STUDIES ox CLUBROOT _OF CRUCIFEROUS PLANTS 427 



spores appeared but there was no further development. They were 

 active for a certain time, and then encysted and remained in that con- 

 dition as long as the cultures were kept. This 

 experiment was performed on four kinds of agar 

 media, on potato plugs, and on healthy cabbage 

 roots. In no case were there any signs of further 

 growth. This, with subsequent infection experi- 

 ments, indicates very strongly, if it does not prove 

 positively, that the swarm-spores never fuse. 

 This is in keeping with what has been found, or 

 > at least suggested, in all other cases of parasitic 

 slime molds, Spongospora subterranea excepted. 

 If spores for germination are taken from roots 



FIG. 98. FLAGELLATE OR- 



that have not previously been disinfected, there GANISMS ASSOCIATED 

 are often found in the cultures flagellate bodies SIC P A L F ASM DIOPH RA 

 which are almost small enough to resemble 



swarmspores. They are larger, however, are more active, and when stained 

 are more or less reniform, having two flagella arising from the concave 

 side (fig. 98). These, as pointed out later, belong to another organism. 



PENETRATION 



In the knowledge of the life history of Plasrnodiophora Brassicae, there 

 has always been a gap between the swarm-spore stage and the amoeba 

 within the cell, the true sequence of development never having been 

 shown. Most writers pass over the difficulty with the mere statement that 

 the organism enters the root and there begins its parasitic life. Woronin 

 (1878), in this as in nearly all other points connected with clubroot, is 

 the only one who has tried to fill in the gap. In a way he succeeded, but, 

 as his plants died before reaching the stage in which invasion of any of 

 the tissue took place, he is not sure that the root hair is the real point of 

 entrance. He placed cabbage seedlings in shallow watch glasses, in water 

 well supplied with spores. For some reason the plants began dying before 

 hypertrophy took place. When the roots were examined microscopically, 

 the root hairs were filled with amoebae but nothing further had happened. 

 The question still remained, whether these infections under normal con- 

 ditions would have been followed later by invasion of the cortical cells, 

 or whether the case was similar to that which Schwartz (1914) found in 

 species of Ligniera. Schwartz thinks that penetration takes place near 

 the apex of the root, so that when the root hairs act as bearers of the 

 amoebae the parasite does not advance farther than the base of the cell. 



Most writers believe not only that the apical cells and the root hairs 

 act as infection courts, but also that the epidermal cells can be infected 



