68 



Mycologia 



filaments in the vascular tissue but they never invade the ground 

 tissue except in the swollen areas noted above. Sections have 

 been taken from the aerial portions, the rhizome, and the roots. 

 At this season of the year (November) underground portions of 

 the plant are abundantly supplied with the fungal filaments while 

 the aerial parts have died down and almost completely disap- 

 peared. It seems highly probable that these filaments persist 

 throughout the winter and begin growth with the aerial portions 

 in the spring. Strength is afforded this hypothesis by the fact 

 that the disease occurs in localized areas while plants somewhat 

 removed are often unaffected. This point however has not been 

 definitely determined. 



The fungus causes decided structural changes in the steles and 

 in the ground tissue immediately surrounding them. These 

 changes are affected only in portions of the stem which become 

 hypertrophied as noted above. The steles are generally changed 

 in outline and frequently become branched. The cambium of the 

 inner face is stimulated to produce new xylem cells and fre- 

 quently a wedge-shaped area results which is greater in extent 

 than the original stele. The cells of this enlarged portion always 

 contain filaments of the fungus (Plate LX, fig. i). 



The loose, lace-like ground tissue surrounding the stele is re- 

 placed by a dark, compact parenchyma with no intercellular 

 spaces. This tissue seems to be made up of cells of both fungus 

 and host but in some cases the host tissue is changed beyond the 

 border of the fungus invasion. This parenchyma develops from 

 the side of the stele directed toward the periphery of the stem, 

 while there is little or none of it produced toward the center. 

 The central stele is also usually affected (Plate LIX, fig. i). 

 Compare cells of the normal ground tissue in parts of Plate LIX 

 with Plate LX, fig. i. 



The fungus, after it reaches the outer part of the stem, 

 forms a layer of rather loose pseudoparenchyma which bursts 

 open the epidermis producing the pulvinate effect already 

 noted. From this tissue the conidiophores arise. The conidio- 

 phore layer is very compact in structure and its outer surface is 

 quite smooth and even. The conidiophores produce numerous 

 crops of spores. In cross section this layer is marked by several 



