440 



BULLETIN 387 



The small offspring at once begins to grow in the newly invaded cell, 

 the process of penetration being repeated while the tissue is still young. 

 From this statement, however, it is not to be inferred that all this occurs 

 while there is no cell division, and that each daughter cell in turn does 

 not become infected. Cell division certainly does take place from the 

 beginning, first in conjunction with penetration and later alone. The 

 result of both methods of invasion is illustrated by figure 100, B (page 430), 

 which shows a row of eight diseased cells. They extend the same length as 

 three healthy cells. Their relative lengths had been attained before 

 infection occurred; therefore the organism must have passed thru at 

 least two walls, while cell division accounts for the remainder. 



This leads the study to the 

 process of " Krankheitsherde " 

 formation. The whole subject 

 has usually been dismissed with 

 the arbitrary statement that a 

 single cell becomes diseased and 

 then a closely packed group of 

 cells finally results by repeated 

 divisions both anticlinally and 

 periclinally. A cursory study of 

 cross sections would naturally 

 suggest such an explanation, for 

 undoubtedly the diseased areas 

 are arranged in more or less 

 distinct groups. But again lon- 

 gitudinal sections of young and 

 recently infected rootlets may 

 be used to clear up the difficulty 

 and show the initial stages of a typical "Krankheitsherde." 



The impression must be avoided that passage thru cell walls is so fre- 

 quent that a single root-hair infection will suffice to spread the organism 

 thruout the entire affected part of the root. There must be repeated 

 infections, since the amoebae never migrate far, as the longitudinal sec- 

 tions show. They may enter in almost a straight path as far as the endo- 

 dermis. The invaded cells may then divide or merely increase in size. 

 Meanwhile the adjoining healthy cells show abnormal division. Nawaschin 

 (1899) explains this hyperplasia on the part of non-invaded cells as due 

 to the mechanical outward pressure of the much-enlarged diseased cells. 

 Eleven rows of uninvaded cells adjacent to a "Krankheitsherde" are 

 shown in figure 106 (page 438); in the healthy part of the root there are 

 only five rows of cells. 



FIG. 1 08. 



HOST CELLS FILLED WITH CLOSELY 

 CROWDED AMCEB^E 



