Strains, the writer would recognize as groups of cultivated plants, derived from a race, which do not differ from the original of the race in visible taxonomic characters. When the breeder, by a careful selection of Blue Stem wheat, produces a sort of Blue Stem that differs from the original race only in the quality of yielding heavily, it would be called a strain of Blue Stem.
Clons are groups of cultivated plants, the different individuals of which are simply transplanted parts of the same individual, the reproduction being by the use of vegetative parts such as bulbs, tubers, buds, grafts, cuttings, runners, and the like. The various sorts of apples, potatoes, strawberries, chrysanthemums, and so on, commonly denominated varieties, in a more restricted sense would be clone. Clons of apples, pears, strawberries, potatoes, and the like, do not propagate true to seed, while this is one of the most important characters of races and strains of wheat, corn, and the like. The term variety would thus be used in a general sense, and would include races, strains and clons.
Factors of breeding.
laws of heredity are of primary importance to the breeder. It is a general principle that ordinarily like begets like, but it is also true that like frequently gives rise to unlike. There are thus apparently two conflicting principles in plant bre ding. On the one hand. the breeder seeks to produce variations in order to get new types as the foundations for improvement. On the other hand, when such a variation from or improvement on the normal type is secured. he then reverses the pro cess and tries to establish heredity and reduce the amount of variation, so that the aphorism, "like begets like," will hold true.
In pedigree or grade breeding, and in breeding to produce new varieties, the importance of hereditary strength, prepotency or transmitting power, cannot be overestimated, as it is only by rendering this power very great that any new form can be brought to what is called a fixed type.
Unity of indiridual.—The unity of the individual is also an important factor in plant-breeding. If, for instance, the breeder is attempting to produce a seedless fruit, it is important that he discover the tendency to seedlessness in the entire individual.
It would not be the correct policy for a breeder to select simply a single fruit which might acciden tally be nearly seedless. He should examine a large number of fruits of different individual plants, and find a plant on which he can discover a general tendency toward seedlessness showing in all of the fruits produced. By selecting seed from such indi viduals, he may be able to find in time one such individual that would transmit to its progeny this tendency to produce few seeds.
While this is certainly generally true, there are some instances in which divisions of the individual are important. As an illustration may be mentioned the case of hybrids between a smooth- and a fuzzy seeded cotton : when one is breeding to produce a smooth, black seed, it may be desirable to select a part of an individual. In this case the writer has found that very frequently a cotton hybrid of the above parentage will produce bolls that vary greatly in the amount of fuzziness on the seed, and that this variation does not seem to be limited to any part of the plant in particular, hut seems to be a variation in certain branches or bolls (Fig. 78), and is thus a sort of bud variation. The writer's experiments have shown that by taking seed from certain bolls in which the seeds are nearly smooth and black, a much larger number of plants is produced the next year with smooth black seeds than are pro duced when bolls are selected in which the seeds have considerable fuzz, although the seed in both cases were borne on the same plant. This illustra tion shows that in some instances it is desirable to select a certain fraction or part of an individual which shows more clearly the character desired.
Variations.— It is well known that all plants vary. Plants differ from each other just as do men. Each plant has a facial expression, as it were, which marks it as distinct from any other plant of the same variety (Fig. 79). These slight fortuitous or individual variations are of the greatest value to the plant-breeder in connection with what may be termed pedigree breeding. By these variations alone, however, we would not expect to produce strikingly new varieties.