). Pollination between plants that have a high degree of heterozygosity results in very different seeds.
Many plants are cross-pollinating, meaning that the pollen comes from a different source than the plants in which seeds develop. This characteristic is desirable in nature because genetic diversity provides better adaptation to changing environments. On a commercial level, however, it is necessary to maintain genetic quality and purity so that seeds will produce the plants desired by the customer.
Any vegetable and herbaceous bedding plants are available as hybrid seed. Hybrids are produced by cross-pollinating two plants, each of which is homozygous and differ in at least one characteristic. All of the seed produced will be heterozygous for differing traits. The production of hybrids has created superior flower types, increased plant vigor, and increased yield and quality. However, because these hybrids are heterozygous, seeds that are saved will produce plants with varying characteristics.