402 Plant-Breeding 



plants may vary in height from 5 cm. to 95 cm. ; to make the 

 classes differ by only 1 cm. would give us 90 classes, which 

 would be very inconvenient to handle mathematically. 



The class limits should be given in all cases, not the mid- 

 point of the class. The magnitude of a class is its value and is 

 designated by the symbol V. In calculations the mid-point of 

 a class is "used as the class value. The number of individuals 

 falling into each class is termed its frequency and is symbolized 

 by the letter /. The accompanying table shows how the various 

 bean lengths are distributed throughout the range : 



V f 



5-14 4 



15-24 72 



25-34 169 



35-44 125 



45-54 64 



55-64 38 



65-74 11 



75-84 11 



85-94 6 



500 



The graph or frequency polygon. It is often desirable to 

 present the data in a graphic way so that the eye can take in at 

 a glance such information as would otherwise require an extended 

 and careful study of quantities of figures. For this purpose the 

 frequency polygon is used. Such a simple diagram or chart 

 presents a picture embodying the chief characteristics of the 

 given population. Its significance is apparent to the student at 

 once. The frequency polygon is made, as explained in Chapter 

 IV, by plotting the class range along the base-line or axis of 

 abscissas. On the vertical axis, or axis of ordinates, are plotted 

 the class frequencies. 



When all the frequencies have been plotted in their proper 

 places on the chart they may be connected by a continuous 



