704 Journal of Agricultural Research voi.xx, N0.9 



Under ordinary storage conditions, the progress of the fungus is 

 closely parallel to that just described, except that the progress is much 

 slower under this different environment. As described before, the first 

 macroscopic symptom of invasion from spots on the dry outer scale to 

 the underlying fleshy scale is a small, yellowish, slightly sunken area. 

 This usually increases in size very slowly in well-ventilated storage. A 

 cross section of one of these spots is illustrated in Plate 83, A, and a de- 

 tailed drawing from a similar section is shown in figure 9. The fungus 

 develops extensively at first just beneath the cuticle, and the softening 

 and lamination of the subcuticular wall is very slight. As invasion pro- 

 gresses, hyphae penetrate this wall directly, evidently by chemical solu- 

 tion rather than mechanical pressure, since the cavity is slightly larger 

 than the mycelium and there is no sign of bulging of the wall before 

 penetration is achieved. The collapse of cells beneath the epidermal 



Fig. 8. — Cross section of epidermis (A ) and underlying parenchyma cells (S) of onion scale inoculated with 

 a suspension of Collelotrichum circinans spores and kept in a moist chamber at room temperature. 

 Note softening and lamination of cell walls by the invading hyphae. Material fixed five days after 

 inoculation. Camera-lucida sketch. A, X 308; B, X 350. 



cell takes place before any appreciable invasion of hyphae occurs. In 

 the section shown in Plate 83, A, two layers beneath the epidermal layer 

 have collapsed, while only an occasional hypha is to be found beneath 

 the subcuticular wall. There is no evidence of softening of the cell wall. 

 Morever, in such lesions mycelium has never been found in the walls or 

 lumina of turgid living cells. This suggests that either the cells are 

 killed in advance of the hyphae or only slight invasion of the wall leads 

 to their collapse. This slow invasion, which prevails even after the cells 

 have become functionless, is surprising in view of what occurs when bulbs 

 are inoculated in moist chambers. Is it possible that the volatile oil 

 present in the onion scale is influential in checking the advance of the 

 fungus ? 



Under moist conditions and optimum temperature the stroma develops 

 very rapidly in the subcuticular wall, and acervuli are formed in five to 



