UTILIZATION OF HYBRIDS IN PLANT BREEDING 357 



therefore, highly desirable to understand the reasons why some crosses give 

 favorable results and others give little or no increase over the yield of the parents. 

 A necessary step in this direction is to develop a reliable method of measuring 

 the effect of crossing, apart from other factors that influence yield. 



"The development of satisfactory methods of comparing the yield of first- 

 generation hybrids with that of their parents has been retarded by (1) a failure 

 to fully appreciate the importance of individual diversity in hybrids, (2) the 

 abnormal behavior of self-pollinated maize plants, and (3) the difficulty of 

 securing for comparison hybrids and parents with identical ancestry. It is 

 believed that the method here described avoids these difficulties and affords 

 more accurate means of comparing first-generation maize hybrids with their 

 parents. 



"The method is illustrated by an experiment in crossing two varieties of 

 sweet corn in which it was found that the progeny from one hybrid ear yielded 

 nearly double that of the other hybrid ear involved in the experiment. To have 

 taken either ear alone would have led to entirely erroneous conclusions regarding 

 the increase secured as a result of crossing. The increase in yield due to crossing 

 as measured by the method here proposed was 31 per cent." 



Collins describes his method as follows: 



"To compare the behavior of two varieties, which may be called A and B, 

 with that of a hybrid between them, two plants were selected in each variety, 

 AI and A 2 in the one variety and BI and 5 2 in the other variety. The following 

 hand pollinations were made: A\ X A 2 , A 2 X BI, BI X BI, and J5 2 X A\. The 

 result is two hybrid ears and one cross-pollinated ear of each variety. It is 

 believed that the mean yield produced by seed from the two pure seed ears gives 

 a fair measure of the effects of hybridization. By making two hybrids involving 

 all the plants used in producing the pure seed ears individual differences that 

 affect the yielding power of the pure seed ears are similarly represented in the 

 hybrids. Thus, in both the parents and the hybrids the average yield represents 

 the mean yielding power of the four parent plants, the only difference being the 

 way in which the individuals are combined. 



"To secure the most accurate comparison of the yield of the four ears, one 

 seed from each of the ears was planted in each hill. The different kinds were 

 identified by their relative position in the hill. To place the seeds accurately, 

 a board 4 inches square was provided with a small pointed peg 2 inches long at 

 each corner. These pegs were forced into the soil at each hill, making four holes, 

 one for each of the four kinds, only one seed being planted in a hole. 

 The board was always placed with two sides of the board parallel to the row. 

 It was necessary to exercise extreme care in dropping the seeds to avoid changing 

 the position of the kinds. The best way to obviate mistakes of this kind is to 

 make ah 1 the holes of a row in advance and to go down the row with one kind of 

 seed at a time. 



"At harvest time the seed produced by .each plant was weighed and recorded 

 separately. All hills that lacked one or more plants were excluded and the com- 

 parison confined to hills in which' all four kinds were represented. The method 

 of handling the yields was to determine the mean yield of the four kinds in each 



