PLANT - BREEDING 



PLANT -BREEDING 



1365 



wheat), etc. The necessity of intelligent selection of 

 plants from which seed is" to be taken can hardly be 

 overestimated. 



In all of our orchard fruits selection has played a 

 most important part, though here the process is slightly 

 different, as the selection is usually limited to a single 

 generation, the best individual among many seedlings 

 being selected and propagated by buds or grafts so that 

 further selection to fix and retain its qualities are un- 

 necessary. In this way was produced the famous Con- 

 cord grape, the Dana Hovey pear, the Wealthy apple, 

 etc. It must not be understood, however, that continu- 

 ous selection would not give valuable results among 

 orchard fruits also. Some of our good select seedling 

 varieties are seedlings of select seedlings, and the pro- 

 cess is thus a continuous one, though carried on largely 

 by different experimenters owing to the long time re- 

 quired to secure fruits. 



The character of the individual as a whole is a factor 

 of prime importance in selection, and should be clearly 

 recognized by every one striving for improved or 

 pedigree plants. The aim should not be to select the 

 single best fruit, but to select fruit from the best indi- 

 vidual plant. If one is selecting to decrease the num- 

 ber of seed he would probably entirely fail should he 

 depend upon examining a number of fruits without ref- 

 erence to the plants on which they grew, and selecting 

 that one for propagation which was found to contain the 

 fewest seed. He should examine the fruit on numerous 

 trees, and then take seed for planting from that tree 

 which by the examination of many fruits is found to 

 show the most decided general tendency toward seed- 

 lessness. 



Limitations of Selection. Selection is by many horti- 

 culturists considered to have been the most important 

 factor in the development of our cultivated plants, and 

 some go so far as to assert that all other factors are 

 of minor importance. Both crossing and selection, 

 however, have their definite and distinct places in any 

 rational system of plant-breeding. When used alone in 

 the improvement of plants, selection depends upon the 

 adding up of small, insignificant variations through 

 many generations, which in the end may possibly 

 result in marvelous differences ; but by this method the 

 breeder has no way to force the change, and must be 

 satisfied with slight variation and long-continued selec- 

 tion. When marked changes and new creations are 

 desired it is to crossing or to chance sports that at- 

 tention must be directed. 



1842. Aquilegia flowers, illustrating the process of 

 emasculation. 



a, mature bud showing stage which should be selected 

 for emasculation; b, similar bud with the tips of the 

 corolla pried apart and the stamens removed; c, a bud 

 opened naturally, too old to operate on ; d, a bud of the 

 stage shown in a and b, with corolla removed to show 

 the internal organs : e, a bud the same as in d, but with 

 the stamens removed; f, the same as e, but older, at the 

 age when pollination normally takes place; g, pistil 

 shortly after fecundation, the remnants of the stamens 

 having fallen away. 



IMPROVEMENT BY MEANS OF CROSSING. Aside from 

 selection, crossing has played the most important role 

 in the formation of the varieties and races of our culti- 

 vated plants, but the results obtained are in many cases 

 closely connected with selection. In the production of 



1843. Plant of Aquilegia with 

 flowers covered with bags 

 in hybridization work. 



new and novel races it is to the crossing of mark- 

 edly different species or races that the breeder must 

 look. In this way plants may be obtained combining 

 the good qualities of two distinct sorts, with an elimi- 

 nation of the undesirable features. In plants which are 

 propagated by vegetative parts, such as buds, grafts, 

 cuttings, etc., a desirable variation obtained by cross- 

 ing may be propagated di- 

 rectly without further im- 

 Erovement. This is the case 

 i apples, pears, oranges, 

 and all orchard fruits. In 

 the case of plants propa- 

 gated by seed, however, a 

 valuable hybrid must be 

 inbred and selected for 

 several generations until a 

 fixed type is produced. 



The process of crossing 

 or hybridizing plants is 

 neither difficult nor mysteri- 

 ous. It is simply necessary 

 to recognize that plants, like 

 animals, bear male and fe- 

 male organs. In plants, 

 however, the male and fe- 

 male elements are most com- 

 monly borne on the same 

 individual and in the same 

 flower. In some cases, as 

 in the castor bean, corn, 

 etc., both sexes are borne 

 on the same plant but in 

 different flowers, while in 

 other cases, as in the date palm, hemp, box elder, etc., 

 the sexes are on different plants. 



If the plant to be operated on has the stamens and 

 pistils (male and female organs) in the same flower, 

 buds must be selected and the stamens removed before 

 they burst and discharge the pollen. This is necessary 

 in order to prevent self-fertilization. In some in- 

 stances, as in the case of the columbine, this can be 

 done very easily by simply selecting a bud just before 

 it opens (Fig. 1842) and prying the tips of the petals 

 apart so that the stamens may be pulled off with small 

 pincers or forceps. The bud should then be inclosed 

 in a small paper bag until the pistil matures normally, 

 when the bag may be removed and the pollen dusted 

 over the pistil (female organ). After this the bag must 

 again be put over the flower to prevent other pollen 

 from being brought in by insects, etc. Fig. 1843. In 

 some cases the pollen may be placed on the immature 

 pistil without injury when the flower is emasculated, 

 and this is a great saving of time when it can be done. 

 However, in experiments conducted by C. P. Hart- 

 ley, of the U. S. Department of Agriculture, it has 

 been demonstrated that in some plants the pollination 

 of immature pistils produces injury and thus cannot 

 be practiced in all cases. 



In some instances, as in the apple, pear, cotton, etc., 

 it is difficult to pry the petals apart to remove the sta- 

 mens, and in such cases the corolla may be cut off 

 without injury, in this way exposing the organs so that 

 the stamens can be easily removed (Fig. 1844). 



In striving to secure a certain combination of the good 

 characters of any two sorts, very numerous crosses 

 between them should be made in order to furnish the 

 breeder greater range of selection. Indeed at every 

 step the success of the plant-breeder depends on han- 

 dling large numbers. True, the desired variation may 

 be secured in a small batch of crosses, but the chance 

 of success is increased in proportion to the number 

 handled. In his experiments Burbank has found that 

 in peaches about one desirable variety of superior ex- 

 cellence is obtained from each 1,000 seedlings tested, 

 while in raspberries and blackberries only about one 

 sort in 20,000 has proved to be worthy of retention. 



In the crossing of races and species the hybrids in 

 the first generation are usually nearly alike, exhibiting 

 in general the same intermediate characters. In the sec- 

 ond generation of these hybrids, however, there is al- 

 most invariably greater variation, and it is usually from 

 the plants of this generation that the most valuable 



