STUDIES ON CLUBROOT OF CRUCIFEROUS PLANTS 



429 



but a more detailed study of the same series showed the invaded cell to 

 be in every case the basal portion df a root hair. This, together with the 

 fact that no new swellings are ever found at any great distance from the 

 region where root hairs might have existed previotisly, has led the writer 

 to believe that seldom, if ever, is there direct penetration into simple 

 epidermal cells. 



This holds true not only for the area above the place where the root 

 hairs have disappeared, but evidently also for the space near the extreme 

 tips where the hairs have not yet been formed. Not only did these slides 

 demonstrate this point : but infection secured under aseptic conditions 

 in test tubes has confirmed it. The small root-tips were so placed that 

 they were the first to come into contact with particles of diseased tissue 

 and the muck-soil filtrate containing free spores. When these rootlets 

 were sectioned and stained, they showed various stages of root-hair 

 invasion, but no 

 amcebas were 

 found in any of 

 the apical cells. 

 The evidence pre- 

 sented in these 

 slides shows that 

 these invasions 

 are not, like those 

 which Schwartz 

 (1914) suggested 

 for Ligniera sp., 



confined alone to the epidermal cells of which the hairs are outgrowths. 

 The passage of amoebae from the epidermal cells into the cortical tissue is 

 demonstrated not only by the position of the amcebas within the paren- 

 chyma cells, but also by actual cell-wall penetration. 



The argument advanced for other species of Plasmodiophoraceae, that 

 infection must take place in the growing tip where cells are dividing 

 rapidly because the organism often occurs in definite rows of the cortical 

 cells, does not necessarily apply to Plasmodiophora Brassicae. A glance 

 at a section of a root tip (fig. 99) indicates the difficulty that a swarm- 

 spore would encounter in entering at this point. The rootcap does not 

 merely protect the root tip, but a row of its cells extends upward almost 

 halfway to the root hairs. The remaining distance is protected by a 

 comparatively heavy cuticle, leaving the root hair as practically the only 

 vulnerable point. Moreover, the presence of the organism in continuous 

 rows of cells can be explained in another manner. The condition shown 



FlG. 99. LONGITUDINAL SECTION OF A CABBAGE ROOT 



This shows the tip of the cabbage root protected by the cells of the root- 

 cap. X no 



