438 Plant-Breeding 



the plants given to them, and sort out the segregated characters. 

 When sorting has been completed, the counts for the whole class 

 may be ascertained. It should serve to illustrate the expected 

 theoretical mendelian ratio. 



Remarks. Timothy affords very good material for this prac- 

 ticum, especially when bundled and mounted specimens, together 

 with photographs, are available. 



Oats exhibit excellently contrasted unit characters, but expe- 

 rience shows them rather poorly adapted for class study, except 

 when mounted specimens are used. The reasons for this are : 



1. Side and panicled characters the specimens are often 

 pressed out of shape, due to drying and storing, and are, therefore, 

 difficult to distinguish. 



2. Color. Black oats crossed with white give oats of inter- 

 mediate color which are often difficult to distinguish from black. 

 White and yellow are impossible of being distinguished by the 

 inexperienced student. Moreover, color in oat hulls varies 

 greatly with the seasonal conditions under which it was grown. 



3. Plants are likely to become broken up in handling, thus 

 spoiling the count when mendelian ratios are expected. The 

 first two of these objections can be obviated by using mounted 

 specimens. Other characters such as naked, hulled, awned, and 

 awnless can be illustrated in this way. Probably a better exer- 

 cise would be given by substituting corn for oats. 



EXERCISE 16 



Mendelian Problems 



Purpose. To enable students to become familiar with what 

 might be called the mechanics of mendelism by working out 

 mendelian problems by the method of squares. 



Problem. Given : Two pairs of contrasted characters 

 Tall vine (T), dwarf vine (t) ; Yellow seeds (F), green seeds (y) . 

 Tallness and yellowness are completely dominant characters. 



