ONION SMUT FUNGUS. 123 



seed and knee. From observation of mature sori, however, it seems prob- 

 able that infection sometimes occurs above the knee. MyceUum in various 

 quantities has been found in the cotyledonary cavity of many plants, even 

 in the youngest stages, and by tracing it to the opening of this cavity it 

 can be seen that it comes in from the outside through the natural opening, 

 but in most cases it has been impossible to trace a direct connection be- 

 tween this mj'^celium and any hyphse inside the tissues between the cells. 

 This mycelium has the size and all the other distinctive characters of 

 smut mycelium, but it is not possible to prove that it is such. It was 

 thought at first that this was the usual infection court, but after it was 

 demonstrated beyond any question that in a large number of cases young 

 infections could have no connection whatever with this cavity, the con- 

 clusion was reached that only a small part of the infection could be ac- 

 counted for in this way. It is still doubtful whether the mj-celium which 

 was found in the ca\dty was always that of LJrocystis, or whether it may 

 have been that of another soil fungus. 



It is probable that all infection takes place Ihrongh the cotyledon . A case 

 was never noted where the leaf became smutted while the cotyledon 

 remained healthy. More careful experiments on this point, however, 

 might show that the leaf does sometimes become infected firjt. It is 

 probable that all infection takes place beneath the surface of the ground. 



Character of the Inoculum. 

 In all literature on onion smut it has been assumed that the spores of 

 the organism must be present in close proximity to the seedling in order 

 that infection may occur. The possibility that the mj^celium might be 

 present and growing saprophytically and indefinitely in the soil, and might 

 infect without the immediate presence of spores, has been left out of con- 

 sideration. In order to determine the abihty of saprophytic mycelium 

 to produce infection, onion seeds were germinated beneath the surface of 

 agar cultures in test tubes in such a way that the developing seedling as 

 it elongated must pass through the mat of mycelium. Over 50 per cent 

 of the seedlings became infected, although no smut spores could have 

 been present. In the stained sections which were studied, in a few cases 

 mycelium was found outside the walls of the epidermal cells where infec- 

 tion has occurred. Onl}^ in one case were spores found in these sections, 

 and at that time there was no infection beneath them. It is probable, 

 however, that spores would usually be removed by the washing process, 

 and this could hardly be adduced as conclusive evidence against the neces- 

 sit}' of spores for infection. It is probable that either sjjores or saprophytic 

 mycelium in the soil can serve as the inoculum. 



Method of Entrance. 

 The infecting hypha enters the epidermal cell by boring directly through 

 the outer wall. Since in the younger infections the stomates are not yet 

 open, and mechanical wounds have not been found, there is no other route 



