.0 



76-100% BARED SURFACE INFECTED 



tiliJlNFECTED 



___MOSTjnf ected 



1914- 1918 1919-1923 1924-1928 1929-1933 



1934-1938 



FIVE YEAR PERIODS 



f 



Differences not s+a+i s+ica I ly sigmf icani- 



FIGURE I.— RELATION OF DEGREE OF INFECTION TO RADIAL INCREMENT BY FIVE YEAR PERIODS 



