STUDIES ON CLUBROOT OF CRUCIFEROUS PLANTS 



439 



Lutman (1913) figures actual passage thru the wall. He believes that 

 the amoebae are transferred in the cortical tissue both by penetration and 

 by division of the host cells. 



It is altogether possible to cut a large number of sections without obtain- 

 ing any definite clue as to the mode of migration from the root hair to the 

 cortex or the medullary ray, for in the later stages the cell wall acts as a 

 perfect barrier. In view of this fact, Nawaschin might have done enough 

 staining to complete his carefully planned cytological problem without 

 once cutting a root so recently infected that the passage from one cell to 

 another could be detected. During the first two years of the writer's 

 study, only roots that showed evident hypertrophy were used and none 

 of these gave any evidence of such a passage. As soon as the smallest 

 rootlets were sectioned longi- 

 tudinally, penetration could be 

 observed. It is true that it never 

 appeared abundantly; yet it 

 might have been there and not 

 noticed, for the opening in the 

 wall is so minute and the strand 

 which passes thru is so nearly 

 hyaline that only deep staining 

 will make it apparent under the 

 microscope (fig. 105, F, page 437). 

 There are numerous cases in 

 which it is probable that such a 

 migration has taken place but 

 the connecting strand cannot be 

 seen (fig. 105, E). 



The objection has been sug- 

 gested that these strands are merely the remains of a thread which 

 was not severed when the wall was laid down between two dividing cells. 

 This may be true in such cases as are represented in figure 107, but in 

 other cases the position of the cells precludes the tenability of such an 

 assumption. 



Another argument in favor of cell-wall penetration is the shape of the 

 amoeba in the initial stages of invasion as compared with that in later 

 stages. When the smallest rootlets, containing only a few diseased cells, 

 are sectioned longitudinally, the amoebae are usually seen to be elongated 

 and often have pseudopodia extending in different directions. This is 

 never true in a more advanced stage. The amoebae are then nearly 

 spherical and remain stationary in the cell. This difference is seen on 

 comparison of figures 105 and 108. 



FlG. IO7. AMCEBA EXTENDING FROM ONE CELL 

 INTO ANOTHER 



