1908.] 



" Corky Scab " of Potatoes. 



597 



at one point to appear at another. These changes are most 

 marked at temperatures ranging between 6o° and 66° F. 

 Below 50 0 F. no movement can be seen, and if a section in 

 damp air on a slide is placed for twelve hours on a block of ice, 

 the plasmodium contracts very considerably and appears to be 

 preparing to pass into an encysted condition. When such 

 chilled sections are placed in water at a temperature of 65 0 F., 

 the plasmodia slowly expand and assume their normal appear- 

 ance. It is highly probable that the plasmodia in the scabbed 

 potatoes become encysted during the winter months, and 

 resume their activity when the potatoes commence to sprout, 

 but at present there is no direct evidence on this point , as 

 diseased potatoes have not yet been examined during the 

 winter months. The substance of the plasmodium is very 

 minutely granular and contains many quite small vacuoles. 

 Numerous minute nuclei are also present. 



In addition to the extension of the parasite from cell to 

 cell in a tuber by means of amoeboid bodies, it also seems 

 certain that the plasmodium in its early condition can migrate 

 from cell to cell, and in some sections an appearance of the 

 continuity of the plasmodium from one cell to another through 

 pits in the cell wall has been observed. This method of migra- 

 tion appears to be followed in those instances where the parasite 

 is extending for the purpose of passing into a resting stage, 

 rather than when spore formation is the immediate object. 

 A possible instance of this mode of migration was clearly seen 

 where the plasmodium was passing from a diseased area into 

 a young sprout. 



The incidence of spore formation is indicated by the vacuola- 

 tion of the plasmodium : at first a central very large vacuole 

 is formed, round which the whole of the substance of the 

 plasmodium is arranged in the form of a shell or crust, not 

 more than 5 to 6 ^ in thickness, then, owing to the con- 

 tinued increase in size of the central vacuole, the bounding 

 wall is ruptured irregularly here and there, with the result 

 that a hollow body is formed, more or less spherical in outline, 

 with a varying number of irregular cavities in its wall. (Fig. 6). 

 When this stage has been reached no further change of form 

 takes place in the plasmodium ; the wall or crust of which 

 becomes simultaneously differentiated into a series of cells, 



