Lewis: Be.ack Knot Disease of Dianthera 67 



of compact tissue. In typical cases there are six such ruptures, 

 one for each of the peripheral steles. In older stages the fungus 

 areas become somewhat confluent, but they always remain more 

 or less separate. The surface becomes distinctly roughened and 

 papillate with age. The grayish color disappears and the entire 

 area becomes jet black (Plate LVIII, figs, i and 2). 



Etiology and Effect on the Host 



In order to facilitate the study of the relationship of fungus 

 and host, sections were prepared from normal portions of the 

 stem and from afifected areas in different stages of development. 

 The material was fixed in chroni-acetic acid fixing solution and 

 imbedded in celloidin. Both longitudinal and transverse sections 

 were prepared and then stained with aniline safranin and Dela- 

 field's haematoxylin. 



A section through one of the affected areas reveals the fact 

 that the fungus bears a close relation to the vascular tissue of the 

 host, and that certain definite structural modifications are caused 

 by it. 



The stem is polystelic, there being seven steles, six of which are 

 disposed in a circle in the peripheral portion while one occupies 

 a position near the center of the stem. The ground tissue is 

 made up of thin-walled parenchymatous cells with large inter- 

 cellular spaces typical of aquatic or semiaquatic plants. The 

 steles are orbicular in cross section and each is surrounded by 

 a thin-walled, completely closed endodermis. Inside the endo- 

 dermis there is a layer of thin-walled stereomatic tissue. The 

 mestome bundles are collateral and arranged in an arch toward 

 the periphery of the stem, while the inner face of the stele is 

 occupied by a pith and a few scattered strands of pure leptome. 

 The cambium lies inside the leptome. 



Sections taken from portions of the stem somewhat removed 

 from one of the affected areas show the same structure as a 

 normal unaffected stem except that the vessels of the xylem 

 contain numerous fungal filaments (Plate LXI, fig. 3; PI. LX, 

 fig. i). In some cases the vessels are completely filled with the 

 filaments of the fungus. 



All portions of the affected plants reveal the presence of these 



