I see the term hybrid applied to many kinds of plants. What is the significance of this term? I am only familiar with hybrid cars.
Hybrid applied to plants is similar to hybrid cars. It indicates that two plants are combined together like two cars (gasoline and electric) are combined together. Hybrid plants are created when two or more unrelated plants of the same species are combined or crossed with each other. Their resulting progeny are F-1 or first generation hybrids.
Unrelated plants have many differences in their gene pools. When this diversity of genes is recombined in the first generation, it produces more vigorous plants, referred to as hybrid vigor.
In nature plants grow in closely related colonies. Random pollination in this colony of related individuals includes both self-pollination and pollination between brothers/sisters and cousins. However, pollen may be carried into one plant colony from a more distant and unrelated colony of plants by wind or insects. This natural hybridization occurs without any intervention by man.
The most popular method of producing new varieties of ornamental and fruit plants (including the new Cosmic Crisp apple) is by propagation without seeds. Many different crosses are made between promising unrelated parent plants. An individual plant from a cross may be outstanding in some way and is selected as a new variety. Plants of this one outstanding plant are produced by cuttings, grafting or more rapidly from tissue culture.
Tissue from the parent plant is grown on agar in a laboratory. Thousands of tiny new plants are then grown in greenhouses. The new plants are extremely uniform because they all came from the same individual plant. They are also vigorous because the parents were unrelated. This process is used by professionals and thousands of amateur plant breeders and hybridizers.
Another method is used to develop new hybrid varieties which are grown from seeds such as field crops, vegetables and annual flowers. Individual plants are self-pollinated for three or more generations to create a uniform gene pool. Then individual plants from two or more unrelated plant lines are crossed. The resulting plants from these crosses are very uniform because they received the same genes from both parents. They are also vigorous because of the diversity of genes.
The hybrid seeds from these test crosses are evaluated and an occasional one is selected and named as a new variety. Then the parent plants are self-pollinated to produce enough plants to produce the seed of the new variety.
Special techniques for seed production are used to ensure that no self-pollination or cross-pollination from outside sources contaminates the seed crop. This may be hand removal of pollen followed by hand pollination of plants grown in greenhouses or the use of male sterility genes. Because of the greater complexity and investment, this process is confined primarily to large companies.
In summary, the greater vigor and uniformity of hybrids usually means they are improvements over older varieties.