Many crops were established by introduction from foreign countries of highly heterogeneous varieties, each with identifiable characters, such as Kherson oats, Turkey wheat, Manchuria barley, and Manchu soybeans. Such varieties conform more nearly to a race rather than a conventional variety. Single plant selections were subsequently made and the seed increased to produce pure line varieties, such as Richland oats, Kanred wheat, OAC 21 barley, and Mandell soybeans.

The term "line," according to the Code, is a "sexually reproducing population of uniform appearance, propagated by seeds or by spores, its stability maintained by selection to a standard."

A pure line variety consists of the genetically identical progeny of a single, self-fertilized, homozygous, or true breeding, plant. Such an individual transmits through its gametes, or germ cells, identical genes hence all its offspring are identical to it and each other. The variety thus remains pure, except for mutations.

Conversely, a plant from the union of genetically unlike germ cells is heterozygous, or hybrid, for each gene in which the gametes differ.

In self-pollinated crops, heterozygosis largely results from controlled or infrequent natural cross-pollination between plants with different germ plasm. Self-pollination in subsequent generations rapidly reduces the amount of heterozygosity, or genetic variability; in fact, it is halved in each generation.

Among all gene pairs, heterozygous in the first hybrid or first filial generation, commonly designated F₁, only half are heterozygous in the F₂, one-fourth in F₃, and so on. Very rapidly, therefore, the population derived from one hybrid plant becomes segregated into many true breeding plants, no two of which are identical.

Most improved varieties developed from hybrids in self-pollinated crops virtually are pure lines, because final selection of the progenitor plant of the new variety is delayed until a high degree of homozygous has been attained. This uniformity has advantages in that, barring mixtures, further outcrosses, or mutations, the germ plasm will not change. As offtypes are usually detectable, the task of maintaining purity of the variety is simplified. Uniformity may be disadvantageous, however. If climate or soil is unfavorable for maximum production or if a disease epidemic occurs, every plant in the variety is equally affected.

Certain genetic variability among plants may provide a buffer against adverse environment or diseases. Some plant breeders have developed varieties from plants selected from populations that have undergone only one or two generations of self-pollination, or they have composited a number of homozygous lines from the cross. The new variety Traveler oats is of this type. Such varieties, in effect, are a composite of many closely related plants but not a pure line. Generally the breeder should limit the range of visible characters so that the variety may be distinguished.

Normally cross-pollinated crops may have still greater range of genetic variability within varieties.

Cross-pollinated crops, frequently called open pollinated, can conveniently be grouped into those in which pollination is subject to control on a large-scale field basis and those in which natural cross-pollination must be relied upon for seed production.

Commercial hybrid varieties are supplanting open-pollinated varieties in crops that are subject to controlled pollination.

In crops such as corn and melons, the bearing of pollen (male) and ovule (female) flowers separately on the same plant permits removal of pollen flowers from seed plants.

In crops such as onions, sorghum, sugarbeets, and castorbeans, pollen of seed plants is eliminated through cytoplasmic or genetic male sterility.

In crops bearing pollen and ovule flowers on separate plants, such as spinach, male plants can be cut out of seed rows.

In crops with high self-sterility, such as cabbage and Pensacola bahiagrass, clones or lines may be interplanted.

Rules and Regulations Under the Federal Seed Act contains this definition of hybrid: "The term 'hybrid' means the first generation seed of a cross produced by controlling the pollination and by combining (1) two, three, or four inbred lines; (2) one inbred or a single cross with an open-pollinated variety; or (3) two varieties or species, except open-pollinated varieties of corn (Zea mays)."

A hybrid as a kind of variety is defined by the Code as "a uniform group which is a first generation hybrid (F₁) reconstituted on each occasion by crossing two or more breeding stocks maintained either by inbreeding or as clones."

When plants of a crop are self-fertile, inbred lines usually are developed through self-pollinating plants of highly heterozygous populations. The first generation of inbreeding from controlled self-pollination is designated S₁, the second S₂, and so on. From a genetic standpoint, the S, corresponds to the F₂ generation following controlled hybridization. This process is analogous genetically to the development of pure lines from an induced hybrid in a naturally self-pollinated crop.

Commercial usage of the term "hybrid" is restricted to the first generation following hybridization. Several types of hybrids, however, are possible.

When inbred lines are the parental stocks, the hybrid may involve two, three, four or more inbreds, which may be indicated by such terms as "single-cross," "three-way-cross," or "double-cross hybrids."

Parental stocks of double-cross hybrids are single crosses. If the single crosses have been self-pollinated for one or more generations, however, the resulting cross is an advanced generation hybrid.

When the parental stocks involve an inbred line and an open-pollinated variety, the product is a topcross hybrid.

Variety classification is extremely variable in normally cross-pollinated crops where large-scale controlled pollination is not possible. These crops include many forage grasses and legumes; some vegetables, such as carrots, celery, and cauliflower; and many flowering ornamentals. Varietal designation often is given to populations, such as a race, that have evolved naturally under certain environmental conditions and have acquired somewhat constant characters.

Seeds of many modern varieties are constituted by interplanting selected clones or seed-propagated lines and allowing them and their progenies to intercross under conditions free from foreign pollen. Varieties so formed are known as synthetic varieties. Seed stocks are increased by permitting random mating within the population in each generation. The first generation of seed resulting from intercrossing is known as Syn 1 and the second as Syn 2.

Syn 1 is called composite by corn breeders because it is in effect a composite of single-cross seeds.

The term "polycross" is at times associated with such varieties; probably the usage stems from the similarity of the random interplanting of clones and the establishment of a poly-cross nursery. Polycross, however, is a term that describes a type of experimental cross to measure the general combining ability of clones and has no commercial usage.

The term "chance hybrid" has been applied to hybrids recently produced without controlled pollination. Equal seed portions of four inbred lines of pearl millet, for instance, are composited for planting as a mixture. The resulting seed is a mixture predominantly of hybrid but with some inbred seed. Plants from the latter are weaker and largely are eliminated by competition during the seedling stage.

Thus, the variety, which is created and maintained by man, is the essential unit of classification in cultivated plants. Though usage differs greatly from crop to crop, varieties are distinguished by constant, inherent characters. Because of its importance, the variety has been called the basis of modern agriculture and horticulture.

MARTIN G. WEISS is Associate Director, Crops Research Division, Agricultural Research Service, Beltsville, Md.

ELBERT L. LITTLE, JR., is dendrologist, Forest Service, Washington, D.C. Both are members of the Department of Agriculture Committee on Plant Nomenclature and members of the International Commission for the Nomenclature of Cultivated Plants.