454 Plant-Breeding 



Enough hills should be provided so that each student may 

 have for himself several tuber-units. Five to ten units to each 

 student will be enough if the student is required to observe and 

 compare a large number as they lie in the field. The complete 

 data for the whole field should be compiled by the class as a 

 whole, and distributed to each student for a comparative study. 



Program. Just prior to the exercise, each hill should be 

 dug carefully and the tubers replaced where they grew, but 

 exposed to sight, especial care being taken that no labels be mis- 

 placed nor lost. The class may then be taken to the field. 

 The instructor should explain the purpose of the exercise, the 

 principles of pure-line selection as illustrated here, and the 

 method of planting a potato-breeding plot by the tuber-unit 

 method. He should give careful instructions for the after- 

 noon's work. The class may then examine and compare the 

 units as they lie exposed in the rows. The instructor should 

 point out such differences as occur. A certain number of tuber- 

 units should then be assigned to each student, and he should 

 be required to take data from these units, as directed on the 

 printed sheets provided. Such data-taking as involves the use 

 of apparatus will necessarily have to be postponed until the 

 following period, when it can be done in the laboratory. 



Each student should carefully preserve his tubers properly 

 labeled for the next laboratory exercise. 



Second Exercise 



Materials. Data taken in Exercise 1 ; the tubers collected 

 in Exercise 1 ; scales ; paper plates (6 for each student) . 



Program. The instructor should first outline the afternoon's 

 work. He should explain the qualities that constitute a good 

 tuber ; also how that ideal form, size, and color differ in various 

 varieties. He should explain a score-card. 



The students may now proceed to finish taking the data on 

 the tubers that they collected at the previous laboratory period. 



