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Plant-Breeding

selection, plants, plant, pollen, method, varieties, individuals and flower

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PLANT-BREEDING. The art of improving plants by crossing and selection. In general plants tend to reproduce their main characters unchanged, hut their long-recognized tendency to vary has prompted much experimentation, which, with the undirected variation, has resulted in the production of many cultivated varieties, strains, and races. In some cases the variation appears on a part of a single individual and can be traced to no apparent cause. such variations are usual ly called sports, and, when desired, are propagated by artificial asexual methods, which consist of the indefinite propagation of a single individual by cuttings, grafts, layers, etc. This method is one of the simplest and in some respects the surest method of plant-breeding, since as a rule sports are less plastic than other variant forms. The principal objection raised against the method is the weakness supposed to follow long continued asexual propagation. Familiar examples of this type of breeding are the weeping willows and elms, cut-leaved birches and maples, variegated foliage plants. etc. Selection. a second method, consists in the saving of seed from only such individuals as exhibit qualities not possessed by all, undesirable forms being destroyed. Plants grown from seed tend to vary greatly because of the action of diverse factors. As with many varieties of grapes, apples, and other fruits, some seedling may exhibit sufficient merit to be propagated without further selection by the asex ual method: mentioned. Usually, however, the progeny shows only a slight improvement over the parent plant, and selection must be continued Un til the ideal is approximated or an improvement secured that is worth commercial introduction.

Associated with selection, and in many cases preliminary to it. is fertilization, which may re sult from the transportation of pollen by wind or insects in a natural cross, as in numerous hy brids of oak, willow, etc., occurring in nature. Or there may be an artificial transfer of pollen for a definite purpose. Variations are often in troduced by this means and the desirable variant propagated and improved by long continued se lection. Since Darwin's time the superabundant nutrition of plants has been held to be a prominent cause of variation. A large part of the work of improving sugar beets, potatoes, tomatoes. cereals. etc.. has been along these lines. The usual meth

od of artificial pollination is to remove the im mature stamens from the flower to be fertilized unless the pistil is known to be sterile to pollen of the same variety of plant. When the stigmas of the emasculated flower are ripe, pollen from a related plant is placed upon them, and the flower covered with fine gauze or a paper bag to prevent an accidental application of other pollen. When the fruits o• seeds are mature, they are collected. and planted when the usual season for planting arrives. From the progeny of such a fertilization individuals that differ from both parents are se lected as the basis of new varieties. When a de sirable form is found, it is subjected to further cultivation and selection, attention being given to keep the selection always along one line. since any deviation will likely result in the ultimate failure of the experiment. In making selections the in dividual plant and not any particular portion of it must be the unit of selection. In following out this improvement thousands of individuals will probably have to be discarded. It is said that an American carnation-grower destroyed more than 00.000 plants that he had reared to flower ing to secure a single new desirable variety.

In plant-breeding the following principles are laid down: Thorough knowledge of the plant. a preconceived ideal established and maintained throughout, large numbers of seedling's should be employed in all seleetion experiments, and fixed characters should be selected as far as possible. A knowledge of the following correlations will aid the plant•breeder; Small foliage is usually asso ciated with small fruit ; dwarf seedlings produce poor plants; pale-eolored foliage is usually au ac companiment of light-colored fruit, flowers, etc.; large individuals are incompatible with numerous specimens, great productiveness with extreme earliness, and very great size with intense colora tion. By paying attention to these principles a skillful plant-hreeder call influence almost at 1,611 the external (diameters of form and color and the internal qualities of flavor, perfume, and chemical composition. It is quite as possible, however, for varieties to degenerate under selection as to im prove. and this fact shows the importance of mak ing the proper selection and constantly keeping it in mind in subsequent selections.

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