CHAPTER X 



SOME RESULTS OF CROSSING AS A MEANS OF 

 IMPROVING SELF-FERTILIZED CROPS 



In the preceding chapters it was shown that the selection 

 and increase of a homozygous individual plant isolated a pure 

 line. No one of these pure lines contains, as a rule, all the 

 characters desired. What usually happens is that one pure 

 line excels in one character, while another is superior with 

 regard to some other character. The only way in which the 

 desirable characters belonging to different strains can be com- 

 bined is by crossing and then selecting the desired segregate. 



To attain success in this field, it is important to use as parents 

 those forms which most nearly approach the combination of 

 characters desired. The old idea of indiscriminate crossiiig in 

 order to procure superior economic characters, such as yield, 

 has been largely abandoned, which is reasonable from our knowl- 

 edge of what selection accomplishes and of Mendel's law of in- 

 heritance in crosses. Love (1914) compared the yield of oat 

 selections with hybrids which were the result of more or less 

 indiscriminate crosses made by J. B. Norton. The average 

 yield of the hybrids was but little higher than the average yield 

 of the selections. It is probable that the comparison would have 

 shown a greater difference if the parents had been chosen on the 

 basis of their performance records. Two forms may be crossed 

 because each possesses to the greatest degree the character sought, 

 with the hope of obtaining transgressive segregation; or a cross 

 may be made to combine different characters. 



The Improvement of Black Oats at Svalof. Nilsson-Ehle 

 (1917) has reported experiments carried on from 1901 to 1917 for 

 the purpose of improving the black oats grown in Sweden. The 

 native oats formerly grown had weak straw and lodged badly. 

 Black Tartarian oats was introduced to overcome this difficulty. 

 Little by little this form mixed with the native oats and probably 

 naturally crossed to some extent. The resultant complex (Svart 

 Tartarish Plymhafre) was especially suitable for selection and the 



132 



