8 H. H. Whetzel and John M. Arthur 



afford a more satisfactory taxonomic placing of the fungus than in the form 

 genus Sclerotium, where it was left by Klebahn. 



In pure cultures on potato agar, the fungus forms an appressed white 

 mycelium which is inclined to a radiate rhizomorphic formation (Plate 



III, A). The medium is soon discolored, often taking on a distinctly 

 reddish tinge which deepens with age. The sclerotia first appear as 

 irregular, white, cottony masses on the surface of the culture, in a broad 

 zone at some distance from the center of the colony (Plate III, A, B, and 

 C). They soon turn to a pale yellowish buff, deepening to a dark reddish 

 brown, and eventually become almost black when dry. The sclerotia vary 

 greatly in size and form. They are, in general, globose to loaf- shaped 

 bodies, 1 to 9 millimeters in diameter, and in many cases several are 

 agglomerated into a large, irregular mass. The surface of the sclerotium 

 is dull, fibrillose, and irregular, as contrasted with the smooth, naked. 

 polished surface of the sclerotia of Botrytis or Sclerotinia. 



A cross section through a sclerotium shows the interior to be light yellow- 

 ish brown in color, but of a much lighter tint than the outer narrow zone 

 forming the rind. This colored medulla is in striking contrast to the white 

 medulla of the sclerotia of Sclerotinia bvlborum or S. sderotiorum (Plate 



IV, A and D), to which these sclerotia bear a superficial resemblance. A 

 microscopic examination of thin sections of the sclerotia show further 

 marked differences, which Wakker (1885:23) first recognized and pointed 

 out, but which Klebahn appears to have overlooked. Instead of the 

 narrow, thick-walled, interwoven hyphae which constitute the medulla 

 of the sclerotia of the Sclerotinias (Plate IV, D), one finds large, irregularly 

 globose, thin-walled cells forming a typical pseudoparenchyma (Plate IV, 

 A). Instead of the distinctly differentiated rind of thick- walled, black 

 cells forming the rind of the sclerotia of the Sclerotinias, there is here a 

 pseudorind differing in structure from the medulla only in that the 

 cells are generally more elongated and the walls are darker in color (Plate 



V, B). As the sclerotium matures and dries, the cells of the pseudorind 

 collapse (Plate V, C). The transition from medulla to rind is gradual, 

 and is marked only by an increasingly deeper tinting of the cell walls. 

 Covering the young sclerotia and arising from the cells on the outside 

 is a loose, wefty covering of septate hyphae, the cells of which are sonic- 

 times slightly barrel-shaped and are branched in the manner character- 

 istic of Rhizoctonias. This fibrillose mycelial coating is very abundant 

 in the early stages of the development of the sclerotium (Plate V, A), but 

 collapses and disappears more or less as the sclerotium matures. The 

 walls of this mycelium also become reddish brown with age. but an 1 of a 

 lighter tint than the cells of the pseudorind. 



A comparison of the sclerotial structure of *S. tuliparum with that of 

 Corticium vagum {Rhizoctonia solani) and Corticium stevensii indicates at 

 once its taxonomic relationships. In the early stages of development of 



