428 BULLETIN 387 



directly up to the time when the epidermal layer is thrown off (Woronin, 

 1878). Somerville (1895) gives an observation as proof of this statement. 

 He often found swellings high up on the roots of turnips, where he declares 

 no root hairs could have been responsible for the entrance of the slime 

 mold, which must have penetrated the thick cuticle. This question of 

 entrance has a direct economic bearing on control, for, if Somerville 's 

 statement is true, Massee's (1903) assumption is certainly erroneous. 

 Massee states that the Cruciferae can be attacked only during seedling 

 time, and that after six weeks they are practically immune. It is doubtful 

 whether either Somerville or Massee interprets the conditions correctly. 

 If infection could not take place after six weeks, the grower could control 

 the disease merely by late transplanting and the proper care of his seed 

 beds; but this has evidently not proved to be the case in practice. 



Maire and Tison (1909, 1911) and Schwartz (1910, 1911, 1914) have 

 done nearly all the work that has been reported on the parasitic slime 

 molds other than Spongospora subterranea and Plasmodiophora Brassicae. 

 It is interesting to note that their conclusions agree very closely, and that 

 they feel sure the amoebae enter oftener thru the apical cells than otherwise, 

 altho the root hairs also may serve as points of entrance. They made no 

 particular study of this question, but were led to this conclusion by finding 

 uninucleate amoebae in the cells near the growing tips. Their opinion is 

 substantiated also by the presence of rows of diseased cortical cells, the 

 divisions of which apparently take place when still very near the initial 

 cells in the root tips. The powdery scab pathogene, Spongospora sub- 

 terranea, passes directly thru and between the epidermal cells into the 

 tuber (Kunkel, 1915). 



There is more or less difficulty in studying the nature of penetration 

 in the case of Plasmodiophora Brassicae, because of the fact that the 

 uninucleate amoebae are so small. They can be recognized only under a 

 very high magnification, and, since they are so nearly transparent, stained 

 sections must be used for all the work. A very large number of both 

 longitudinal and cross sections were prepared, the thickness ranging from 

 three to fifteen microns, and the staining was done with the combination 

 stains of safranin, gentian violet, and orange G. These proved best for 

 differentiating the parasite from the host, especially when orange G was 

 used in excess. 



There is no possible stage in penetration that was not represented in 

 the preparations. Large, as well as very small, roots were sectioned, 

 and a great number of epidermal cells showed amoebae. But in a careful 

 study of almost three hundred slides, none of these cells showed that 

 penetration had taken place directly thru the cutinized wall. In a number 

 of cases this appeared to be true when the sections were first examined, 



