BREEDING 



BREEDING 



557 



generation, when he should be able to select a pure 

 type with reference to both characters. 

 " If, as frequently occurs, neither character of an 

 allelomorphic pair is dominant, but gives in the hybrid 

 an intermediate form, the fixation becomes simple, as in 

 such cases those hybrids in which either character is 

 pure can be recognized. 



While these methods appear very complex at first, 

 they will be easily understood with careful study, and 

 are far simpler than the methods breeders were com- 

 pelled to employ in fixing hybrids before they had an 

 understanding of Mendel's law. 



When more than two characters are concerned in the 

 recombination, the process becomes more difficult, and 

 indeed one cannot limit one's consideration to 

 two characters in practical breeding unless one 

 is combining standard varieties where all charac- 

 ters are good. As in simple selection work, one 

 must necessarily consider all important characters 

 that go to make up a good variety, and usually 

 one will be able to recognize Mendelian segrega- I 

 tion only in a few prominent differential charac- 

 ters. The breeder should use the knowledge of 

 inheritance that he possessess with all characters 

 which he can recognize, but at the same time the 

 plants which he inbreeds to secure purity of type 

 should be perfect plants of all-round good type, 

 and in every generation of the hybrids grown he 

 should exercise his best judgment in selecting 

 the best plants for seed-bearers. 



In the fixation of cotton hybrids, the 

 policy was pursued of selecting for in- 

 breeding the most fruitful and best-shaped 

 plants of those hybrids having the desired 

 characters, using very large numbers of 

 hybrids from which to choose. The self- 

 fertilized seed of a certain type was then 

 planted by the plant-to-row selection 

 method in an isolated plat, in order to 

 give an opportunity to select not only 

 the pure combination of the desired 

 characters but the best all-round plants. 

 As soon as the plants in such an isolated 

 plat were sufficiently developed to show 

 their characters and it could be recognized 

 that certain ones had inherited the de- 

 sired qualities, the fields were carefully 

 searched and all plants not true to type 

 were pulled up, leaving only a few good 

 plants of the correct type. This process 

 of roguing, as the seedsmen call it, insures 

 that at least the greater part of the seed 

 developed would be fertilized with pollen 

 of similar plants of good type. This sort 

 of selection and purification of type will 

 probably in most cases be found neces- 

 sary even after such Mendelian charac- 

 ters as can be recognized have been 

 secured in a pure state. 



The inheritance of many fundamental 

 characters will doubtless remain obscure 

 for many years. 



The use of impure first-generation 

 hybrids. 



In the case of very many of the most 

 important horticultural crops, fortu- 

 nately, it is possible to use hybrids with- 

 out the necessity of purifying or fixing 

 them as described in the last section. 

 Plants such as apples, pears, oranges, 

 grapes, roses and strawberries, which are 

 grown as clonal varieties, being pro- 

 pagated by buds, grafts or slips, are 

 merely parts of one individual and it does 

 not matter whether they are germinally 



648. Pyrus baccata above, 

 and three named crosses 

 with forms of P Malus, 

 Alberta, Columbia and Robin 

 respectively from top to 

 bottom. (XJi) 



pure, as seeds are not needed. This makes it possible 

 to use Fi hybrids and, as hybrids are notoriously vigor- 

 ous, this is a factor of very great importance. Again, 

 characters which blend and give intermediates in the F, 

 generation may, in such cases, prove very valuable. 



The work that has been carried out by the Depart- 

 ment of Agriculture in the breeding of citrus fruits very 

 clearly indicates that valuable intermediates may some- 

 times be secured. The writer, in conjunction with 

 Walter T. Swingle, hybridized the hardy cold-resist- 

 ant trifoliate orange (Poncirus trifoliata) with several 

 varieties of the tender sweet orange, and as a result at 

 least five different varieties of hardy oranges or citranges 

 have been produced. These hybrids are nearly inter- 

 mediate between the two parents, having the 

 characters in the first generation nearly blended. 

 The leaves are trifoliolate, but are much larger 

 than the leaves of the ordinary trifoliate orange 

 tree, and show a tendency to drop off, the lateral 

 leaflets producing an unifoliolate leaf. The tri- 

 foliate orange is deciduous, while the sweet orange 

 is evergreen. The hybrids are semi-deciduous, 

 holding a large share of their leaves through the 

 winter In hardiness they also seem to be inter- 

 mediate, being much more cold-resistant than 

 the ordinary orange, but not so hardy as the tri- 

 foliate orange. They are sufficiently hardy so 

 that they doubtless may be grown with safety as 

 far north as South Carolina, or 300 to 400 miles 

 north of the present orange region. Some of the 

 fruits produced are as large as the ordi- 

 nary orange, but most of them are very 

 nearly intermediate in size. They are very 

 variable, however, in the first generation. 

 At least five of the fruits that have been 

 produced are juicy and valuable. It is not 

 probable that they would be reproduced 

 true to seed, but orange varieties are 

 clons, and the different types will, of 

 course, be normally reproduced by buds 

 or grafts, so that from a practical stand- 

 point it does not matter whether or not 

 they would reproduce true through the 

 seed. In the second generation it is prob- 

 able that these different characters 

 would split up, possibly according to 

 Mendel's law, and it is likely that still 

 more valuable varieties will be secured 

 when a second generation has been 

 grown. See Citrange. 



Similar groups of valuable intermedi- 

 ate types of fruits have been produced by 

 Wm. Saunders, until recently the Director 

 of the Canadian Experimental Farms, by 

 crossing varieties of the ordinary apple, 

 such as the Pewaukee and Wealthy, with 

 a very hardy cold-resistant crab (Pyrus 

 baccata). Saunders has produced already 

 numerous hardy intermediate types 

 which bid fair to be of very great eco- 

 nomic value, particularly in the cold 

 regions of Manitoba and Saskatchewan 

 (Fig. 648). Second generation seedlings of 

 these valuable types may be expected to 

 yield still more important improvements. 

 The reproduction of such unfixed hy- 

 brids may be said to form the basis 

 of fruit-culture, as all of the apple, 

 peach, pear, plum, orange, lemon and 

 grape varieties, as well as the varieties of 

 small fruits, are of mixed parentage and 

 do not reproduce true to seed. Most of 

 the varieties oi these fruits are either 

 known to be hybrids or are superior seed- 

 lings that have been selected and propa- 

 gated. These latter, doubtless, in the 



