35 



THE WHEAT PLANT 



exact nature and causes of the relationships are not understood in all cases, 

 and satisfactory explanations of them cannot always be given. 



Where the variations are capable of being quantitatively expressed, 

 the amount of the correlation between a pair of characters can be repre- 

 sented as a coefficient calculated from a correlation table, in which is 

 displayed the frequency distributions of the simultaneous variations of 

 the two characters concerned. 



The coefficient of correlation (r) lies between o and i . It is a measure 

 of the extent to which one character varies in unison with another. 



When r=i complete correlation is indicated, the variations being 

 controlled by exactly the same causes ; this degree of correlation is, 

 however, rarely, if ever, found between any of the characters of living 

 organisms. 



When r = > -9 correlation is almost complete. 

 ,, r= -6-'9 there is very high correlation. 

 ,, r = '3-'6 there is distinct correlation. 

 r =o there is no correlation. 



CORRELATION BETWEEN THE NUMBER OF SPIKELETS AND THE LENGTH 

 OF THE EAR IN " SWAN " WHEAT 



Length of the Ear (mm.). 

 Average Lengths of Classes. 



