been possible to establish strains of wheat that combine immunity to rust 

 with other desirable qualities. For, as we have seen, it is necessary to breed 

 a sufficient number of hybrids only into the next generation in order to get a 

 complete segregation of the various dominant and recessive characters, in all 

 their possible combinations. In a third generation we can begin to select off- 

 spring with the desired characteristics in a pure dominant or pure recessive con- 

 dition. Experiments are under way to develop wheat varieties that can resist 

 more severe winters. Crosses between wheat and rye promise to yield valuable 

 results. Some of the many varieties that appear after the hybrids are inbred 

 have valuable wheat quaUties combined with the rye's resistance to cold. 



Practical Breeding The failure of their hybrids to breed true was the 

 despair of plant and animal breeders in past centuries. Only a few, like Luther 

 Burbank, were successful. Burbank was patient enough to try out vast num- 

 bers of hybrids. And he was keen enough to detect the rare individuals that 

 would probably breed true with regard to the desirable combinations of quali- 

 ties. With our present knowledge of heredity it becomes possible to produce 

 almost any combination of useful or fancy characteristics that we may desire. 

 This does not mean that new characters are produced by these methods. When 

 Burbank produced a "white blackberry" he did not get a plant with a new 

 character, in the biological sense. He combined a plant having pale-yellow 

 berries, of no value as fruit, with one having large, black berries — the Lawton 

 blackberry. From the hybrids he obtained segregating offspring. And from 

 the segregated lines he was able to fix the strain that lacked pigment and had 

 other desirable qualities in a "pure" state — that is, had only recessive genes 

 or only dominant ones from both parents. 



Every year experiment stations and private gardens of seed-producers, 

 nurserymen, and horticulturists offer us "new" flowers, fruits and vegetables. 

 Many of these new varieties are hybrids which cannot breed true. Such plants 

 are propagated by means of cuttings or grafts or by means of bulbs or tubers. 

 The Burbank potato, for example, which originated as a seedling and has been 

 one of the best-known potatoes in this country, has to be propagated by means 

 of the tuber. Seedless varieties of grapes, apples, oranges, and so on, would, of 

 course, be propagated by grafts or cuttings. But all cultivated fruits are 

 propagated vegetatively even when they have seeds. Since they are hybrid, 

 their seedlings would "split up" the combination of qualities that is of value. 



Novel combinations in annual plants, which have to be grown from seeds 

 every year, present special difficulties. But the breeders are offering more 

 and more varieties of hybrid seeds for field and garden. These seeds will grow 

 into plants having the desired combinations of characters. But the seeds of 

 these plants will "throw back" into the numerous ancestral types; that is, 

 they will segregate. 



If one wants to continue growing plants with the same qualities, he has to 



497 



